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pulpy brain,) you may look on the deceaseof the foetus as certain. That the motherhave not felt her child for weeks together, isno decided proof. This mobility of the bonesalone deserves no reliance whatever. Cuti-cular desquamation itself (possible in conse-quence of cutaneous disease) is an ambi-

guous indication ; the total dissolution andbreaking up of the bony structure of the cra-nium is the best, and perhaps the only cer-tain sign of death. Many a child, rashlypronounced to be dead, breathes and criesimmediately on leaving the vagina ; and therecollection of these acknowledged truthsmay, I trust, hereafter paralyse some pru-rient murderous hand, too eager for the per-forator.

I shall conclude my remarks upon thosefostal heads which deviate from the standard,by observing that we sometimes meet withheads without brains, and I here show aspecimen, unique, no doubt, and the onlyone to be met with in this theatre. (Laugh-ter.) By the Gexznans, in consequence oftheir resemblance, these crania are calledcat’s head, a denomination by no means in-appropriate. The bones of the occiput, front,sides, and summit, are wanting, while thosewhich form the basis cranii are perfectenough. This defective organisation I therather notice, because where it occurs, andwhere the accoucheur is not in full posses-sion of the confidence of the family, it leadssometimes to an ill-grounded suspicion, thatthe cranium has been laid open. Observe,therefore, the specimen here submitted toinspection, and recollect that this is nothingmore than a particular variety of monstro-sity, on the whole not unfrequent. Withinthe circle of my own obstetric acquaintance,four or five examples of this brainless mon-ster have occurred, and in two instancesgave rise to unpleasant and unjust sur-

mises.-Et hœc huctenus.Presentation and Situation.—Before I enter

on the next important topic, I mean the pas-sage of the fall grown foetus through the pel-vis, it may not be amiss that I should ex-

plain to you the meaning of two obstetricterms of frequent use—presentation, I mean,and situation. By presentation, the accoucheur,accurate in his language, understands thatpart of the child which is found lyingover the centre of the pelvis. Thus, inthe illustrations I here give you, the arms,the face, the breech, the legs, and so on,constitute the presentations, for these are

lying successively over the centre of the

pelvis. By the situatiorc of the child, whenspeaking of its passage through the pelvis,we mean the place of it, with respect to thesurrounding bones. Thus, in the illustra-tion here given, the vertex of the child pre-.senting one ear is situated on the symphysispubis, and the other on the sacrum; the

face on one side of the pelvis, and the occi-put to the other. Again, the arm present-ing as here ; the head is situated on the oneos innominatum, the body on the other; theabdomen in front, and the back in the pos-terior part of the uterus. So, then, to dropa more extended exemplification, in the ac-curacy of obstetric language, by presenta-tion we mean that part of the child which islying over the centre of the pelvis; bysituation, the place which the child holdswith respect to surrounding bones.When we meet again, 1 shall resume.

LECTURES ON CHEMISTRY,

BY

PROFESSOR BRANDE.

Delivered at the Royal Institution of GreatBritain.

LECTURE V.

On the Thermometer, and the Nature of Heat.

GENTLEMEN,—The last Lecture was oc,cupied by endeavouring to make you under-stand the action of heat, in producing theexpansion of bodies ; at present, we shallconsider the apparatus usually employed forthe admeasurement of the degrees of heat-namely, the thermometer.Now, the thermometer, in itself, is a

very simple instrument; a certain quantityof a fluid is enclosed in a vessel, allowingits expansion or contraction to take placewith facility, and it is generally found, thatif we mix equal quantities of the same fluid,at different temperatures, the cold portionwill expand as much as the hot portion con-tracts, and that the resulting temperaturewill be the mean ; so that, it appears, thatas much heat is gained by one portion as islost by the other. Upon this principle, ther-mometers are constructed. It is assumed,that if 10° of heat be taken from a cold, andadded to a heated body, that the expansionof the one will be equal to the contractionof the other ; simply supposing, that theincrease of bulk is equal to the increaseof heat, between 32° and 2120, which isthe ordinary range of the thermometricalscale. Without taking you far into the his-tory of the thermometer, I may just ob-serve, that the first which was constructed

upon truly scientific principles, was thatinvented by Sanctorio, of Padua. He con-fined a portion of air in tubes, which, byits contraction and expansion, was used to

