PKESIDENTIAEi ADDBESS — SECTION B. 87 



quantity of the solution which contains unit weight of the 

 substance, and M and T as before. E is still a constant for all 

 substances; but has it a different value in this new connection? 

 Well — and this is our fourth point — -experiments have proved 

 (4) that E has the same value for the dissolved state as it has 

 for the gaseous state. To put this in words, we may say that 

 the osmotic pressure of a substance in solution is equal to that 

 pressure which the same quantity would exercise if it could be 

 confined as a gas at the same temperature and within the 

 same space as its solution now occupies. 



It is hardly necessary to say that the gaseous laws are not 

 absolutely and rigidly true in this new connection, any more 

 than when applied to gases and vapours themselves ; but I shall 

 have occasion to say a good deal about one class of exceptions 

 — or apparent exceptions — that would at first sight appear to 

 contradict the laws altogether. First, however, let us consider 

 the rule, before we go to the exceptions. Eemember that all 

 that I have told you has been proved by exact experiments 

 with Pfeffer's semi-permeable cell. But it is interesting to 

 find that, as usual, Nature forestalled man by many ages with 

 this invention. It has been shown that plants are provided 

 with cells which act on exactly the same principle ; in which the 

 sap is enclosed by a membrane through which water can pass 

 but the dissolved matter cannot, and this is enclosed by an- 

 other membrane permeable to both water and dissolved matter. 

 Certain ingenious experimenters (particularly de Vries) have 

 turned such delicate organic apparatus to account in the labo- 

 tory, and have thus conducted reseaches, the general result of 

 which has been to amply confirm the laws of osmotic pressure 

 as deduced from Pfeffer's work. 



Now, if you accept the laws as expressed in the equation 

 2>.r.M = E.T, an important result at once follows. If 

 we have a substance whose percentage composition is known, 

 but whose molecular weight is not known, we may deduce the 

 molecular weight from the measurement of its osmotic pressure 

 in a solution of ascertained concentration at a definite tem- 

 perature, for then in the equation M will be the only unknown 

 quantity. I need hardly remind an assemblage of chemists 

 that hitherto the only reliable method of determining the 

 molecular weight of a body has been the measurement of its 

 volume, temperature, and pressure in the gaseous state, and 

 that there are a vast number of compounds which cannot 

 be vaporised by heat without decomposition. The extension 

 of the same method to these very substances in solution is 

 the direct consequence of the discovery that solution is 

 vaporisation, and is as important practically as it is interest- 

 ing theoretically. 



There are drawbacks, however, to this method from a prac- 



