242 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1925 



gesterl that the separation of the flakes of mica exposes the alkaline 

 part of the molecules, which would be more readily attacked by 

 water than the siliceous part. It will be interesting to see whether 

 the X-ray examination of mica confirms this arrangement. Again, 

 however, a word of warning as to the effect of possible impurities 

 must be uttered. Natural minerals are not pure, and any uncom- 

 bined alkaline salts present might well segregate along cleavage 

 planes in the proces.s of crystallization, and so give rise to the effect 

 noticed above, but the figures recorded by Tammann are striking and 

 suggestive. 



In this hurried review of a large field it may seem that I have 

 presented rather our ignorance than our knowledge, my intention 

 having been to show how much remains to be done before wc can 

 understand the chemical relations of solids as we do those of liquids 

 and gases. One department of research is, however, more advanced 

 than might have been supposed from my brief references to it. 

 That is the study of the internal changes in metallic alloys as re- 

 vealed by the microscope and by thermal and electrical methods 

 Metallography has made wonderful progress since the days of 

 Sorby, and it would repay students of physical chemistry to give 

 some attention to its main results, even though they ma}^ not intend 

 to make a special study of the subject. Nowhere are the benefits of 

 the doctrine of phases of "Willard Gibbs to be more clearly traced, 

 whilst the recognition of every change of phase by microscopical 

 examination, making use of a technique which has been brought tn 

 a high state of perfection, gives concrete reality to the study by 

 direct verification of its conclusions. 



To understand more thoroughl}^ the mechanism of these changes 

 in alloy's and to extend its application to salts, minerals, and 

 rocks, we need a fuller knowledge of the relation between crys- 

 tal structure and chemical behavior. Research on the mechanical 

 side is discovering the direction of planes of slip in the atomic space 

 lattice under stress, and it remains to determine the corresponding 

 planes of greatest and least chemical activity towards a given re- 

 agent. Next follows the still unsolved query as to the nature of the 

 intercrystalline boundary, and the solution of these two problems 

 will make it possible to define exactly the chemical character of a 

 given aggregate of crystals. The results will be of extreme interest 

 for the study of metallurgy, of mineralogy, and of petrology, be- 

 sides filling a serious gap in chemistry, serious because of the extent 

 to which solids compose the world around us, and of the part which 

 they play in our daily life. 