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exhibit the alternations of heat and cold.This instrument was very imperfect, andextremely inconvenient, from the rapid andgreat dilatability of air, by moderate changesof temperature ; but principally on accountof the pressure of the atmosphere on thecolumn of fluid at different altitudes. TheFlorentine academicians were the first to

employ a solid body instead of air, the ad-vantage of which is very evident ; but,upon the whole, it was an useless instru-ment, if we speak of it in comparison withthose now constructed. This instrumentwas a bulb containing spirit of wine, andclosed so as to exclude the influence of air,and to prevent the co-operation of this fluid.Now, this served to show, very well, theamount of increase or decrease of tempera- iture in a given body, but it did not allow ofany accurate comparison being made withothers; for there being no fixed point ateither extremity of the scale, the graduation I

was arbitrary, and no two instruments,when placed in the same temperature, indi- cated the same degree of heat. You willfind, in the proceedings of the FlorentineAcademicians, drawings of various instru-ments made on this principle, but they wereof no use, except by direct comparison.All these, however, have been far sur-

i

passed, by contrivances of a later date ; sothat, now, thermometers are constructedwith an extreme degree of nicety. Thethermometer, as now constructed, consistsof a glass tube, having a bulb at one ex-tremity, for the fluid to be employed, andhermetically sealed at the other. In select-ing a tube for the manufacture of a ther-mometer, care should be taken to see that itis perfectly cylindrical and perfectly clean ; ’and if the column of mercury inserted, beequal in all parts of the length of the tube,it shows that it is quite straight. Then a

ibulb is blown by heating the end of thetube, and by forcing air into it from a

caoutchouc bottle, attached to the other ex- litremity of the tube, until the bulb becomesof a sufficient size. The caoutchouc is ne-

cessary, because the moisture of the breath ,,would destroy the tube for the purpose in-tended. The next point is to get the fluidinto the bulb, which is intended to be usedto show the degrees of expansion, whichwill be hereafter explained. Now twofluids are, at present, usually employed forthis purpose, alcohol and mercury, theformer being very well suited for the indi-cation of extreme cold, from the fact of itsnot freezing ; and mercury, as it boils onlyat a very high temperature, is a very goodsubstance for showing great degrees of heat.The main difficulty to be overcome in esta-

blishing what states of temperature shouldbe considered as the fixed points from whichto reckon the degrees, remained to be over-

come ; and Mr. Boyle has the merit of hav.ing suggested that point, at which wateris converted into ice, as the zero; andMartin gives Dr. Halley the merit of fixingupon the boiling point of water as a standardof graduation, which elevates the quicksilverin the thermometer tube always to a givenpoint, under given barometrical pressure,and the mean pressure of thirty inches isgenerally understood to be that at which ourthermometers are graduated.Now, the three thermometric scales at

present most in use in Europe, are those ofFahrenheit, Reaumur, and the centigrade.Fahrenheit’s scale commences at the tem-

peratnre produced by mixing snow and salt,being at 32° below the freezing of water,which point, therefore, is marked 32°, andthe boiling point’212°, the intermediate spacebeing divided into 172°. In Reaumur’s scale,the freezing point is zero, and 80° the boil-ing ; and in the centigrade, the space betweenthese two is graduated into 100°. Now itwill frequently happen, that during yourchemical studies, you will find temperaturesmentioned, reckoned according to one or

other of these scales ; therefore, it is verynecessary that you should know how to

compare these, so as to ascertain the equiva-lent degree on the one scale, for any givendegree on another. There are some ther-mometers, on which the degrees are markedoff, according to these three scales ; but ifyou should not be in possession of such, youmay easily reduce Fahrenheit to Reau-mur, if you recollect that each degree ofthe former is equal to four-ninths of a de.gree of the latter; and that, if you take anydegree of Fahrenheit, subtract 3io, multiplyby four, and divide by nine, and the resultwill give you the corresponding degree ofReaumur. To reduce Reaumur to Fahren-

heit, you multiply the degree by nine, aitttdivide by four ; to the product, 32° must beadded. Each degree of Fahrenheit is equalto five-ninths of a degree on the centigradescale ; so that, if you multiply any degreeof Fahrenheit, having first substracted 32°,then multiply by five, and divide by nine,you will obtain the corresponding degree onthe centigrade scale-and vice versa.

