BISCHOF CHEMICAL GEOLOGY, 7 



volume in question extending to not less than 952 pages of large 

 octavo size ; the bulk would have been still further increased if all the 

 analyses contained in the first edition had been given in full, instead 

 of being merely referred to in foot-notes, as is done in several in- 

 stances. While in the first volume the general chemical principles 

 involved in the changes of the earth's crust are set forth, the second 

 treats of the salts occurring in the mineral kingdom, taking the widest 

 sense of the term and according to the ordinary chemical classification. 

 The volume is divisible into three parts, the first comprising the Haloid 

 salts, the second the oxy-salts of acids which are easily soluble or 

 miscible in all proportions with water, and the third those of the 

 nearly insoluble silicic acid, or silicates. A fourth division treats of 

 the most important of the minerals which, in the author's opinion, 

 have been produced from the decomposition of silicates. The volume 

 may in fact be regarded as a purely chemical mineralogy, in which, 

 without referring to their external characteristics, the composition, 

 formation, metamorphism, replacement, and decomposition of the non- 

 metallic minerals is considered, the metallic minerals being reserved 

 for a third volume. 



The division into chapters is continued from the first volume, 

 commencing with chapter xvii. and ending with chapter xlii. The 

 first chapter (xvii.) contains general remarks on the alkaline and 

 earthy chlorides, bromides, iodides, and fluorides, and on their presence 

 in crystalline rocks and lavas. Chapter xviii. is devoted to chloride 

 of sodium, under the headings of rock-salt and salt-lakes ; the former 

 is shown to be deposited from sea-water by slow evaporation, on 

 account of its great purity, rock-salt being perhaps the nearest 

 approach to a pure chemical compound occurring in nature, some 

 varieties yielding by analysis 100 per cent, of chloride of sodium. 

 The salt from brine-springs, on the other hand, being produced by 

 rapid concentration of the solution, decrepitates when heated, portions 

 of the mother-hquor containing uncrystallizable chlorides being en- 

 tangled among the crystals. The great difi'erence in the relative 

 proportion of gypsum and common salt existing in sea- water and in 

 sahferous rocks is explained by showing that, although sea-water 

 contains 16-4 times more salt than gypsum, the smaller absolute 

 amount of the latter salt represents 63 per cent, of the total necessary 

 for saturation, while the chloride of sodium only amounts to 7 per 

 cent. It is evident therefore that the solution will be sufiiciently 

 concentrated to allow of the deposit of gypsum when 37 per cent, of 

 water has been evaporated, whereas it will be necessary to remove 

 not less than 93 per cent, before any salt can be separated. In this 

 way the great masses of gypsum and anhydrite accompanying the 

 salt-masses of the Alps may be supposed to have been formed, by 

 partial concentration of sea-water which was removed before the salt 

 had time to be deposited. The notices of salt-lakes contain numerous 

 interesting particulars of the composition of the water of the Elton 

 Lake in the Crimea, which is remarkable for the large amount of chlo- 

 ride of magnesium, and the variable amount of common salt, that it 

 contains at difierent seasons of the year. 



Chapter xix. treats of fluorides, the author noticing the combinations 



