DISCUSSION OF EVIDENCE. 155 



of mass; the more water there is present the more will be combined 

 with the dissolved substance. The more complex the hydrate the 

 greater the change in the complexity of the hydrate with rise in tem- 

 perature. Since the magnitude of the temperature coefficients of 

 conductivity seems to be a function of the change in the complexity 

 of the hydrate with rise in temperature, it follows, from the hydrate 

 theory, that the temperature coefficients of conductivity for any given 

 substance should be greater at the higher dilution than at the lower. 



A comparison of the results at the two dilutions for any given sub- 

 stance in table 48 or table 49 will show that the above consequence of 

 the hydrate theory is confirmed by the facts. The temperature coef- 

 ficients are larger at the higher dilution for every substance recorded 

 in both tables. 



One other relation should be pointed out before leaving the discussion 

 of the temperature coefficients of conductivity. We have seen that 

 the hydrates are unstable, and that with rise in temperature they 

 break down. The higher the temperature to which they are heated 

 the more unstable they become. We should, therefore, expect the 

 hydrates to break down more rapidly as the temperature goes higher. 

 If this were the case, the higher the temperature of the solution the 

 larger the temperature coefficients of conductivity. If we compare 

 the results for any given substance in table 48 or 49 we will find that 

 such is the case. The temperature coefficients for any given dilution 

 are higher between 50 and 65 than between 25 and 35. 



The above four conclusions from the solvate theory of solution, as 

 far as aqueous solutions are concerned, are confirmed at every point 

 by the results of measuring the temperature coefficients of conductivity. 

 Without this theory it does not appear to be simple to explain the 

 above relations. The agreement between the four deductions from 

 the theory and the experimental results is so satisfactory that it is 

 regarded as strong evidence in favor of the general correctness of 

 the theory. 



RELATION BETWEEN THE HYDRATION OF THE IONS AND THEIR IONIC VOLUMES. 



Jones and Pearce 1 worked out the approximate composition of the 

 hydrates formed by a large number of salts, using the freezing-point 

 and conductivity methods already referred to. They found the 

 following relation between the volumes of the ions and their power 

 to form hydrates. The atomic volume curve is obtained by plotting 

 the atomic weights of the elements as abscissa against the atomic 

 volumes as ordinates. This curve, as is well-known, contains well- 

 defined maxima and minima. At the maxima are the alkali elements, 

 the three with the largest atomic volumes being potassium, rubidium, 



Carnegie Inst. Wash. Pub. No. 180, p. 57; Amer. Chem. Journ., 38, 736 (1907). 