You will understand then now, uponwhat principles the thermometers are gra-duated, and how far they are to be dependedupon as measures of heat. Mercury beingvery equal in its expansion, is for generalpurposes preferred ; and spirit, as it is neverfrozen, being well adapted for the admea-surement of low temperatures. Linseed oilwas employed by Sir Isaac Newton to fillhis thermo metrical tubes, and he assumedthe boiling and the freezing of water as thefixed points; it is a liquid, however, veryinconvenient on account of its viscidity, andfrom the irregularity of its expansion. A

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great variety of thermometers are in use,but for common purposes, you cannot havea better than the spirit thermometer, whichis a very simple instrument; the bulb is

usually protected by a brass shield, but formeasuring the heat of fluids, the bulb shouldbe bare. You may have the scale markedupon the tube, or you may have the scaleinserted into the tube, containing the spiritor mercury. (Mr. Brande then showed theclass a great variety of these instruments,marking the gradual improvement that hadtaken place in their construction.)Now of course the indications of the

thermometer become unavailing, when wearrive at temperatures at which mercuryboils, which is about 500° ; the thermome-ter is unavailable, because the tube con-

taining the mercury would burst, and wemust have recourse to another mode of

ascertaining the increase of temperaturebeyond that point. Such a mode was longsince suggested by Mr. Wedgewood, who,observing the differences produced in thebulk of clay on exposure to a strong heat,took a piece of clay, measured it, and thensubmitted it to a powerful heat; and bycomparative experiments of this kind, heendeavoured to establish, by the degree ofshrinkage of a given bulk on exposure to agiven heat, the absolute heat of any furnacebeyond that point by the comparative shrink-age. But as Sir James Hall has correctlyobserved, the shrinkage of the clay is thesame when submitted to a red heat for aquarter of an hour, as when exposed to awhite heat for five minutes; and hence agreat door was opened at once to fallacy inarriving at any conclusions fiom this testof heat. The shrinkage, as it will be seen,depending rather upon time, than upon thedegree of heat to which the clay was sub-mitted. The metals are not, however, liableto that objection; and here is an instru-ment invented by Mr. Daniel, which I willexplain to you. A platinum bar is insertedin the centre of a cylinder of black leadware, as it is called, which bears a great de-gree of heat, and from the top of the pla-tinum bar, a wire was passed over the axisof an index. He established by various ex-periments a sort of accordance between ascale which he attached to the index andthe scale of Fahrenheit, and he considersthat the boiling point of mercury would beabout 92° upon his pyrometer, or equal to644° of Fahrenheit; that the point of fu-sion of tin would be about 630, or equal to4410 of Fahrenheit; bismuth 66°, or equalto 462° ; lead 870, or equal to 609°; castiron 497°, or equal to S477° of Fahrenheit.

If you take a bar of brass and steel, rivetthem together, and lay them on a flat sur-

face, the brass contracts more rapidly thanthe steel, so that in cold weather the steel

will be bent upwards, and in hot weather inthe other direction. This principle has beenapplied to the manufacture of the pendu-lum of clocks, and also to the balances cfwatches and chronometers ; and when we

consider the great minuteness of watch-work,we cannot but feel surprised at the great de.gree of perfection at which this compensa-tion balance, as it is called, has been

brought. The chronometer taken by Capt.Parry to the North Pole, varied a very fewseconds only during the voyage, on accountof the compensation of the balance by thedifferent degrees of expansion and contrac-tion of the metals of which it was composed.These things belong, however, more to themechanical than to the chemical philoso.pher.Here is an instrument, which has been

an apple of discord in the philosophicalworld, especially between Count Rumfordand Mr. Leslie ; Count Rumford said, hehad discovered it, and he called it a ther-moscope, and VIr. Leslie called it a differentialthermometer. Van Helmont, however, hasdescribed a similar instrument, and hasmade the same application of it as thesetwo philosophers have done. It has beencalled a differential thermometer, on ac-

count of its showing the difference of tem-perature between the two balls. Now, re-member that this thermometer merely indi.cates the degree of heat given off by bodies,and thus becomes a measurer of that quan-tity of heat, which bodies are capable of

absorbing or communicating under ordinarycircumstances ; but it is by no means a

measurer of the quantity of heat containedin bodies. It consists of two large glassbulbs containing air, united by a tube twicebent at right angles, containing coloured

sulphuric acid. When a hot body ap-proaches one of the bulbs, it drives thefluid towards to other. Its principal ad-

vantage is, that in making delicate experi-ments, the general changes of the atmos-phere do not affect it ; but you must remem-ber, that it only indicates the differenceof temperature between the two balls.Sometimes a simpler form of air tliermome-ter is used, consisting merely of a bulbwith a tube at one extremity, which is

plunged into coloured water, that is madeto stand at any convenient height in thetube, by expelling a portion of air by theapplication of heat. As the air in the bulb

may be heated or cooled, the fluid will riseor fall in the tube, and such variations maybe marked by affixing a scale on the instia-ment.

