on Atomic Volume and Specific Gravity. 529 



lowing law in respect to them: — The volume occupied by an 

 equivalent of any salt whatever in solution in water, is always 

 an exact multiple of the number 9 representing the atomic 

 volume of water. 



It is difficult to conceive whence this simple relation be- 

 tween the volume of the salts and the volume of the water 

 arises ; nevertheless, if by adopting this hypothesis we were 

 led to represent by similar formulae the volume of analogous 

 compounds this law would be interesting, but the following 

 examples will suffice to show how many anomalies we meet 

 with. We will here compare the number of volumes of water 

 which some groups of analogous compounds occupy. 



But let us pass over these objections and see whether really 

 the circumstance indicated as fact is sufficiently established by 

 experiment. The atomic volume of a salt in solution is not a 

 constant magnitude ; it varies with the temperature and with 

 the relative proportions of the salt and of the water ; we will 

 examine successively these two influences. That of the tem- 

 perature is very considerable; this is proved by the experiments 

 of Messrs. Playfair and Joule, who measured the volume oc- 

 cupied by a salt at different temperatures. The following are 

 some of their results : — 



Now at what temperature should these volumes be com- 

 pared with one another ? We are totally unable to answer this 

 question, but certainly nothing authorizes us to choose for 

 each salt a different and arbitrary temperature, as Messrs. 

 Playfair and Joule have done, by taking only the numbers 

 inscribed in the second column, because these alone satisfied 

 the law which they wanted to prove. For other bodies, how- 

 ever, they have admitted experiments made at low tempera- 

 tures; thus for the alums of iron and chromium the experi- 

 ments were made at 2|°, for the sulphate of alumina at 10°. 



