336 REPORT—1890. 
observed, with respect to the theoretical foundation of the osmotic pres- 
sure law, that the action on a semipermeable diaphragm is due, partly to 
the shock of the dissolved molecules, partly to the difference of forces 
acting upon them, from the solvent on one side, and from the solution on 
the other. Now, the result of the shock is directly proportional to the 
concentration, whereas that of the attraction is proportional to the square: 
thus in very dilute solutions the second action vanishes when compared 
with the first, and the shock is alone the origin of pressure as it is 
in gases. However, he insisted on these views as more intended to popu- 
larise than to prove the laws in question. If we want to do the last on 
kinetic grounds, we must take everything into account—movement of the 
molecules of the two substances mixed, action on themselves and on each 
other. Now, this has been just recently done by van der Waals, and 
the result is a very complicated formula, simplified, however, for dilute 
solutions into this statement, ‘that the dissolved molecules act on a semi- 
permeable membrane with strictly the same force as they would do on an 
ordinary membrane in the gaseous state.’ So from a kinetic point of view 
the law of Avogadro and the ‘ osmotic pressure’ law stand on the same 
basis. 
Mr. Pickering commits a fundamental error in supposing that the 
osmotic pressure theory arrives at 0°63 as the number with which we 
had to multiply the solvent’s molecular weight in order to get the so- 
called ‘ constant of depression.’ Such conclusion was never drawn from 
oes fhat was dedeendeeliiie 
the theory in question ; it was the formula 
value 0°63 was an empirical one, introduced by Raoult. This difference 
has urged Professor Hykman to a very extensive experimental research, 
the conclusion of which was so evident that in the July number of the 
‘Annales de Chimie et de Physique’ Raoult openly accepts the value 
0:02T? 
wo 
gélation produit par une molécule dissoute dans 100 molécules dissolvantes 
2 
est, d’aprés M. van’t Hoff, donné par l’expression a = 0°02 ee 
quelle T est la température absolue de congélation et L sa chaleur latente 
moléculaire de fusion.’ In addition, on p. 361, he says: ‘ L’accord entre 
Vexpérience et la théorie est donc, sur tous les points, aussi complet 
qu’on peut le désirer en pareille matiére.’ No one now defends the value 
0:63, anda good deal of the objection which Mr, Pickering directs against 
it has no bearing on the osmotic pressure theory itself. 
On p. 359 Raoult states: ‘L’abaissement a du point de con- 
1, dans la- 
Mr. W. N. Suaw remarked that the meaning of the term solvent used 
by physicists when referring to water, alcohol, and the like, is somewhat 
widely extended when it is understood to include 100(H,SO,.H,0), and 
the other equally complex solvents of Table I. of Mr. Pickering’s paper. 
An ordinary solvent could not fairly be regarded as being ‘ inert’ and 
‘having no action whatever’ when it was claimed that the solvent caused 
the dissociation of a large portion of the dissolved salt. The action is in 
fact most remarkable, and is the important point now requiring investiga- 
tion and explanation. This action has been clearly illustrated by Mr. W. 
Coldridge (‘ Phil. Mag.’ May 1890, p. 383), who has endeavoured at Cam- 
bridge to ascertain the circumstances under which stannic chloride can 
