126 SCIENCE PROGRESS 



been collected. Graham thought that the colloid or crystalloid 

 condition was characteristic of a given substance, that colloids 

 and crystalloids were two separate classes of natural substances. 

 Soda was essentially a crystalloid, gelatin a colloid. This 

 view has changed slowly. It was gradually realised that bodies 

 might occur in either state, and eventually, in 1 9 1 4, von Weimarn 

 showed, from experiments with a variety of materials, that, 

 by the suitable choice of conditions, a given substance could be 

 obtained in either state desired. 



The condition in which a body will be formed depends upon 

 its solubility, the excess concentration of substance that will be 

 thrown out of solution, the size of the particles in solution, and 

 the viscosity of the solution or rate of diffusion of the particles. 

 A large solubility and small excess concentration cause large 

 crystals. When the particles in solution are not aggregated, 

 their constitution is simple and they are able to diffuse freely ; 

 again, large crystals tend to be formed. All these condi- 

 tions are fulfilled in the case of soda. On the other hand, 

 a substance with a very small solubility, like barium 

 sulphate, occurs in the form of very small crystals. By 

 simply increasing the supersaturation as much as possible, 

 the crystals of barium sulphate become so small as to be 

 beyond the range of the microscope and a translucent jelly is 

 obtained. These principles have been embodied by von 

 Weimarn in a simple formula, which, although not quite 

 exact, proves most useful as a working rule. Should a 

 varnish prove permeable to moisture, von Weimarn's formula 

 indicates the remedy. If the products of a manufacturing 

 process are difficult to deal with, the same expression shows 

 how to modify them. 



It has now been demonstrated that gelatin and other sub- 

 stances that occur naturally as colloids obey von Weimarn's 

 law. Gelatin has a small definite solubility which increases 

 rapidly with rising temperature. Its particles in solution are 

 very complex and diffuse very slowly. A hot solution contains 

 very much more gelatin than it can retain in the cold. All the 

 conditions, therefore, contribute to the formation of particles 

 below microscopic size. 



Gelatin has rarely been prepared in another state. But, if 

 no more is dissolved than corresponds to its true solubility in 

 water, gelatin forms a clear solution. With gradually increas- 

 ing concentration the solution first becomes supersaturated 

 and then deposits a precipitate, of which the particles decrease 

 in size and increase in bulk until, when sufficient gelatin is 

 present, the precipitate fills the solution and forms the 

 ordinary well-known jelly. In this state, the mother liquor, 

 from which the gelatin has been deposited, is held by capillary 



