180 SCIENCE PROGRESS. 



a voltaic cell, the other plate being copper, and the electro- 

 lyte being a solution of stannous chloride in hydrochloric 

 acid. The electromotive force of each combination is then 

 determined and the results plotted. The curve thus obtained 

 shows that so long as the alloy contains less than 40 per 

 cent, of tin, the electromotive force is small, somewhat ir- 

 regular, and does not vary much from *i volts, but that as 

 soon as this percentage is exceeded, it runs up to a much 

 larger and very constant value of '56 volts, a value which is 

 independent of the excess of tin present. The change pro- 

 bably occurs with a slightly smaller percentage of tin, very 

 near the composition Cu 61 "64 per cent., which corresponds 

 to the formula Cu,Sn. He considers that the change is due 

 to the first appearance of crystals of free tin in the solid 

 alloy, the tin when in less quantity being all in combination 

 with the copper as Cu,Sn. Of the other body Cu 4 Sn, which 

 Riche believes to exist, he found no indication. 



In the copper-zinc alloys a similar change in electro- 

 motive force was found to occur when the composition 

 CuZn, was reached, and in the gold-tin series at about 

 AuSn. These experiments belong to a large class, in- 

 cluding those of Matthiessen, Oliver Lodge, Wiedemann, 

 and others on electrical conductivity, those of Roberts Austen 

 with the induction balance, and numerous experiments on 

 the density of alloys. In all these the physical properties 

 of a series of alloys have been shown to experience an ab- 

 rupt change at a particular composition. That these facts 

 point to real and important differences between the alloys 

 on either side of the critical percentage is certain, but it 

 would be premature to conclude that their origin lies in 

 chemical combination. Other explanations certainly can 

 be offered. For example, it has been suggested that the 

 much finer grain in the crystalline structure of alloys be- 

 tween certain percentages might prevent the electrolyte 

 from attacking the more positive metal except superficially, 

 and so cause the electromotive force after a very short time 

 to become that of the more negative metal. 1 



1 Foerster, Naturmnssenschaftliche Rundschau, ix., 36. 



