AVOGADRO'S LAW FOR SOLUTIONS 31 



dissolved nitrogen, we get a ratio between pressure and 



concentration 



C = kP, 



exactly as if the nitrogen were dissolved in the form N 2 

 instead of N 2 . H 2 O. Hence the presence of a hydrate has 

 no influence on the law of absorption ; and generally that 

 of any hydrate N a (H 2 O) 6 does not differ from solution of 

 the water-free molecule N a . 



We may therefore conclude that gases which on solution 

 follow Henry's law, possess the same molecular weight in 

 the solution as in the gaseous state, with only the possible 

 / exception that hydrate formation, always without change 

 in size of the gaseous molecule, may occur. As already 

 remarked, nearly all the gases whose absorption has 

 been studied belong to this category: N 2 , H 2 , O 2 , CO 2 , 

 CO, N 2 O, CH 4 , C 2 H 4 , H 2 S, NO, C 4 H 10 , C 2 H 6 in water and 

 alcohol ; C0 2 in carbon disulphide and chloroform ; C 2 H 2 

 in acetone 1 . Indications of a different behaviour occur 

 with NH 3 and S0 2 in water, while HC1 in water departs 

 so far from Henry's law that an entirely altered molecular 

 magnitude on solution is to be concluded. 



2. Avogadros Law for Dissolved Substances. 



What for our purpose is essential in the preceding 

 argument is not that the molecular weight of some ten or 

 twelve gases in solution is known, but the principle that 

 if Henry's law of absorption answers to the facts, then 

 a gas does not change its molecular magnitude on going 

 into solution. On account of the limited data available on 

 absorption, and especially on account of the fact that most 

 substances are too involatile to give gases or vapours of 

 measurable density, the principle discussed is of minor 

 importance in its direct application. We have rather to 

 consider the question without assuming any experimental 

 data on absorption what property must any dissolved 



1 Compt. Rend. 124. 988. 



