148 DISCUSSION OF EVIDENCE. 



occur at greater concentration than in the freezing-point curves. 

 What is the meaning of this relation? To see this we must first call 

 attention to one property of hydrates which has thus far not been 

 referred to. They are very unstable, and readily break down with 

 rise in temperature. This is easily seen if we consider the facts in the 

 case of a salt like calcium chloride. At ordinary temperatures there 

 may be as much as 30 molecules of water combined with 1 molecule 

 of the salt; while at the boiling-point of the saturated solution all of 

 the water can be removed except the 6 molecules with which the salt 

 crystallizes. The higher the temperature to which a solution is heated 

 the less the hydration in such a solution. Solutions, of course, boil 

 higher than they freeze. There is therefore less hydration in the 

 boiling than in the freezing solution. Consequently, to produce enough 

 hydration to give the minimum in the curve would require a greater 

 concentration of the solution at its boiling-point than at its freezing-point. 

 The fact is, then, not only in accord with the hydrate conception, 

 but could readily have been predicted from it, as a necessary conse- 

 quence of the theory. 



RELATION BETWEEN WATER OF CRYSTALLIZATION AND TEMPERATURE OF 



CRYSTALLIZATION. 



Jones and Bassett 1 worked out the approximate composition of the 

 hydrates formed by a large number of substances, and also the following 

 relation. The hydrates, as we have seen, are very unstable systems. 

 They are readily broken down in solution with rise in temperature. 

 The hydrates which exist in solution at ordinary temperatures are 

 much more complex than those which the salts can bring with them 

 out of solution as water of crystallization. The hydrates are more 

 stable and more complex the lower the temperatures. We were, how- 

 ever, surprised, on examining the literature, to find the large number 

 of examples on record of salts crystallizing with varying amounts of 

 water, depending on the temperature at which the crystals were formed. 



A few examples will be given to bring out the general relation that 

 the number of molecules of water of crystallization is larger the lower 

 the temperature at which the salt crystallizes. 



iCarnegie Inst. Wash. Pub. No. 60. Amer. Chem. Journ., 33, 534 (1905) ; 34, 291 (1905). Zeit. 

 phys. Chem., 52, 231 (1905). 



