494 Mr. Pickering on the Theory of Osmotic Pressure 



Direction of the Deviations with Dissolved Substances 

 which give Abnormal Values. 



The abnormally low values (i. e. half values) which some 

 dissolved substances give are attributed, as has been men- 

 tioned, to the polymerization of their acting molecules, and 

 the abnormally high values which some solvents give are, in 

 the same way, attributed to a polymerization of the solvent 

 molecules. This view is accepted by both the supporters 

 and the opponents of the osmotic pressure theory. 



In the case of dissolved substances which give abnormally 

 high values (?'. e. double values), the supporters of the theory 

 hold that these substances are dissociated into their ions, and 

 in this view their opponents cannot agree with them. 



The first point to settle, is whether there are any abnormally 

 high values (given by dissolved substances), and it appears to 

 me that there certainly are none. 



In the case of the five organic solvents examined, the 

 substances giving the higher and lower values are in the 

 proportion of 100 to 16, and here there can be (and is) no 

 question as to which are the normal ones; even if in the case 

 of water these proportions were reversed, it would be incon- 

 sistent to term the lower values the normal ones ; but the 

 proportions are actually not reversed, the substances giving 

 high values being to those which give the lower ones in the 

 proportion of 100 to 76. I fail to see therefore what grounds 

 there are for saying that there are any abnormally high 

 (double) values at all, or what need there is for calling in 

 such a theory as that of dissociation into ions *. 



Indeed, there is a distinct difficulty, if not impossibility, in 

 calling the lower values (l o, 03) the normal ones ; for the 

 normal value with other liquids is o, 63, and the excess in the 

 case of w r ater could only be explained by the water molecules 

 being more complex than those of other liquids in the pro- 



* Van't Hoff's statement as to Raoult's position respecting the high 

 and low values of substances in water is certainly remarkable ; he says 

 (Phil. Mag. 1888, xxvi. p. 99), "Raoult did not discover the existence of 

 so-called normal [meaning small] molecular depression of freezing-point 

 and lowering of vapour-pressure until he investigated organic compounds ; 

 their behaviour is almost without exception regular." As a matter of 

 fact, Raoult investigated organic substances first (Ann. Chim. Phys. 

 1883 [5] xxviii. p. 133), and when he subsequently (ibid. 1884 [6] xi. 

 p. 81) found that inorganic substances gave higher values, he did not 

 hesitate to call these latter the normal ones. 



If we reverse the application of the word normal, we shall have to 

 admit that of the 150 substances examined in water, not one gave an 

 abnormally low value. A most improbable admission. 



