in the Analogy between Solutions and Gases. 83 



The great practical advantage arising from this method of 

 regarding the behaviour of solutions, which leads at once to 

 quantitative conclusions, consists in the fact that the applica- 

 tion of the second law of thermodynamics to liquids is rendered 

 exceedingly easy ; for reversible processes, of which this law 

 treats, can now be very simply conceived. It has already 

 been mentioned that a piston and cylinder with semipermeable 

 walls, placed in the solvent, permits of alteration of concen- 

 tration of the solution contained therein, by alteration of the 

 pressure on the piston, in exactly the same way as a gas can 

 be rarefied or compressed ; except that in the former case it 

 is the solvent which escapes through the semipermeable walls 

 on increase of pressure. Processes of this kind can always be 

 made reversible, if care be taken that the pressure on the 

 piston is made equal to the opposed pressure, that is, in the 

 case of solutions, the osmotic pressure. 



We shall make use of this practical advantage in the fol- 

 lowing pages, particularly in investigating the laws of " ideal 

 solutions;"" that is to say, solutions so dilute as to be com- 

 parable with " ideal n or " perfect M gases, in which the action 

 on one another of the dissolved molecules, as well as their 

 actual volume compared with that of the space they inhabit, 

 is so small as to be negligible. 



IL Boyle's Law for Dilute Solutions. 



The analogy between dilute solutions and gases acquires at 

 once a quantitative form, if it be noted that in both cases 

 alteration of concentration exercises a similar influence on 

 pressure, and is in both cases proportional to the pressure. 



This proportionality, which for gases goes by the name of 

 Boyle's law, may be proved experimentally for liquids, as well 

 as deduced theoretically. 



Experimental Proof (Determination of osmotic pressure 

 for solutions of various concentrations). — Let us first adduce 

 Pfeffer's determinations (Osmotische Untersuchungen, p. 71) 

 of the osmotic pressure (P) in sugar-solutions at the same 

 temperature (13°*2 to 16 0, 1), and with varying concentra- 

 tions (C): — 



c. 



P. 





P 



1 per cent. 



535 



milliin. 



535 



2 „ 



1016 



)) 



508 



2-74 „ 



1518 



» 



554 



* n 



2082 



)) 



521 



6 „ 



3075 

 G2 



') 



513 



