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44 THE MECHANISM OF LIFE 



been defined by Fick as the quantity of a solute which in one 

 second traverses each square centimetre of the cross section of 

 a column of liquid 1 centimetre long, between the opposite 

 sides of which there is unit difference of concentration. Nernst 

 in his definition substitutes " unit difference of osmotic pressure " 

 for " unit difference of concentration." 



Until recently it was generally believed that diffusion took 

 place in colloids and plasmas just as in pure water. This is, 

 however, by no means the case : the differences are considerable. 

 When a solute is introduced into a colloidal solution, the 

 greater the concentration of the colloid the slower will be 

 the diffusion. This may be shown by a simple experiment. 

 Several glass plates are prepared, by spreading on each a 

 solution of gelatine of different concentration, to which a few 

 drops of phenol plithalein have been added. If now a drop 

 of an alkaline solution be placed on each plate, we can see 

 that the drop diffuses more slowly through the more con- 

 centrated gelatine solution, since the presence of the alkali is 

 rendered visible by the coloration of the phenol plithalein. 

 A similar demonstration may be made by allowing drops of 

 acid to diffuse through solutions of gelatine made slightly 

 alkaline and coloured with phenol plithalein. In general, 

 we find on experiment that when similar drops of any 

 coloured or colouring solution are left for an equal time on 

 plates of gelatine of different degrees of concentration, the 

 greater the concentration of the gelatine the smaller will be 

 the circle of coloration obtained. 



We may show that the rapidity of diffusion diminishes 

 as the gelatinous concentration increases, by another experi- 

 ment. If we put side by side on our gelatine plate a drop of 

 sulphate of copper and another of ferrocyanide of potassium, 

 the point of contact of the two fluids will be sharply marked 

 by a line of precipitate. We find that under similar conditions 

 the time between the sowing of the drops and the formation 

 of this line of precipitate is longer when the gelatine is more 

 concentrated. 



Oxmosis. — In 1748, FAbbe Nollet discovered that when a 

 pig's bladder filled with alcohol was plunged into water, the 



