DISPERSOIDOLOGY 271 



solution cooled to room temperature. In 10-12 days a fine- 

 grained precipitate of A1 2 3 . 3H 2 had been deposited, which, 

 under a 15,000 magnification, was observed to consist of spear- 

 shaped needles 3-4 mm. long. 



The stage of so-called amorphous precipitates begins with 

 aluminium hydroxide at about N/500. At N/i coarse-celled 

 gels are formed. And at 3N/1 the mixture sets to a solid gel. 



By application of similar principles a typical crystalloid, 

 such as common salt, can be obtained in the colloid form. For 

 this purpose it is merely necessary to choose a medium in which 

 the solubility is sufficiently small. By pouring 4 c.c. of a 

 saturated solution of NaCl in methyl alcohol into a mixture 

 of 30 c.c. propyl alcohol and 10 c.c. ether, a precipitate in the 

 form of growth figures is obtained. If a mixture of equal 

 parts of propyl alcohol and ether is employed, the precipitate 

 appears amorphous under a 300-times magnification. With 

 30 c.c. ether and 10 c.c. propyl alcohol a voluminous precipitate 

 of fine-grained amorphous structure is obtained. Finally, by 

 pouring 4 c.c. of the saturated methyl alcohol solution into 

 40 c.c. ether, a strongly opalescent sol is obtained, with a 

 characteristic reddish-brown colour by transmitted light, and 

 which deposits only a minimal precipitate in 24 hours, re- 

 maining otherwise unchanged. 



These researches explain the natural occurrence of large 

 crystals of insoluble bodies either by deposition after many 

 years from enormous volumes of very dilute solutions, by the 

 gradual diffusion of the reacting solutions into gels, or by the 

 slow crystallisation of the latter. 



