152 PHYSIOLOGY 



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

 28-4 parts of water to 100 parts of dried gelatin would require a pressure of 

 over two hundred atmospheres. The imbibition pressure of colloids in- 

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

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

 of common salt. It is impossible to draw any hard line of distinction between 

 imbibition pressure and osmotic pressure, or 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 sub- 

 stances in question. Thus all the crystalline 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 condensa- 

 tion 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 turpentine often to an indefinite extent, while they are un- 

 touched 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 Overton 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. 