Speaking of the quantity of heat, it mustbe borne in mind, that there must be twiceas much heat in a quart of water as in apint, yet the thermometer will only indi-cate the same degree of heat; the quantities

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of lieat which bodies, in the same state, re-quire to raise them to the same theimcme-tric temperature, is called their specific heat,and those bodies requiring most heat forthat purpose, are said to have a greater ca-yacity for heat. Dr. Black, of Glasgow,first showed. in his lectures in 1762, thatthe quantity of heat in different bodies ofthe same temperature, was different.

It has been said, in proof of the accuracyof the thermometer, that the mixture of

equal volumes of the same fluid at differenttemperatures, will give the arithmeticalmean of the temperature of the two ; andthat a mixture of a pint of hot and coldwater will afford the mean temperature ofthe two, as indicated by the thermometer

previous to mixture ; but if you take a

quart of oil, and a quart of boiling water,as indicated by the thermometer at 212°,and mix them, you will find if you attendcarefully to what takes place, that the mix-ture will not possess the arithmetical meanof the two, but that there will be a certainquantity of heat absorbed. Now, it was todetermine how this happened, that La-voisier and La Place commenced a series ofinteresting experiments. They took a ves-sel properly constructed, into which theyput a quantity of ice ; and into the centreof this vessel, they put a given quantity ofboiling water ; the whole was then’covered

up, and so contrived, that the quantity ofice thawed in the cooling of the water, fromthe boiling to the freezing point, could beascertained; and it was found, that twelveounces of ice were thawed. They thentook a quantity of oil heated to the sametemperature as the boiling water, 212°, andthey found that, on cooling the mass downto 320, only six ounces of water ran out ofthe ice; so that they inferred, that the

quantity of heat in the water, to that of theice, was as 2 to 1. There are experiments,which might be made to show the same

thing, namely, that the degree of heat, asshown to belong to bodies by the tlxer-

mometer, differs from the absolute quantityof heat they contain. You will see, fromthis table, (pointing to one behind him,)the resulting temperatures of a mixture ofbodies at different temperatures. Thus two

portions of water mixed together, at

50°&} afford a mean of 75°.10Uo

Water at 50°, } a mean temperatureMercury at 100°, } a of 70°.A pint of water at 50°, } a mean of oA pint of oil at 100°, a mean of 80°,

The capacity of bodies for heat is verydifferent, and when you change the form ofbodies you change their capacity for heat ;

and this is the case with regard to solids,liquids, and gases. On condensation, solidslose their capacity for heat; by hammeringiron on an anvil you squeeze out the heat fromit, to use a vulgar phrase; but if you putit into the fire it again expands, and if allowedto cool gradually it again retains, when cold,the same degree of heat as before it washammered. Now if you mix together twofluids, so that by their admixture their densityshall be increased, as by mixing together oilof vitriol and water, a quantity of heat willbe evolved, or the capacity of the bodies forheat, in other words, is diminished. Onthe contrary, if you suddenly expand or rarefyair, it becomes cold; the air in expandinghas its capacity for heat increased, andwhence is it to get its supply of heat butfrom itself’! It is, therefore, clear, that if

you compress air you produce proportionablya quantity of heat, and with a syringe ofthis kind by a small stroke of the piston youmay set free so much heat from the aircontained in the cylinder as to set fire to apiece of tinder attached to the piston.Now it is easy to see how these facts bear

upon natural temperature ’ as the air ascendsit becomes rarefied, and cooling, deposits itsmoisture on the mountain tops ; and on theother hand, as it approaches the earth, it be-comes more dense, its capacity for heat isgreater, it absorbs moisture from the earth’ssurface, which being conveyed in its turnto the upper regions of the atmosphere, isagain deposited in the form of rain anddew, or snow and hail, as the temperatureof the medium through which it has to passmay be hot or cold. The temperature ofmines may be easily accounted for by the in-creased pressure to which the atmospheieof the mine must be subjected by the weightof the column of air above it.

