On the Action of Copper-Zinc Alloys on Nitric Acid. 231 



tried ; for I find that in some cases the branches only approach 

 considerably towards each other, while in other cases they 

 merely cross each other, but never entirely reverse their relative 

 positions as expected. 



It seems, then, that I was able to show by the above expe- 

 riments, defective and incomplete as they were: — 



(a) That in soft iron of certain kinds there exists the pheno- 

 menon of thermo-electric hysteresis with respect to twist (at 

 least when combined with longitudinal tension) ; 



(b) That, other things being equal, the hysteresis is reversed 

 at a certain twist ; 



(c) And that mechanical agitation has its own effects, which 

 are reversed as the hvsteresis is reversed. 



XIX. On the Want of Uniformity in the Action of Copper- 

 Zinc Alloys on Nitric Acid. By Dr. J. H. Gladstone, 

 F.R.S.* 



MUCH attention is being directed at the present time to 

 methods by which it may be possible to determine the 

 heat evolved in the formation of alloys. Among these methods 

 is that of the dissolution of filings of the alloys of copper and 

 zinc in nitric acid, as compared with the dissolution of mixtures 

 of the two metals in the same proportion. This method has 

 been objected to by more than one chemist on the ground that 

 there is no assurance that the same substances, and in the 

 same relative proportions, are produced in each case. As my 

 name appears on the British Association Committee which 

 has adopted this method, 1 have felt myself under an obliga- 

 tion to make some direct experiments on the point. 



In the Report of this Committee, " On the Heat of Com- 

 bination of Metals in the Formation of Alloys," read at the 

 Dover Meeting of the British Association (Report, 1899, 

 p. 246) a table is given containing details of a long series of 

 observations, which show the heat developed during the action 

 of nitric acid on copper and zinc in various proportions, both 

 when the metals are mixed together and when they are in 

 combination as alloys. The difference of heat between the 

 solutions compared has been regarded as a measure of the 

 heat evolved by the combination of the metals in forming the 

 alloy. 



The alloys experimented upon by Dr. Gait are twenty-two 

 in number. The results are set out in a table, and plotted in 



* Communicated by the Physical Society : read May 11, 1900, 



R2 



