240 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1925 



Bruni has shown ami Ve<rar(l has confirmed the observation by 

 the X-ray nietliod, that true interdilTtision occurs between potassium 

 and sodium chlorides when mixed and heated in the solid state. 

 Electrolytic transport is observed in the solid halidee of silver and 

 in mixtures of silver and copper sulphides, but the modern view of 

 the sti'uctui-e of such substances represents them as built up of ions 

 rather than of neutral atoms, and this must be taken into account in 

 any interpretation of the facts. The apparent absence of diffusion 

 in minerals which have once solidified, even when fjiven freolo^ical 

 periods of time, is a .serious difficulty in the way of any general 

 theory of diffusion. Such examples of the passage of alkali metals 

 through quartz and other silicious minerals under the influence of 

 a difference of electric potential are probably not examples of true 

 diffusion at all, but merely of the passage of traces of iiu))urities 

 through a mass which is not completely imi^ervious. 



A new field of investigation has been opened up by Tamnumn in 

 his attempts to determine the arrangement of the atoms in solid 

 solutions by purel}'^ chemical means, by studying the action of 

 chemical reagents on the solid. It is a familiar fact that the 

 " parting " of silver and gold in assaying, which consists in dis- 

 solving out the silver from the alloy by means of nitric or sulphuric 

 acid, is only possible when the silver for'ms more than 60 per cent of 

 the alloy. When gold is present in excess of this proportion, only 

 a little silver is removed from the surface, and the action then comes 

 to a standstill, the acid being unable to penetrate to the interior. 

 Assuming the alloy to be completely crystalline, the atoms of silver 

 and gold will occupy the points of the space lattice, and as the two 

 metals have face-centered lattices of only slightly differing dimen- 

 sions, the amount of distortion will be small. There are, however, 

 different ways of arranging the two kinds of atoms. They may be 

 distributed at random, or they may be so regularly arranged as to 

 form two interpenetrating cubic lattices. 



The two forms of distribution may be distinguished by means of 

 the X rays, but Tannnann has also drawn conclusions on the point 

 from the action of various reagents on the alloys. He finds that 

 each reagent which attacks silver ceases to act on the alloys when 

 the proportion of gold atoms in solution exceeds a certain limit, 

 which is not the same for different reagents, but he states that it is 

 always capable of being expressed as one-eighth, two-eighths, three- 

 eighths, and so on, of the total number of atoms. The limits so 

 found are not consistent with the distribution according to the laws 

 of probability, but they may be accounted for by a regular distribu- 

 tion on the assumption that a certain number of inactive atoms is 

 necessary to protect each atom of silver. 