In the next lecture the various conductingpowers of- bodies in regard to heat will beconsidered, and also the nature of latentheat.

LECTURE VI.

On Heat.

GENTLEMEN,—It happens, that’.’.’hen dif-ferent bodies are exposed to the same sourceof heat, they allow it to pass through themwith unequal velocity, and the body is saidto have a greater or less conducting power inproportion to the celerity or tardiness withwhich it becomes cooled down to the tem-

perature of the surrounding atmosphere.Among the solid bodies, down, feathers,

wool, cotton, and other light and porous ar-ticles of clothing, are bad conductors ofheat, and are therefore chosen for that pur-pose ; next to them we may rank lightearthy bodies and wood, and from some ex-

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periments of Professor Mayer, of Erlangen,it appears that the conducting power ofdifferent woods is in proportion to their den-sity, as you will see by the table behindme. Thus, assuming water to have the

specific gravity of 1.000, and its conductingpower 10, the conducting power of ebonywood will be 21.7, and its specific gravity1.054; apple tree 27.4 and 0.639; ash30.8 and 0.631 ; beech 32.1 and 0.692, andso on. Next to them we may name siliciousand hard stony bodies in general ; then

glass, the topaz, and the diamond. But ofall solid bodies, the metals are best conduc-tors, and silver, gold, and copper are bet-ter conductors than platinum, iron, andlead.

Liquids are bad conductors, and on thissubject we may advert to the experimentsof Count Rumford. He took a large glassjar of this kind, having filled it with warmwater, he put at the bottom of it a lump ofice ; the water about the ice formed a stra-tum of ice-cold water, as was shown by im-mersing a thermometer into it, and it re-quired a long time to make that water warm.But there is a number of simple experi-ments to show the same thing. In the pro-cess of distillation, you know that the steamenters the worm tub at the upper surface ofthe water, and the water in the upper partof the tub will become almost boiling hot,whilst the water in the bottom of the tubremains quite cold ; therefore, when wewish to heat a mass of water, we do not ap-ply the heat at the top, which would be anendless job, but we apply to the bottom ofthe vessel containing it, and the ascent anddescent of the heated and cold portions ofwater constitute the phenomena of boiling.Now here is an apparatus with which the

bad conducting power of water may be easilyshown : we have an air thermometer im-mersed in a vessel of water, and you willfind, that although we apply heat to thesurface of this liquid, we shall in vain lookfor any elevation of temperature in the wa-ter beneath. The water about the fire be-comes very hot, and that stratum of thewater will remain hot without communicat-ing the heat downwards, and consequentlywithout producing any effect upon the airthermometer below. I might show you anumber of experiments to prove, that iffluids do conduct heat, thev conduct it very Iimperfectly. You might boil the surface ofwater in a vessel in which ice was at the ibottom. Another mode of determining thesame fact is this ; you take a glass cylinder, Iin which an air thermometer has been: iplaced, and you pour some boiling oil on,the surface of the water ; the thermometer ildoes not rise, although it may be held very Inear to the surface of the water. It hasbeen found, however, by experiments made I

with extreme caution, and taking into ac.

count the conducting power of the sides ofthe vessel, that heat does ultimately makeits way downwards through the liquid ; butit must be still considered that fluids havea very weak conducting power.

Air is an equally bad conductor as water,and indeed worse ; you may make substancesred hot in the air contained in the upperpart of a vessel, yet the air below will re-main cool ; this experiment, however, isdifficult to show in a class room. We ar-rive, then, at this general conclusion, thatheat passes through solid bodies with va-rious degrees of retardation of its progress,and that it passes through them equally ina-11 directions. We find that different solidsretard the heat very differently, as shownin their different conducting powers ; andin regard to liquids, we find that they areall very bad and very imperfect conductorsof heat, the exact conducting power of eachnot having been determined, and that air isa still worse conductor of heat. I may refer

you to the experiments -of Count Rumford,described in the Philosophical Transactionsof 1792, respecting the conducting power ofdifferent materials used as clothing. Heheated the thermometer to 100°, and hav-

ing enveloped the bulb in different kinds ofmaterial, he measured the time it requiredto come down to the temperature of the at-mosphere, and he found that that time wasdirectly according to the conducting power ofthe substance in which the thermometer was

enveloped. The relative conducting powersof wood, metal, and other substances, are

shown in a number of familiar applications,as when wooden handles are fixed to metallic

