338 MR W. S. TUCKER ON HEATS OF 



water, i.e., from NaOH . 5H 8 O to NaOH . 9H 2 O, 979'2 calories are produced. For the 

 same range THOMSEN gives 962. This agreement is as good as the difficult nature of 

 the experiment will allow, under the conditions chosen. 



(7) HEAT OF DILUTION OF CALCIUM CHLORIDE SOLUTIONS. 



All the above solutions yield two ions to the molecule. A fourth solution, that of 

 calcium chloride, was chosen, not only because of its existence in a high condition of 

 concentration, but also because it yielded three ions to the molecule. The con- 

 centrations were obtained from . the density table given by PICKEBING, and the 

 molecular weight is taken as 111. 



Tables IX. and X. give the data obtained in experiments in which the procedure 

 was very much the same as in the above solutions. 



The curve for calcium chloride is of a different character. A small change of slope 

 occurs for all concentrations. For a limited distance at high concentrations the curve 

 appears to be nearly a straight line which would cut the axis of concentration, if 

 produced at about 7 '69 molecules per 100 molecules. This would make a limiting 

 concentration of CaCl 2 . 13H 3 0. 



Many attempts were made to get an equation connecting cZQ/c?N and n/N, but 

 without satisfactory result. The total heat of dilution can, however, be obtained 

 graphically by plotting dQ/c?N and N/n, as in the case of hydrochloric acid, when the 

 total heat is measured by area. 



The recorded temperature changes on dilution were smaller than in the above 

 solutions, and the degree of accuracy was consequently not so high. 



(8) VARIATION OF HEAT OF DILUTON WITH TEMPERATURE. 



The above heats of dilution were determined under nearly isothermal conditions 

 and at air temperatures. 



Very little work has been done to determine the variation of heat of dilution with 

 temperature, COLSON* lias obtained a so-called " dead point " for various solutions. 

 He found certain temperatures at whicli the heat of dilution vanishes for certain 

 solutions. Taking one of these solutions he found the dead point the same for all 

 concentrations. Later on, however, when experimenting with sodium sulphate, he 

 found the dead point to vary with varying concentration. He concluded that those 

 solutions giving a constant dead point contain solutes in which the state of aggrega- 

 tion is independent of the concentration, and cited sodium sulphate as a substance 

 whose composition varies with concentration. 



MAGiEf found a change in heat of dilution for barium chloride from positive 

 to negative, giving a zero effect at temperatures which decrease as the dilution 

 increases. 



* 'Comptes Kendus,' vol. 134, pp. 1496-1497, 1902. 



t ' Amer. Phil. Soc. Proc.,' vol. 51, 1912. 



