408 Dr. A. Gait on the, Heat of Combination of 



The absolute amount of heat evolved in dissolving 1 gramme 

 of metal is calculated from the following formula: — 



K = t{{vps) + c\j 



where t — increase of temperature, in centigrade degrees, of 

 the acid used per gramme of metal dissolved, 

 v = volume of the acid in cubic centimetres, 

 p = density of the acid, 

 s = specific heat of the acid, 

 c = water equivalent of the apparatus. 



The accompanying tabular statement (p. 410) shows the quan- 

 titative composition of eac 1 ! of the twenty- two alloys of zinc and 

 copper, the quantities of metal and acid used in each set of 

 experiments, the mean increase of temperature of the acid 

 per gramme of metal dissolved, and the absolute amount 

 of heat evolved in dissolving this weight of alloy and 

 mixture. 



Assuming the products of solution of each mixture and 

 its corresponding alloy to be the same *, the difference between 

 the absolute amount of heat of solution of 1 gramme of each 

 mixture and its corresponding alloy indicates the heat of com- 

 bination of the metals in forming 1 gramme of alloy. These 

 differences are shown in fig. 2, and they indicate that the 

 heat of combination is at first negative, this negative value 

 reaching a maximum when the alloy contains about 16 per cent, 

 of copper. With greater percentages of copper the negative 

 value of the heat of combination rapidly falls to zero, and then 

 becomes positive. The maximum positive value is very 

 soon reached at about 38 per cent, copper, which is near the 

 formula Cu 2 Zn 3 . Beyond this point the heat of combination 

 gradually becomes less, until at 90 per cent, copper it almost 

 vanishes. 



Further experiments were made with the alloy which gave 

 the greatest negative heat of combination, and also with the 

 alloy which gave the greatest positive value. In each case 

 when the solution of the mixture and that of the alloy had 

 reached room- temperature, they were mixed together; but no 

 appreciable change of temperature was observed. 



* Chemical investigation would be necessary to test the truth of this 

 assumption. It is possible that there may be differences in the chemical 

 products ; but it seems scarcely probable that such differences, if they 

 exist, can largely modify the thermal results which are the subject of the 

 present communication. 