tea-pots. The experiments made by thelate Lord Stanhope may be mentioned as

showing the different conducting powers ofwood and the metals. He placed a piece ofpaper around a metallic cylinder, and held itover the flame of a candle. (Mr. Brande re-peated the experiment.) It will be long be-fore the paper will burn, and the reason is

very easy to see, the heat is diffused overthe surface of the metal, ::’.s fast almost, as itis communicated to the paper ; but if youenvelop a piece of wood with paper, andexpose it in the same way to the flame of a

lamp, the heat is not rapidly distributed,and therefore the paper speedily burns.We now proceed to another very import-

ant part of the subject of heat, namely, thatwhich relates to its effect upon the state or

form of bodies.We purposely avoid entering here into the

state of heat, as connected with decomposi-tion, or chemical change. It is very gene-rally known, that when solids are heatedthey have a tendency to become liquid, andliquids to be converted into gaseous or aeri-form bodies; cool a liquid and it becomes

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solid; this’is generally the case, and youhave a familiar illustration of this fact inthe case of water and ice. Heat water and

you convert it into steam ; the steam, as itcools, becomes converted into water, andthe water, on further cooling, into ice. Weare indebted to the late Dr. Black, of Glas-gow, for an important series of observationsupon these subjects, and his researches de-serve to be ranked among the most admira-ble efforts of experimental philsophy. Ishall endeavour to give you an abstract onlyof these experiments, because to enter intodetail would require too much time. In

prosecuting his investigations, Dr. Blackwas forcibly struck with the extreme slow-ness with which ice thaws. If, for instance,you bring ice into a warm room, it is a longtime before it begins to run down into theliquid state; and there is a still more re-

markable fact, which is this, if you put thebulb of the thermometer into the ice, it willfall to 32°, and will remain there until thewhole of the ice is melted. Now it is clearthat heat is communicated to the ice fromthe surrounding atmosphere of the room,and yet the effect of that heat is not toelevate the thermometer, but to melt theice. Suppose you make the experimentwith water instead of ice ; you take a por-tion of water at 32°, and put into it a pieceof ice ; you bring the vessel, containing thewater and the ice, into a room, the tempe-rature of which is at 60° ; you will find thatthe temperature of the water will ascendgradually to 60°, but the temperature of theice will not ascend until the whole is melted,and then it rises. The Doctor called theheat, absorbed during the melting of theice, the heat of fluidity, and having observ-ed it to affect the state of the body withoutraising the thermometer, lie called it latent

heat, or the quantity of heat conducing tomake the ice liquid. He then proceededby a new series of experiments, to deter-mine the quantity of heat that became latentduring the conversion of ice into water, andhe found it very considerable. He did thisin two ways: in the first, he brought theice into a warm room, and ascertained thetime required to thaw it; he then estimatedthe quantity of heat which had entered theice in that time. ’

In another suite of experiments, he mixeda quantity of snow and finely powdered icewith a quantity of boiling water, and he ob-served that a quantity of heat suddenly dis-appeared to liquify the ice. Here are theresults of one or two of his experiments, setdown on these tables. He mixed water at320 with water at 213°, and he founn, as hesuspected, that the mean was 1220; but onmaking the experiment under the same cir-cumstances, but taking ice at 32° and waterat 21x°, he found that the mean temperature.

was only 52°; so that, you observe, a largequantity of thermometric heat disappearedin order to melt the ice. He then found,that when he took ice at 32° and water at

172, the result was a quantity of ice-coldwater at 32. Here the water was cooled140°, while the temperature of the ice wasunaltered, that is, 140° of heat disappeared,the effect being not to increase temuerature.but produce fluidity. You see what animmense loss of heat there is in the conver-sion of ice into water ; and you observe thatthe thawing of ice must, under ordinary cir-cumstances, be only a slow process, andhence it is that natural temperatures arechanged by very slow degrees; you havenot a sudden transition from heat to cold,but a gradual and slow change of tempera-ture.

