632 SCIENCE PROGRESS 



the sugars, is non-permeable to gelatine, starch, and various 

 other colloids. A definite osmotic pressure was found for 

 gelatine, gum acacia, and the serum proteids, while starch 

 and gum tragacanth give rise to no measurable osmotic 

 pressure. A 10 per cent, solution of gelatine gives an osmotic 

 pressure of about 70 mm. of mercury at 30 C, rising to about 

 140 mm. at 8o°. This increase is greater than it would be if the 

 osmotic pressure were proportional to the absolute temperature, 

 and it is exceedingly probable that the large "solution aggre- 

 gates " present at lower temperatures are subject to dissociation 

 as the temperature rises. Re-cooling does not seem to lead 

 at once to the re-formation of the large aggregates, for the 

 osmotic pressure at 30 of a gelatine solution which has been 

 heated to 8o° for a short time is higher than before heating, and 

 returns only gradually to its former value. Prolonged heating 

 at 8o°, on the other hand, leads to a permanently higher value 

 of the osmotic pressure, a phenomenon that is accompanied also 

 by changes in the physical properties of the gelatine solution. 



The objection might be raised that the osmotic pressure 

 exhibited by gelatine solutions is due to the inorganic matter 

 invariably associated with the colloid ; but it must be remem- 

 bered that parchment paper is permeable for crystalloids, while 

 Moore and Roaf have shown that the maximum osmotic 

 pressures observed for gelatine solutions at a given temperature 

 remain steady for days together. 



The Nature of Osmotic Pressure 



It is not proposed to discuss here at any length the various 

 views that have at one time or another been advanced as to the 

 cause and nature of osmotic effects. No general agreement has 

 been reached on the matter, and it is noteworthy that some 

 of those who contribute most to our experimental knowledge 

 of osmotic pressure are the least ready to dogmatise on the 

 theoretical side of the question. 



This much may be said, however, that the older way of 

 regarding osmotic pressure as due to the impact of the dissolved 

 molecules on the semi-permeable membrane has been generally 

 abandoned. This point of view was the result of pressing too 

 far the undoubted analogy between gases and solutions, and 

 makes too little of the part which must be played by the 



