144 HYDRATES IN AQUEOUS SOLUTION. 



very different amounts of water, e. g., H 2 SO 4 .H 2 O; H 2 S04.2H 2 0; H 2 SO4.6H 2 0; 

 H 2 S04.150H 2 0. These hydrates and no intermediate ones exist. 



According to the present theory, a compound such as calcium chloride 

 can form all the possible hydrates with water, from one or a jew molecules of 

 water up to at least 30 molecules. The composition of the hydrate is con- 

 ditioned by the concentration of the solution, temperature being constant. 



It is possible, indeed quite probable, that several hydrates having different 

 compositions exist simultaneously in every solution. We can not, of course, 

 distinguish between this possibility and the simpler one that at any given 

 concentration only one hydrate, with a perfectly definite composition, exists. 



SUMMARY AND CONCLUSIONS. 



A brief summary of the relations established, and the conclusions reached 

 as the result of this part of the work, is given below. 



1. This investigation has already been extended to 98 compounds, includ- 

 ing salts, acids, and bases, and between 1,400 and 1,500 solutions have been 

 studied. 



2. The freezing-points, the conductivities at 0, and the specific gravities 

 of all the solutions have been measured, and the ref inactivities of a large num- 

 ber of them. 



3. Three lines of evidence bearing upon the hydrate theory have already 

 been deduced, and all of these point to its general correctness. These are: 

 The relative positions of the minima in the freezing-point and in the boiling- 

 point curves; the relation between water of crystallization and lowering of 

 the freezing-point; and the effect of temperature of crystallization on the 

 amount of water of crystallization. 



4. The approximate composition of the hydrates formed by a fairly large 

 number of electrolytes and some non-electrolytes has been calculated. 

 Those substances that crystallize without water have little or no hydrating 

 power. In general, the larger the number of molecules of water of crystal- 

 lization the greater the hydrating power of the salt. 



5. The total amount of water in combination with the dissolved substance 

 increases with the concentration of the solution. The number of molecules 

 of water in combination with one molecule of the dissolved substance usually 

 increases with the dilution of the solution. 



6. In some cases, as with the more common acids, the amount of water in 

 combination with one molecule of the dissolved substance passes through a 

 maximum as the dilution is increased. A possible explanation of this fact 

 has been suggested. 



7. A number of organic compounds have been studied, but only a few of 

 these have been found to have any marked hydrating power. 