Having made these experiments to de-termine the quantity of heat, or the ther-mometic quantity of heat in water, it nextoccurred to Dr. Black, that the liquifac-tion of other solids ought to be obedient tosimilar laws. He took a piece of lead, andhe found that, until all the lead was melted,he could not raise the temperature of themelted lead beyond a certain point; but assoon as the whole mass was melted, he couldconvert it into vaponr by increasing thetemperature. There is, of course, a diffe-rent degree of heat necessary for the liqul-faction of different substances. In the case

you have just seen, of mixing a quantity ofpowdered ice with hot water at 172°, youperceive that there is a sudden loss of 140degrees of heat; but whenever you liquifya body, it must of course produce cold, andif you set out with bodies already very cold,you will still lower their temperature, andwe often produce artificial cold of greatintensity by the rapid solution of certainsaline substances in water. Mix togethercommon salt and ice ; in consequence of theaction of the salt upon the ice, a quantity ofwater will be immediately produced, and aportion of the salt be dissolved, formingbrine ; if we examine the temperature ofthe brine, we shall find that it is many de-grees below the previous heat of the ice,many degrees of heat having been takenfrom the ice by the liquifaction of the salt;and this is the common process of obtain-

ing a large degree of cold, artificially, bypurging mixtures, and so on. If you dis-solve a salt in water, you will find that thereis a great lowering of its temperature. Hereis a mixture of sal-ammoniac and nitre, andif you dissolve them in water, especially ifthe water be cold, there is a sudden lower-ing of the temperature, in consequence ofthe sudden liquifaction of the salts, and youmay, without difficulty, freeze small portionsof water in this way. Here is nitrate ofammonia, which dissolves rapidly in water,

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and produces a considerable degree of cold.Take equal portions of nitrate of ammoniaand water, the cold produced on the dis-solving of the salt is very considerable, andof course the colder the vessel the moreimmediate the effect would be. I shouldobserve here, however, in making these

experiments, that you should use the saltsrecently crystallised and finely powdered.This table shows the results of some expe.riments made on this subject by Mr. Wal.ker ; he found that, on dissolving the salts,here set down in water, the thermometersinks as indicated.

MIXTURES. THERMOMETER SINKS.

PartsMuriate of ammonia ..... 5

*)Nitre .................. 5} From 50° to 10°.Water .................. 16Nitrate of ammonia........ 1 From 50° to 40.Water ..... 1}

Sulphate of soda.......... 5 From 50° to 3°.Diluted sulphuric acid ..... 4

snow ..... From 32° to 0°.Common salt ............. 1 }

’ Thus you see that considerable degreesof cold may be artificially produced, and by Imixing together some substances, in diffe- irent proportions, you may carry the tempe-rature down to 90° below zero, as bv mix-ing ten parts of diluted sulphuric acid witheight parts of snow, the thermometer sinksfrom 680 to 910.Now it becomes necessary to show you

the converse changes, namely, to show youthat heat would be evolved when fluids areconverted into solids. When you freezewater, you must let out a certain quantityof heat, and whenever you condense a liquid,or bring a liquid into a solid state, there isa great evolution of heat. I need only topresent you with a single instance : if youcondense water by pouring it upon quicklime, it enters into composition with thelime, and in consequence of this rapid con-densation, you know that a great degree ofheat is evolved, often sufficient to set fireto shavings of wood, and other combustiblebodies, which may happen to be. near. Thecrystallisation of Glauber’s salt is attendedwith a sudden evolution of heat, and this ’

happens in all cases where you convert n I

liquid into a solid; congelation, therefore,is to the surrounding bodies a heating pro- ’,cess, and liquifaction a cooling process. ’,

Dr. Black having made out these facts, ’!in regard to the conversion of liquids

into solids, and solids into liquids, then

proceeded to examine the effects of heat

upon liquids themselves; and here thereis a circumstance closely correspondingto that already pointed out, in regard tothe thawing of ice in a warm room; if youput a portion of water over a lamp, and im-merse a thermometer in it, you wiil find thatthe thermometer will go on rising until thewater boils, but the moment it boils, thethermometer shows no further increase oftemperature. Dr. Black proceeded to ex-amine the cause of this interesting phe-nomenon. There is an increase of heat ap-plied to the vessel, but there is no increasein the temperature of the water, the effectof that surplus heat being to convert thewater into steam, which contains a largequantity of latent heat, but condense thesteam into water, and a large quantity ofheat is given out. Before I state to youDr. Black’s experiments, to determine thequantity of heat thus believed to be con-sumed in the conversion of water into steam,it will be well to advert to the boiling ofliquids generally. Now they boil at differ-ent temperatures, and yet the boiling point@f water, for example, is often spoken of, asif that were an invariable point of tempe-rature, whereas nothing can be more vari-able ; it depends very much upon the pres-suie to which they are subject; and when

