228 Prof. E. F. Herroun on the Duiergence of 



to the production of cuprous chloride on its surface, and 

 further, as Thomsen found for the heat of replacement of tin 

 by zinc from solution of its chloride numbers varying from 

 28,940 to 33,510 calories (a variation of 4570 calories, or 

 •098 volt), a determination of the previously unknown heat of 

 formation of stannous sulphate was made with the object of 

 using a tin-copper sulphate cell. 



A solution of copper sulphate, each cubic centimetre of 

 which contained •015875 grm. of copper, corresponding to 

 •45Cu SO4 IOOH2O, andi containing free sulphuric acid to the 

 extent of 2 per cent., was treated in the calorimeter with 

 excess of pure granulated tin ; the copper was speedily de- 

 posited, and the following heat measurements were made : — 



Weight of Copper. Heat evolved. Heat of Reaction. 



•635 grm. 190-36 grm." C. 19,036 calories. 



•635 „ 193-14 „ 19,314 „ 



'635 „ 193-98 „ 19,398 „ 



Mean . . 19.250 „ 



As Thomsen finds for the heat of formation of copper sul- 

 phate 55,960, this number added to 19,250 gives the heat of 

 formation of [Sn, 0, SOsAq] = 75,210 c. 



In the above reaction there was no evolution of hydrogen 

 gas, even after the whole of the copper had been deposited, 

 and the conditions were as similar as possible to those ob- 

 taining in the case of the same replacement in the voltaic cell. 

 But in the next experiment, which was conducted partly as a 

 control on the previous values, a small amount of hydrogen 

 was produced towards the end of the reaction. The solution 

 of stannous sulphate obtained in the previous reaction was 

 decomposed b}^ placing in it strips of pure cadmium, and 

 the heat of the reaction Sn S04Aq + Cd = CdS0iAq + Sn w^as 

 accordingly measured, giving 15,210 c. as the result. Since 

 [Cd, 0, SO3 Aq]= 89,880, the diflference 89,880-15,210 

 = 74,670 represents the heat of formation of [Sn, 0, SO3 Aq], 

 as measured in this way, and agrees fairly well with the 

 result obtained by the replacement of copper by tin, but is 

 slightly lower, probably owing to the above-mentioned slight 

 evolution of hydrogen, and therefore excess of the heat of 

 replacement of tin by cadmium. 



As a further check upon the general method I determined 

 the heat of replacement of copper by zinc, using the same 

 solution, viz. •45CuS04 IOOH2O, but, of course, free from 

 sulphuric acid, and found the heat of reaction to be 50,980 c., 

 the number calculated from Julius Thomson's values being 



