Geological Society. 441 



more than half an hour — a period sufficient for thirty compa- 

 risons ; and most precipitates will refuse to fall for hours, 

 sometimes for days, together. Traces of argentic chloride 

 will remain unprecipitated in this liquid for months. The 

 operator has therefore only to select such a strength of 

 standard precipitate as shall give him not too great an amount 

 to suspend, and an opacity equal to about fifty scale-divisions. 

 If the substance precipitated should be soluble in the solution 

 of gelatine, that solution should be saturated, before use, with 

 the precipitate in question. 



The estimation of turbidities will doubtless prove of much 

 value in water-analysis, in field work, in the valuation of 

 pharmaceutical extracts precipitable by water (hitherto an un- 

 approachable subject), in watching the variations in composi- 

 tion of well-water for brewing-purposes, in the systematic 

 examination of the atmosphere's impurities, in Eggertz's car- 

 bon process, and in many similar lines of research. 



The colorimeter is an instrument admirably adapted for use 

 in comparatively unskilled hands, and especially in those in- 

 dustrial analyses where one class of product is constantly 

 tested by a single person. 



LXYIII. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 



[Continued from p. 370.] 



April 30. — Henry Clifton Sorby, Esq., E.R.S., President, in the 



Chair. 



T^HE following communications were read : — 

 ■*- 1. "A Contribution to the History of Mineral Veins." By 

 John Arthur Phillips, Esq., E.G.S. 



In this paper the author described the phenomena of the depo- 

 sition of minerals from the water and steam of hot springs, as illus- 

 trated in the Californian region, referring especially to a great 

 " sulphur bank " in Lake County, to the steamboat springs in the 

 State of Nevada, and to the great Comstock lode. He noticed the 

 formation of deposits of silica, both amorphous and crystalline, en- 

 closing other minerals, especially cinnabar and gold, and in some 

 cases forming true mineral veins. The crystalline silica formed 

 contains liquid -cavities, and exhibits the usual characteristics of or- 

 dinary quartz. In the great Comstock lode, which is worked for 

 gold and silver, the mines have now reached a considerable depth, 

 some as much as 2660 feet. The water in these mines was always 

 at a rather high temperature ; but now in the deepest mines it issues 

 at a temperature of 157° Eahr. It is estimated that at least 

 4,200,000 tons of water are now annually pumped from the workings ; 



