THE ENERGY OF MOLECULES IN SOLUTION 187 



OSMOTIC PRESSURE. If we place two gas jars together, mouth 

 to mouth, as in Fig. 19, the upper jar containing hydrogen and the 

 lower jar some heavier gas, such as oxygen or carbon dioxide, 

 within a very short time the gases will have become intimately 

 mixed, and each jar will contain an equal amount of both gases. We 

 say that each gas has diffused into the other, and ascribe the diffusion 

 to the movement of the gaseous molecules. In closed vessels the 

 rapidly moving molecules are continually impinging on 

 the walls of the vessels and rebounding, and it is this 

 bombardment by the gaseous molecules which is respon- 

 sible for the pressure exerted by a gas on its containing 

 walls. If we double the amount of gas in a given space, 

 we double the amount of molecules which strike a unit 

 area of the wall in unit time, and therefore double the 

 pressure exerted by the gas on the vessel wall. In this 

 way we may explain the law of Boyle that the pressure 

 of a gas is inversely proportional to its volume, or the 

 product of pressure and volume at a given temperature 

 is a constant, PV = C, or since the energy of the mole- 

 cules is proportionate to the absolute temperature, 

 PV = RT, the familiar gas equation. 



The molecules of substances in solution behave, 

 within the limits of the solution, in a manner precisely CO 

 similar to the free molecules of a gas. Thus, if a vessel 

 be half filled with a 10 per cent, solution of sugar and 

 be then filled up by carefully pouring distilled water, 

 so as to form a distinct layer on the heavier sugar 

 solution, the sugar at once begins to move upwards 

 into the distilled water. In consequence of the FIG. 19. 

 resistance offered to the movement of the sugar 

 molecules through the water, this process of diffusion is slow, but if 

 the vessel be left undisturbed and free from any agitation for two 

 or three months the sugar will be found to spread gradually throughout 

 the liquid, so that at the end of this time all parts of the fluid contain a 

 uniform amount of sugar. 



This process of diffusion, like that of gases, must be ascribed to a 

 continuous translatory movement of the dissolved molecules. Since 

 the molecules possess mass and are endowed with a velocity, it is 

 evident that they can exercise a pressure on any membrane or dividing 

 surface which tends to hinder their free passage within the limits of the 

 solvent. Thus if we take a pig's bladder containing a 20 per cent, 

 solution of dextrose and immerse it in distilled water, water will pass in 

 and distend the bladder to such an extent that it may burst from the 

 rise of pressure in its interior. This swelling of the bladder is due to 



