442 OSMOTIC PRESSURE. 



have the same vahie under similar conditions of temperature. 

 If the volume of the vessel is i' litres, the pressure produced 

 will be 22"23 per gram molecular weight, and if to is the actual 



'V 



mass of gas and /// its molecular weight, the pressure in atmos- 

 pheres will be 



, 7C' X 22*32 



/;/ X c' 



According to our theory, we assume that when a solid is 

 dissolved in a liquid, the particles of the solid diffuse through- 

 out the liquid, giving rise to a pressure upon all the surfaces 

 of the liquid of exactly the same magnitude as would he the 

 case if the solid could be completely converted into a gas, a)id 

 confined in a rigid I'csscl of the same capacity as the volmne 

 of the liquid in zvkich the solid is dissolved. 



We now consider the evidence upon which this statement is 

 based. It has long been known that many animal membranes 

 exert a selective action upon solutions enclosed within them, 

 as shown by the classical experiments of Graham, in which it 

 was demonstrated that common salt and sugar in solution 

 would pass through such a membrane, wdiilst gelatine or silica 

 would not. A membrane of this character is thus permeable 

 to " crystallisible," but not to "colloidal" bodies. There is, 

 however, another class of membrane which exists naturally 

 in the living cell-walls, but which can be formed by artificial 

 means. This is known as the " semi-permeable " membrane. 



The properties of the " semi-permeable " membranes are 

 very different from those of the permeable membranes, for if 

 a solution of salt in water is enclosed in such a membrane, and 

 the latter immersed in pure water, the salt will not pass out 

 through the membrane, but water ivill pass in, continuing to 

 do so until some stable equilibrium is reached. 



Now, why should the water pass in? The answer is that it 

 is due to the " osmotic " pressure, exerted by the salt upon the 

 boundaries of the solution. Each ion of the salt in solution 

 behaves as though it were a gaseous particle, and therefore is 

 in a state of constant motion, ions colliding with other ions 

 and with the walls of the vessel, or, perhaps, more strictly 

 speaking, with the surfaces of the liquid in contact with the 

 walls, so that a pressure will be set up within the confines of 

 the liquid in the same way as a small quantity of gas enclosed 

 in an otherwise empty space will develop a pressure upon the 

 walls of the containing vessel. This is known as the osmotic 

 pressure. 



A mass of gas is capable of expanding indefinitely, but a 

 limit is put to the volume it will finally occupy, according to 

 the final conditions of temperature and pressure. 



In the same manner a mass of solid dissolved in a liquid must 

 of necessity be contained within a fixed volume, and the osmotic 

 pressure developed within the liquid should have a finite value. 

 Since liquids are practically inextensible, the osmotic pressure 

 developed by a solid dissolved in a definite volume of liquid 

 will be proportional to the amount of the solid. The pressure 



