406 Dr. W. A. Tilden. Influence of Temperature [June 18, 



Mean Results. 







Heat of dissolution. 



No. in table. 



lemp. 









approx. 













A. 



B. 



C. 







11, 14 



Q1 ° 



ol 





1270 





19, 20 



32 







1146 



1, 2, 3 



33 



1387 







4, 5, 6, 7 



35 



1412 







8, 9 



40 



1494 







10a, 13b 



44 



1494 



1703 





14, 15, 16b, 21, 22c 



45 





1713 



1340 



23 



60 







1869 



17b, 24, 25c 



64 





2020 



1945 



18 



68 





223 6 





From these figures it appears that although the manner in which 

 the salt has been prepared influences the result very notably, yet in 

 each series, comparing together the effects of dissolving the same 

 kind of anhydrous sodium sulphate, the heat of dissolution increases 

 progressively with rise of temperature. That this would be the case 

 might be predicted by applying the principle originally indicated by 

 Person,* and since discussed by Berth elotf and by Thomsen,J and 

 embodied in the general equation Q T =Q^ + U— V, where Q T and 

 Qt are the quantities of heat evolved in the act of solution at the 

 temperatures T° and t°, and U represents the sum of the capacities 

 for heat of the salt and the water, and V represents the capacity of 

 the solution between the temperatures T° and t°. 



The salt A must be regarded as being in the most stable condition 

 of the three, and the following comparison of the numerical values of 

 the thermal change observed and calculated, applies to the A series 

 of experiments : — 



U=(18rc+C)(T-0. 



n is the number of water molecules. 



C is the molecular heat of the salt. 



V=(18^+K)(T-0. 



C = 32-2 (Kopp). 



18>i + K=:18i5 (Marignac). 



Take at 34° =1400, which is the mean of numbers given above 

 for 33° and 35°, 



* " Ann. Chim. Phys." [3], xxxiii, 449. 



t " Ann. Chim, Phys." [4], vi, 329. 



1 " Thermochemische Untersuchunsen," I. 



