624 THE MECHANISM OF ABSORPTION 



solution an electrolyte, the degree of ionisation of a given solu- 

 tion can be measured by observing its electrical conductivity, 

 i.e. the resistance offered by the solution to the passage of an. 

 electric current. 



Diffusion. When the concentration of a dissolved sub- 

 stance is not the same at two points in a solution, the difference 

 of concentration, i.e. of osmotic pressure, between the two 

 points moves the substance from the point of higher to the 

 point of lower osmotic pressure by the process of diffusion, 

 and at a velocity proportional to the difference of pressure. 

 Graham divided substances into crystalloids, which diffuse 

 rapidly, and into colloids, which diffuse extremely slowly. A 

 little consideration shows that differences in diffusibility must 

 be a property of molecular weight. For, according to Avogadro- 

 van 't Hoff's law, 1 grm. mol. of a substance dissolved in water 

 and made up to a volume of 22'4 litres exerts at C. an osmotic 

 pressure of 760 mm. Hg ; and therefore it follows that the 

 greater the molecular weight the less is the osmotic pressure 

 exerted by each molecule. Now, colloids are substances with 

 very high molecular weight, and therefore they depress the 

 freezing-point of a solution but little, and have a correspondingly 

 low osmotic pressure and diffusion velocity. They are non- 

 conductors. While it is roughly true that crystalloids diffuse 

 through colloidal membranes, such as animal or gelatin mem- 

 branes, &c., and that colloidal solutions do not, yet there can 

 be no very sharp line between the two ; for the diffusion of 

 many crystalloids is hindered by colloidal membranes, and 

 there is evidence that colloids can diffuse into colloids. In 

 the body simple diffusion between two solutions without an 

 intervening membrane is rare. A membrane would complicate 

 diffusion only if it were not equally permeable to all diffusible 

 substances in solution. In the body this complication is added, 

 and it is clear that diffusion must be greatly influenced by the 

 particular permeability of the membrane present. 



When we turn to the investigation of the osmotic properties 

 of such physiological fluids as blood, urine, &c., we see that 

 the physical methods yield information which we could not 

 otherwise obtain. Chemical analysis alone cannot yield 

 definite information about the osmotic properties of such solu- 

 tions ; for, their osmotic pressure depends upon the number 



