98 G. D. Hubbard — Colloids in Geologic Problems. 



in the gel condition or colloidal state, thus showing that 

 the silica gel is not purely an artificial product. If the 

 ancient scientists who gave the name "quartz" to rock 

 crystal had found this vein and watched it through its 

 transformation to crystalline quartz, they might have had 

 some ground for their theory that it was made of water 

 which had become so thoroughly frozen in a very cold 

 winter that its solid state had become permanent. 



The_experiments of Liesegang, and of many other col- 

 loid chemists who have followed him, suggest a method 

 for the formation of the quartz-gold vein. They have 

 shown that if a gold solution, e.g. AuCl 3 , is mingled with 

 a silica gel when the latter sets, and then is treated with 

 a reducing agent diffusing through the gel, crystals of 

 gold will form in the gel. Oxalic acid placed on top may 

 serve as this reducing agent, H 2 S in water, S0 2 , or CO 

 will serve the same purpose. Sodium sulphite, and 

 especially FeS0 4 , do occur in solutions in nature and 

 would be suitable reducing agents there. 



Holmes 7 has shown that a preparation of silicic acid gel 

 made 0.1N with respect to potassium iodide, and then 

 covered with 0.5N mercuric chloride, gives in the gel after 

 a few days bands of red crystalline mercuric iodide. In 

 some cases as many as 40 rather sharply marked bands 

 occurred in a distance of 8 cm. In a similar manner, 

 bands of copper chromate, cuprous oxide, basic lead 

 iodide, basic mercuric chloride, and other substances were 

 formed. Bands of colloidal gold with scattered crystals 

 of gold developed in a preparation of gold chloride and 

 H 2 S0 4 with oxalic acid above the gel. 



If the silica gel were in one crevice and an intersecting 

 crevice should be conveying one of these reducing solu- 

 tions, the conditions of the laboratory would be very 

 nearly duplicated. The only difference would be that the 

 solution need not become continually more dilute, but 

 might remain essentially the same in composition for 

 years. In such case there might be no occasion for the 

 banding of the salts, or if bands developed, they would 

 no doubt be evenly spaced. Banding is explained 8 as a 

 result of a reduction in concentration of the solution in 

 the gel just in front of the ring, so that the diffusing 



7 Holmes, H. N., Jour. Amer. Chem. Soc, 40, 1187-1195, 1918. 

 "' Holmes, ibid. 



