270 Frederick Guthrie on Salt Solutions 



§ 89 . Remarks on Table — The above Table contains the whole 

 of the salts which I have as yet examined fully. The interesting 

 group of the chlorides of the alkaline earths, including magne- 

 sium and the no less interesting group of the perchlorides of 

 aluminium and iron, have presented difficulties with which I am 

 still contending. The same is the. case with the nitrate of cal- 

 cium and the chloride of copper, Cu Cl 2 . 



From the evidence before us I think, however, that I may 

 venture to enunciate the general law, that if we define as similar 

 salts either (1) those which consist of the same acid united with 

 bases belonging to the same chemical group (ex. Na 2 S0 4 ,K 2 S0 4 ), 

 or (2) those which consist of the same base united with acids 

 belonging to the same group (ex. KN0 3 , KC10 3 ), or (3) those 

 whose bases belong to the same group, and whose acids be- 

 long to the same group — then, of similar salts, the one which 

 produces the greatest cold when used in a freezing -mixture unites 

 as a cryohydrate with the fewest molecules of water. And to the 

 following law there seems to be only one pronounced exception : 

 The temperature at which the cryohydrate is formed is the same 

 as the temperature of the corresponding freezing -mixture. This 

 latter law, however, has to be taken with reserve as far as those 

 salts are concerned which, like A1C1 3 and MgCl 2 , decompose 

 water, and also in regard to those bodies which, like CaCl 2 , 

 unite with water under the liberation of much heat. These I 

 shall consider in my next communication to the Society. 



Cryohydrate of Et hylic Alcohol. 



§ 90. Of very great interest is the behaviour which is shown 

 by mixtures of ethylic alcohol and water when deprived of 

 heat. This interest extends itself in a practical direction, in 

 consequence of the use of alcoholic liquids in regions of ex- 

 treme cold. We have here at once a new element for consi- 

 deration. The two liquids are miscible in all proportions. This 

 means that any possible hydrate of alcohol is soluble at or- 

 dinary temperatures both in water and in alcohol. I shall 

 use the word alcohol to denote absolute alcohol, C 2 H 6 0, and 

 the word " spirit " for a mixture of this with water. 



§ 91. The fact so long known, that heat is liberated and 

 volume finally lost when ethylic alcohol is mixed with water, has 

 silently pointed to the conclusion that there must be at least one 

 definite hydrate of alcohol. It is sufficiently clear that if one were 

 forced to the alternative of relying either upon the amount of 

 heat liberated or upon the loss of volume^ the former rather than 

 the latter would be the most trustworthy. 



§ 92. Historical. — A useful historical summary of much of 

 what has been previously done in France in this direction of 