173

we say that water boils at 2H°, we meanthat it does so under the pressure of an

atmosphere equal to a column of thirtyinches of mercury, or thereabouts, equal toabout 14 or 15 pounds upon each squareinch. Of course, if you diminish the pres-sure, you diminish the temperature of theboiling point; and if you increase the pres-sure, and put two atmospheres upon thewater instead of one, the boiling point willbe increased. Accordingly, as you find thatthe atmosphere is always varying in its

pressure on our earth, the boiling point ofwater is always changing, sometimes beingat 2080, sometimes at 210°, sometimes at

212°, and so on. Another cause which in-fluences the boiling point, is the nature of!the vessel in which the liquid may beheated. In glass vessels, liquids boil at twoor three degrees higher temperature than inmetallic vessels. because slass is a more

irregular, and a worse conductor of heatthan metal. But if you introduce a badconductor into the vessel, as a few shavingsof wood, you will find that the boiling pointwill be lessened, that is to say, under themean atmospheric pressure. If then youascend a mountain, of course you have aless atmospheric pressure, and the water willboil at a less temperature. Saussure accord-ingly found, that on the summit of MontBlanc, water boiled at 187°; and, on the

contrary, in a deep mine, it will boil at a

high temperature, because then the atmus-pheric pressure is greater, in proportion to itsdepth. The Rev. Mr. Wollaston constructeda thermometer so nicely, that he could tellthe difference of temperature in the differ-ent stories of a house, and he proposed todetermine the heights of mountains, byforming a scale from it. The thermometeris so delicate, that it is seldom employedin barometrical experiments generally re-sorted to.Now, to show you that pressure influences I

the boiling point with extreme facility, wemay introduce under the receiver of an air-pump a small quantity of warm water, andwe shall find that the water will boil at amuch less temperature, than when exposedto ordinary atmospheric pressure. Again,if we increase the pressure, it will boil at a agreater temperature ; and it is of importanceto us to kuow, that the temperature of thesteam is always the temperature of theliquid, at the time of boiling; and we shallbe able, by and by, to show you that waterboils at a temperature below the freezingpoint. Other liquids, such as the spirit ofvitriolic sether, and so on, will boil at thecommon temperatures of the atmosphere,when placed in an exhausted receiver of anair-pump ; and were it not for the pressureof the atmosphere, we should have all

liquids converted into a gaseous or aeriform

state. We use the term boiling, therefore,to express the conversion of a fluid into

vapour. -

There is an experiment called the chemi-cal paradox ; because the manner in whichthe experiment is made, and its effect, arevery paradoxical. Here is a Florence flask,to which is appended a stop cock ; you putsome water into it, and cause it to boil ;you have then an atmosphere of steam uponthe water which is invisible, you turn thestop-cock, and the boiling ceases. You

plunge the vessel into cold water, the waterwill again boil, because the steam has beencondensed, and the pressure of the atlnos-

phere of steam is taken off from the surfaceof the water.There are a few facts remaining to be no-

ticed, which must be deferred to the nextlecture, when the circumstances under whichheat disappears will.be pointed out, and thesubject concluded.

WESTMINSTER MEDICAL SOCIETY.

H. MAYO, Esq. in the Chair.

AFTER the minutes of the preceding’meeting had been read, and some privatebusiness transacted,

Mr. DUNCAN rose, to give an account ofthe highly interesting case of aneurismaldisease in the temporal arteries, at presentin the Hospital of Surgery, and which hasbeen already so fully detailed in the pre-vious Numbers of THE LANCET, in which hemade a very full, clear, and candid state-ment of the appearances of the disease, andof the history and progress of the patient;and after relating Pelletan’s opinions andpractice, in a similar instance, proposed,as questions of great importance for the con-sideration of the Society ; lst, What are thetrue anatomical characters of this tumour ?M, Does it consist of mere dilatation of ar-terial branches, of a varicose state of the

temporal veins, or like n2eviis maternus, isthere aay distinct parenchyma throughwhich the blood circulates 1 3d, Is the lossof the eye referrible to the operation, or tothe disease of the brain? and 4th, What isthe plan of treatment most advisable at

present, the tumour having again increasedconsiderably in size ? To these interestingpoints, Mr. Dunuan begged to call the at-tention of the Society, hoping that themembers would be able to throw some lighton them, which might be of use to the pa-tient, at the same time acknowledging hisown inability for the task.