THE PROPERTIES OF COLLOIDS 169 



pressure, as a 67 per cent. NaCl solution. A 67 per cent, cane-sugar 

 solution has, however, the same osmotic pressure as an 18| per cent, 

 solution of common salt. It is impossible, therefore, to draw an} 7 

 hard line of distinction between imbibition pressure and osmotic 

 pressure In the same way it is impossible to say where a fluid ceases 

 to be a solution and becomes a suspension. All grades are to be found 

 between a solution such as that of common salt with a high osmotic 

 pressure and optical homogeneity, and a solution such as that of 

 starch, which scatters incident light and is therefore opalescent, and 

 has no measurable osmotic pressure. 



The close connection between the processes of imbibition and of 

 solution is shown also by the fact that the latter occurs only 

 in certain media, the nature of the media being dependent on the 

 chemical character of the substances in question. Thus all the crystal- 

 line carbohydrates such as grape sugar and cane sugar are easily 

 soluble in water, only slightly soluble in alcohol, and practically 

 insoluble in ether and benzol. The amorphous carbohydrates, which 

 must be regarded as derived by a process of condensation from the 

 crystalline carbohydrates -e.g. starch, cellulose, gum arabic, &c. 

 have a strong power of imbibition for water. This power may be 

 limited, as in the case of cellulose, or may be unlimited, as in the case 

 of gum arabic, so that a so-called solution results. On the other hand, 

 they swell up but slightly in alcohol, and are unaffected by ether and 

 benzol. In the same way proteins all take up water, and in many cases 

 form a so-called solution, but are unaffected by ether and benzol a 

 behaviour which is repeated in the case of the amino-acids, out of 

 which the proteins are built up, and which are easily soluble in water, 

 but are practically insoluble in ether and benzol. On the other hand, 

 india-rubber and the various resins take up ether, benzol, and tur- 

 pentine often to an indefinite extent, while they are untouched by 

 water. With this behaviour we may compare the easy solubility of 

 the hydrocarbons, the aromatic acids, and esters in ether and benzol, 

 and their insolubility in water. As Overtoil has pointed out, the power 

 of amorphous carbohydrates to take up fluids is modified by alteration 

 of their chemical structure in the same direction as the solubility 

 of the corresponding crystalline carbohydrates. Thus, if the hydroxyl 

 groups in the sugars be replaced by nitro, acetyl, or benzoyl groups, 

 they become less soluble in water, while their solubility in alcohol, 

 acetone, &c., is increased. In the same way the replacement of the 

 hydroxyl groups in cellulose by N0 2 groups diminishes the power 

 possessed by this substance of taking up water, but renders it capable 

 of swelling up or dissolving in alcohol and acetone. 



