124 



PHYSIOLOGY 



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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 responsible 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 molecules 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 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 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 con- 

 tinuous translatory movement of the dissolved molecules. Since the mole- 

 cules 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 the fact that the molecules of sugar pass 

 through it only with difficulty, and therefore in their passage outwards 

 towards the confines of the water exert a pressure on the walls, driving them 

 apart and so causing a distension of the bladder. It is impossible, however, 

 by this means to obtain the full osmotic pressure due to the pressure exerted 

 by the sugar molecules, since the bladder wall itself is not absolutely im- 

 permeable to sugar. If we imagine the sugar solution confined in a cylinder 

 and covered with a layer of distilled water, the movement of the sugar 



FIG. 19. 



