C'HAP. i.] TISSUES AND MECHANISMS OF DIGESTION. 521 



passes with a certain rapidit}*- from the stronger to the weaker 

 solution, and water passes from the weaker solution to the 

 stronger; if, to begin with, simple water be substituted for the 

 weaker solution the effect is at first still more striking. Peptone 

 passes in the same manner but as we have seen much more slowly. 

 The process is spoken of as a physical one since it is not accom- 

 panied, necessarily, by any chemical change in the diffusing 

 substance, nor is there any necessary change in the membrane or 

 partition. The rate at which a substance diffuses, and the total 

 amount of diffusion which can take place, are determined by 

 certain qualities of the substance (which we may call physical 

 though they depend on the chemical nature of the substance) in 

 relation to certain qualities of the membrane ; thus two salts may 

 diffuse through the same membrane at different rates, with 

 different rates in the associated current of water, the osmotic 

 current as it is called, from the weaker to the stronger solution ; 

 and the same substance may pass at different rates through 

 different membranes. By a number of observations, in which 

 various substances in solution and several known membranes or 

 partitions have been employed, a certain number of " laws of 

 diffusion " have been established. 



Now if by the statement that diffusible substances pass by 

 diffusion into the blood-capillaries of the intestine we are led 

 to expect that the passage takes place exactly according to the 

 laws established by observations on ordinary membranes we should 

 be led into error ; for the disappearance of these substances from 

 the interior of the intestine does not take place according to the 

 laws which regulate their disappearance from one side of an 

 ordinary diffusion septum. This can be ascertained by introducing 

 solutions of the substances, of various strength, into a loop of 

 intestine, isolated in the living animal by the method described in 

 250, and watching their disappearance by analysis of the contents 

 of the loop. No very large number of experiments have been made 

 in this way, but such as have been made all shew the difference 

 on which we are dwelling. For instance, sodium sulphate passes 

 through an ordinary diffusion septum with a rapidity rather 

 greater than that of dextrose, whereas dextrose disappears from 

 the intestine distinctly more rapidly than sodium sulphate; 

 peptone which diffuses very slowly indeed through an ordinary 

 diffusion septum disappears rapidly (though not so rapidly as 

 dextrose) from the intestine ; and when the details of the disap- 

 pearance from the intestine of weak solutions of two salts which 

 diffuse through an ordinary membrane at different rates, which 

 have as it is said different osmotic equivalents, are studied, these 

 details are quite different from those of ordinary diffusion. The 

 more the matter is studied the more decidedly apparent becomes 

 the difference between ordinary diffusion and the absorption of 

 diffusible substances from the intestine. 



