1886.] 



Conduction and Molecular Comjiosition, fyc. 



283 



given by Professor Lodge as representing the specific conductivities of 

 the copper-tin alloys (" Phys. Soc. Proc," 1879-80, 3, p. 158). He 

 examined five alloys, containing respectively 80*8, 38"2, 31" 7, 12*6, 

 and 9*7 per cent, by weight of tin, which were prepared by Professor 

 Chandler Roberts ; the dotted curve represents the results obtained 

 by Matthiessen, who did not examine any alloy between those con- 

 taining 16*4 and 85'1 volume per cent, of copper. The comparison of 

 Professor Lodge's curve with that given by F. Kohlrausch for mix- 

 tures of sulphuric acid and water — which I also append — appears to 



me to be in the highest degree suggestive. In the case of the latter, 

 it will be observed that, starting from S0 3 on the one side and H 2 on 

 the other, minima occur at points on the curve corresponding to com- 

 pounds of the formula H 2 S0 4 and L^SO^H^O ; it is, however, well 

 known that such compounds are unobtainable at ordinary tempera- 

 tures, and it is highly probable that if the pure compounds could be 

 examined, the minima would touch the base line, as in the case of 

 water.* The point of maximum conductivity does not correspond 

 to any known hydrate, but as I have elsewhere remarked it is 

 almost coincident with that of maximum heat evolution on mixing 

 sulphuric acid and water, and it is therefore doubtless the point 

 at which the maximum chemical change occurs. On reference to the 

 alloy curve, it is seen that the addition of quite a small amount 

 of tin to copper produces a very marked effect just as does the addi- 

 tion of a small amount of water to sulphuric acid, the effect being, 

 however, to diminish conductivity in the one case but to increase it 

 in the other ; after the addition of only a moderate amount of tin, a 



* The acids richer than H 2 S0 4 in S0 3 have been examined by W. Kohlrausch 

 (" Wicd. Ann.," 1882, xvii, p. 69). It is especially noteworthy, as I said when reading 

 this paper, that the hydrate H 2 S 2 7 is a much worse conductor than either of the 

 hydrates H 2 S0 4 or H 2 S0 4 OH 2 , and that the former of these conducts less readily 

 than the latter, and in this connexion to remember that the compound H 2 S 2 7 is the 

 most definite and easily obtained in a crystalline shape, and that the hydrate 

 H 2 S0 4 OH 2 is the least definite of the three : the evidence that conductivity depends 

 on absence of homogeneity is overwhelming in this case. — [May 26, 1886.] 



