of the Theory of Solutions. 



479 



Substance. 



Temperature. 



Pressure. 



S0 4 Na a . 10H 2 

 S0 4 Fe.7H 2 



O 



9 

 25 



65 

 50 



600 atra. 



510 „ 



245 ., 



1100 „ 



1730 „ 



S0 4 Fe.7H 2 



S0 4 Cu.5H 2 



S0 4 Cu.3H 2 





i. e. in order to prevent Na 2 S0 4 from taking up its water of 

 crystallization, for example in Pfeffer's apparatus, a pressure 

 of 600 atmospheres at 9° is necessary and sufficient. 



We must now return from Mitscherlich's affinity problem 

 of the attraction for water of crystallization, where we are 

 obviously dealing with enormous concentrations, to the ques- 

 tion of dilute solutions. Hitherto I have been drawing your 

 attention to what appeared in my Etudes de dynamique chi- 

 mique, and now I would like to refer to a publication in the 

 Archives Nderlandaises. There I gave the proof of my 

 equation, 



dlK _ _q_ 



dT ~2T 2 ' 



which also occurred in my Etudes. 



The introduction of this equation here is only of secondary 

 importance. The chief point to be noted is, that I was able 

 by means of reversible cycles to deduce this equation for 

 dilute gas systems. I should now like to show its applicability 

 to dilute solutions. 



It occurred to me that all the reversible cycles which so 

 greatly simplify the use of thermodynamics in treating of 

 gases, are also applicable to dilute solutions, provided that a 

 semipermeable wall is employed, as may be seen from the 

 following figure. 



This was a decided advance, and it clearly followed that 

 the osmotic pressure for dilute solutions must obey Gay- 

 Lussac's law, or, in other words, must vary with the tempe- 

 rature in the same way as the gas-pressure. A direct 

 comparison is simplest. 



The left side of the figure represents the well-known cycle 

 for a gas. The heat supplied at T which for dilute gases 

 corresponds only to the external work PAY, on lowering the 

 temperature by dT is represented by the work VdP. 



