COLLOIDS, OR THE MATERIAL OF LIFE 127 



attraction and by the molecular forces exerted at the surfaces 

 of the enormous numbers of invisible particles. 



A gel, then, is merely a mass of crystals of below microscopic 

 size, in which the mother liquor is retained. Here, again, the 

 enormous specific surface confers upon the condition peculiar 

 properties. The most noticeable of these is a remarkable 

 attraction for water and other substances. Ordinary air-dry 

 gelatin still contains about 1 5 per cent, of water. And this can 

 be driven off only by heating for hours. In fact this water is 

 held as firmly as the water of crystallisation in many salts. 

 When the dried gelatin is exposed to the air, the moisture is 

 absorbed so vigorously that the gelatin rapidly increases in 

 weight. A piece of ordinary gelatin placed in water gradually 

 drinks it up and swells until it has absorbed as much as ten 

 times its weight. The energy of combination with the water 

 is shown by the considerable heat evolved and by the fact 

 that the swollen gelatin occupies less volume than the total 

 volume of the gelatin and water before swelling. The tre- 

 mendous pressures developed during this contraction are 

 demonstrated by the force needed to squeeze out the water. 

 Forty atmospheres pressure reduce the water content only to 

 about 30 per cent. It can be calculated that tens of thousands 

 of atmospheres would be necessary to force out all the water. 

 This swelling process is the mechanism by which the growing 

 plant cells are able to take up the moisture they need from the 

 ground. 



Suppose, however, a gelatin jelly is immersed in water which 

 has been coloured with a suitable dyestuff. Then the colouring 

 matter is extracted from the solution until it is nearly colourless. 

 The same thing occurs with charcoal, silicic acid gel, and kaolin. 

 It appears to be characteristic of colloid bodies. With textile 

 fibres it becomes the process of dyeing. Here again the 

 mechanism of the action has been the subject of endless dis- 

 cussion. As in the case of catalysis, we are concerned with the 

 molecular forces operating at the colloid surface. There are 

 separate schools which favour dissolution of the dyestuff in the 

 fibre, chemical combination, electrical attraction, and adsorption. 

 Since the forces involved are probably the same in each case, 

 there is much to be said for each of the different theories. 



One striking property of gels must be referred to in con- 

 clusion. In many cases, when a reagent is allowed to diffuse 

 into a gel containing a substance with which a precipitate can 

 be formed, the precipitation does not take place continuously 

 as would be expected, but the precipitate is deposited in bands 

 separated by more or less clear spaces. This curious pheno- 

 menon, first observed by Lupton, was rediscovered and investi- 

 gated by Liesegang, and is frequently known as Leisegang's 



