168 PHYSIOLOGY 



distributed equally and homogeneously among the molecules of the 

 solvent. In the various grades of solution a colloid solution or 

 hydrosol may be assumed to begin when the size of the molecule is 

 increased out of all proportion to that of the molecules of the solvent. 

 The ' dissolved ' molecules now begin to have the properties of 

 matter in mass and to present surfaces with all their attendant 

 attributes. The same sort of solution may be formed with smaller 

 molecules, such as Si0 2 , when these are aggregated together with 

 adsorbed water into huge molecular complexes, or, as in metallic sols,, 

 by the division of the solid metal into ultra-microscopic particles. The 

 distinguishing features of a colloidal solution are due to this lack of 

 homogeneity, and to the fact that in every solution there are two 

 phases a fluid phase, and a second phase, which is either solid or a 

 concentrated or supersaturated solution of the colloid. The huge 

 size of the molecules and the development of surface not only determine 

 the formation of adsorption combinations, but, on account of the 

 inertia of the system, cause a delay in changes of state, and tend to 

 the formation of false equilibria dependent on the past history of the 

 system. 



IMBIBITION 



All colloids, even those such as starch or gelatin, which are insoluble 

 in cold water, exhibit a phenomenon, viz. ' Quellung ' or imbibition, 

 which in many cases it is impossible to distinguish from the process of 

 solution. This phenomenon, which was long ago studied by Chevreul 

 and has lately been the subject of a series of careful experiments by 

 Overton, is exhibited by aft animal tissues and all colloids. Thus 

 elastic tissue dried in vacua absorbs from a saturated solution of 

 common salt 36-8 per cent of water and salt. If dried colloids be 

 suspended in a closed vessel over various solutions, they will take up 

 water in the form of vapour from the solution, and the osmotic pres- 

 sure of the solution in question will inform us as to the amount of work 

 which would be necessary in order to separate the water again from 

 the colloids. 



Thus it has been reckoned that to press out water from gelatin 

 containing 284 parts of water to 100 parts of dried gelatin would 

 require a pressure of over two hundred atmospheres. The imbibition 

 pressure of colloids increases rapidly with the concentration of the 

 colloid and at a greater rate than the latter. In this respect, however, 

 imbibition pressure resembles osmotic, or indeed gaseous, pressure. At 

 extreme pressures the pressure of hydrogen rises more rapidly than 

 its volume diminishes. In solutions this effect is more marked the 

 larger the size of the molecule. Thus a 6-7 per cent, solution of cane 

 sugar has the same vapour-tension, and therefore the same osmotic 



