*28 PROCKEDINGS OF the American academy. 



In taking up the subject <>f BolatioDB it will be necessary to consider 

 their probable nature. Although there La Bome evidence that the phe- 

 nomenoD of solution is accompanied by a molecular change, t ) j « - prepon- 

 derance of evidence seems to 1"- in favor of the theory that the molecule 

 <>i a Bubstanoe in Bolutiou is free, and not combined in any chemical way 

 with the molecules of the solvent. If this is the case, certain proper) 

 oi the Bolute Bhould remain unchanged regardless of the nature of the 

 solvent In the following work we will assume that, for a dissolved 

 substauce, the quantity h as defined above and the quantity c, will be the 

 same in any solvent. If this supposition, which seems eminently proba- 

 ble, proves to be not entirely correct, then the equations developed below 

 will only be approximations to the truth. It is to be hoped that in any 

 case their application will conversely give us information concerning the 

 nature of Bolution. 



otic Pressure. — The simplest phenomenon of a homogeneous so- 

 lution is that of osmotic pressure, which may be determined in the fol- 

 lowing way. The change of free emrg\ on addition of an infinitesimal 

 amount of solvent to a solution containing one BTam-moli rule of Bolufr 

 equal to the sum of the changes of free energy in the two constituents of 



the system. From equation (5), if —2 = 0, and "' becomes the 



volume correction, which, as is shown by Nernst,* is lor each constituent, 

 in the case of osmotic pressure, only the correction for the space occupied 

 by the molecules of the constituent in question, 



d A = -^-r dv x - dll, + " - \ dv a -,/U . 



where the Bubscripl 1 refers to the Bolute, subscript 2 to the solvent. 

 dv l} the change in molecular volume of the solute, is also the change in 

 volume of the system, and dA =pdvn where p is the osmotic pressure. 



Therefore 



irr ,in , » /rr ,i,-, 



where represents the heat produced by the addition of d '•, of the 



solvent. Except in cases of solution of such great concentration thai the 

 molecular volume of the solute and that of the Bolvent are not greatly 



different, : is entirely negligible, and the equation for osmotic pres- 



sure becomes 



* Theor. (hem., p. 209. (References to thi6 book are to the first edition.) 



