492 Mr. Pickering on the Theory of Osmotic Pressure 



show are such as could be reasonably attributed to increasing 

 the pressure from one to three or four atmospheres. 



Influence of the Nature of the Dissolved Substance. 



The half values obtained in many cases are explained by 

 the duplication of molecules, owing to the solutions not being 

 sufficiently dilute ; that is, an additional pressure of two or 

 three atmospheres will cause the vapour-densities of hydro- 

 chloric acid *, methyl, ethyl, amyl, and butyl alcohols f , 

 benzol, and many other substances to double themselves 

 (assuming that they retain their gaseous condition). This 

 may be so, but direct determinations cannot be, or have not 

 been, made. 



Leaving these abnormal values for the present, we find 

 that the normal ones exhibit the following variations: — 



Solvent. 



Number of 



instances 



investigated. 



Variation in the value 

 of the depression \. 





57 

 9 

 41 

 13 

 5 

 59 

 29 



30 per cent. 

 14 „ 

 12 „ 



7 



2 „ 

 60§ „ 

 40 „ 



Formic acid 



Benzene 



Nitrobenzene 



Ethylene dibrornide 



Water 



„ (with organic substances) ... 



These variations appear somewhat too large to be attributed 

 to the fact that we are working at two or three more atmo- 

 spheres of pressure than we should be. 



It is noticeable that the organic solvents, except acetic 

 acid, give comparatively constant results, but that, as soon as 

 we come to the inorganic solvent water (which we know shows 

 a much greater tendency to form "molecular" compounds 

 than the other liquids) , we get a much larger variation, and 

 the following values || will justify the assumption that when 

 other solvents of a similar nature are examined, still greater 

 variations will be observed. 



* Solvent, Acetic acid (Raoult, Ann. Chim. Phys. [6] xi. p. 72, from 

 which paper this and other values quoted here are taken). 



t Solvents, Benzene, &c. 



% Percentage on the lowest value. 



§ Omitting borax, where the value is exceptionally high. 



|| Chem. Soc. Trans. 1890. The values selected refer to solutions of 

 the same strength as those used \>y Raoult. The results with calcium 

 nitrate and chloride are as yet unpublished. 



