Further Progress in Crystal Analysis Source: The Scientific Monthly, Vol. 29, No. 2 (Aug., 1929), pp. 187-190 Published by: American Association for the Advancement of Science Stable URL: http://www.jstor.org/stable/14681 . Accessed: 03/05/2014 05:25 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp . JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. . American Association for the Advancement of Science is collaborating with JSTOR to digitize, preserve and extend access to The Scientific Monthly. http://www.jstor.org This content downloaded from 130.132.123.28 on Sat, 3 May 2014 05:25:18 AM All use subject to JSTOR Terms and Conditions
Transcript
Further Progress in Crystal AnalysisSource: The Scientific Monthly, Vol. 29, No. 2 (Aug., 1929), pp. 187-190Published by: American Association for the Advancement of ScienceStable URL: http://www.jstor.org/stable/14681 .
Accessed: 03/05/2014 05:25
Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at .http://www.jstor.org/page/info/about/policies/terms.jsp
.JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact [email protected].
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American Association for the Advancement of Science is collaborating with JSTOR to digitize, preserve andextend access to The Scientific Monthly.
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eration will be afforded between the asso- ciation and the International Geological Congress in Pretoria and a Pan-Africain and Departmental Agricultural confer- ences in the same city. All the sections of the association will hold meetings in Cape Town and in Johannesburg, and the various scientific interests of these cities and their neighborhoods will be explored. After the meetings in Johan- nesburg and Pretoria, most visiting members will take advantage of one or other of a series of tours which are of- fered by the travel and tourist depart- ment of the South African Railways. These will give occasion for visits to sev- eral of the other important cities of the UTnion, such as Durban, Bloemfontein and Pietermaritzburg, and it is likely that lectures will be giveni in these places
and elsewhere by distinguished scientific visitors. Parties also are expected to proceed to Louren<:o MIarques in Portu- guese Territory, to Rhodesia and the Victoria Falls, with calls at Bulawayo and the antiquarian remains at Zim- babwe. These last are to be reinvesti- gated in advance, and an important re- port upon them is expected at the meet- ing. Lastly a selected party will visit the colony of Kenya by invitation- of its government. Arrangements so compre- hensive can- not fail to leave a permanent mark upon the development of scientific research in South and East Africa, and (speaking more gen-erally) to alle- viate that sense of' remoteness anid aloofness which even yet, in the minds of so many people, attaclhes to southern Africa.
FURTHER PROGRESS IN CRYSTAL ANALYSIS
IN an address before the Royal Insti- Lution of Great Britain, Sir William Bragg, Fullerian professor of chemistry At the institution, explained that alloys hiave played a great part in the history )f mankind. The qualities of pure metals are rarely desirable, but the properties of alloys cover a far wider range, within which can be found every variety of usefulness. Pure copper is too soft for most purposes, but when varying quantities of tin are introduced [he bronzes so formed have many appli- ?ations. At one stagc of human develop- rnent bronze was all-important. It is still largely used. The copper coinage is only slightly alloyed; the useful "'gun- metal" contains a larger quantity of tin; bell-metal contains more, and speculum )r mirror-metal more still. Small quan- tities of other substances, especially zinc, are often inserted into bronzes; and the influence of minute quanitities of such foreign substances is remarkable. Sinlce many substances can be used in making alloys, two or more at a time, and since even minute quanitities of a component often change the properties entirely, it
will readily be understood that the pos- sible variations are almost infinite. Among these metallurgists seek for those which can be put to practical use. Great advances have been made in recent years, and such terms as chrome steel, manga- nese steel, duralumin and the like, have become common.
The reasons for this variety are most obscure, anid great interest attaches to any method which can help to bring order and understanding into the com- plexity. The X-rays have come to give assistan-ce of a novel kind. They reveal the modes in which the atoms are ar- ranged in solid substances, provided that any regularity of arrangement ex- ists, and in general this is the case. It turns out that the atoms in the different phases of a mixture are put together ac- cording to different patterns, and the properties of the substances are obvi- ously connected with the pattern. In pure copper the atoms are piled together in close packing, like splerical slhot; each sphere then touches twelve neighbors. When a small number of zinc atoms are added they distribute themselves at rani-
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dom amongst the copper atoms without disarranging the pattern very much. But there is a limit to this addition. If too much zinc is put in a new pattern is formed, in which each atom now has only eight ncighbors. Next comes a remarkable change as more zinc still is put with the copper. A very compli- cated pattern is formed of which the unit is twenty-seven times as large as in the precedinig case, and there are fifty- two atoms in it. This alloy is vcry hard and brittle. Curiously enough there is an alloy of copper withl aluminium, and again of copper with tin, in which the same properties are exhibited, the same pattern is founid, and the same number of atoms in the pattern. Mlorcover, what is still more interesting is that there is the same number of free electrons. The free electrons are those which a metal can shed comparatively easily: a zinc atom can slhed two, an aluminium atom three and a tin atom four. These curi- ous alloys are composed of five atoms o-C copper to eight of zinc, nine of copper to four of aluminium, and the third, very approximately, in the ratio of thirty-one of copper to eight of tin. In eaclh case there are thirteen atoms to twenty-one electrons.
Professor Bragg says that these new results, which are most interesting from all points of view, are due mainly to the work of Owen and Preston, Bradley, and Bernal in England, Westgren and Phragmen ill Sweden. They open up new ideas of the conditions in the alloy. They suggest that we oughlt not merely to tliink of an alloy as a mixture of atoms, but in some cases at least as a mixture of electrons witlh atoms, the latter having considerable latitude as to nature. Somewhat similar conclusions have been reached in regard to the sili- cates composin-g by far the major part of the earth's crust.
In a different directioni an important step forward has been made in Alrs. Lonsdale's (Mvliss Yardley) determinia-
tioni of the dispositioni of the atoms in the organic compound hexamethyl ben- zene. The application of the X-ray methods to organic structures has always becn very tempting, because the proper- ties of the organic molecule depend so remarkably on the mutual arrangement of the atoms of which it is composed. This has of course been long known, and it has been found possible to arrive at some knowledge of the particular designs by studies of the chemical reactions pe- culiar to them. But the X-rays may be expected in the end to furnish quantita- tive, as against qualitative, details of the molecular structure, and to give the rela- tive positions of the atoms in space. The long chain compounds, whiclh con- stitute a very important section of the organic substances, have already been attacked with success, but the other im- portant section, consisting of substances founded on the benzene ring, have lnot hitherto proved so amenable. They are more complicated, and their analysis is more difficult. If any one of them can be worked out in detail the whole prob- lem will be simplified. It appears that Airs. Lonsdale's solution in the case of hexamethyl benzene has actually pro- vided this initial success. The molecule consists of the hexagonal benzene ring of carbon atoms, to each of which is at- taclhed a methyl group (CH3). The unit of pattern contains only one molecule. It is triclinic, that is to say, there are no planes or axes- of symmetry; there is, however, a center of symmetry. As there is only one molecule in the unit cell, this center is found in the molecule itself, thle only symmetry which it possesses. In certain ways the cell very nearly pos- sesses other symmetries, and by a skilful use of these approximations as measured by the curious effect which they have on the relative intensities of reflection by different sets of planes within the crys- tal, AlMrs. Lonsdale has been able to place every carbon atom in the molecule.
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