46 THE MECHANISM OF LIFE 



The diffusion of molecules through a colloid, a plasma, or 

 a membrane is governed by laws precisely analogous to Ohm's 

 law, which governs the transport of electricity. The intensity 

 or rapidity of diffusion is proportional to the difference of 

 osmotic pressure, and varies inversely with the resistance. 



In the case of molecular diffusion, however, the rapidity of 

 diffusion depends also on the size and nature of the molecules 

 of the diffusing substance. The theory of the resistance of 

 the various plasmas and membranes to diffusion has been 

 but little understood; we can discover hardly any reference 

 to it in the literature of the subject. 



The laws of diffusion apply equally to the diffusion of ions. 

 Nernst has shown that there is a difference of electric potential 

 at the surface of contact of two electrolytic solutions of different 

 degrees of concentration. Both the positive and negative ions 

 of the more concentrated solution pass into the less concen- 

 trated solution, but the ions of one sign will pass more rapidly 

 than those of the other sign, because being smaller, they meet 

 with less resistance. 



The resistance of the medium plays a most important part 

 in all the phenomena of diffusion. When two solutions of 

 different concentration come into contact, the interchange of 

 molecules and ions which occurs is unequal owing to the 

 differences in resistance. Hence both solutions become modified 

 not only in concentration but also in composition. It has 

 lonij been known that diffusion can cause the decomposition 

 of certain easily decomposed substances, and it would appear 

 probable that diffusion is also capable of producing new 

 chemical combinations. 



The separation of the liberated ions in consequence of the 

 unequal resistance which they meet with in the medium they 

 traverse often determines chemical reaction. This ionic 

 separation is a fertile agent of chemical transformation in the 

 livinir organism, and may be the determinant cause in those 

 chemical reactions which constitute the phenomena of nutrition. 



When different liquids come into contact there arc two 

 distinct series of phenomena, those due to osmotic pressure 

 and those due to differences of chemical composition. Even 



