INTRODUCTION. xxvii 



from 27 to 3'1. The hardness ranges between 3*5 and 4*5, except in 

 Pyrophyllite, in which it is only 1'5. 



Chlorite, Ripidolite, Clinochlore, and Pyresclerite. — In these minerals the 

 laminse are flexible, but not elastic, and they often have a slightly greasy 

 feel. 



They afford no water in a tube ; fuse at the edges more easily, before the 

 blowpipe, than the preceding group, but give no blue colour to cobalt solu- 

 tions. They are slightly acted on by acids, giving mostly dark green solutions, 

 except in the case of Pyrosclerite, which is often reddish. 



Talc resembles the preceding group In most characters, but the lamlnas 

 are much softer and more greasy, but are not so thin, and the colour is 

 generally pale green. It is infusible before the blowpipe, and insoluble in 

 acids. With cobalt solution it gives a reddish colour, with some difficulty. 



Brucite bears some resemblance to Talc in its whitish and greenish colour, 

 and in being infusible before the blowpipe. When heated in a tube, it gives 

 off water, and is entirely dissolved in acids without effervescence. 



Diallage, B7'onzite, HyperstJiene, Clirdonite, Chloritoid, though sometimes 

 approaching to micaceous in structure, are, more correctly speaking, foliated. 

 The laminae are brittle, and not easily separated. Marmollte differs from 

 the above in having a greasy feel, and in bearing some resemblance to Talc. 



Gypsum, or sulphate of lime, is very soft, and may be scratched with the 

 nail. It differs from the carbonate (or Calc Spar) in not effervescing with 

 acids. It turns white before the blowpipe and crumbles, but is fused only 

 with difficulty. When crystallized it is generally colourless, often trans- 

 parent, and separable into thin laminae, which can scarcely be bent without 

 breaking. 



Uranite, Bed Zinc Ore, and Copper Mica. — The first is of a bright green 

 or yellow colour, and crystallizes in square tabular crystals ; the second (an 

 oxide of zinc) is bright or deep red ; and the third, a deep green, crystal- 

 lizing in hexagonal crystals, which give the reaction of copper. 



In the following work, under the head of Camp. (^Composition'), is repre- 

 sented, by means of a formula, the chemical composition of each mineral, 

 supposing it to be perfectly pure or free from foreign admixtures. The 

 meaning of the symbols employed to denote the simple substances forming 

 the components is explained further on, at p. xliv. The chemical formula 

 is (in most cases) followed by the per-centage amount of each ingredient 

 present, on the above-mentioned supposition of their being altogether free 

 from extraneous matter. 



Perhaps, however, it should be stated here that each equivalent of oxygen 

 is represented by a dot placed over the symbol of the substance with which 



it is combined : thus Fe represents the metal Iron ; Fe, the protoxide of that 

 metal (or the combination of one atom of the metal with one atom of 



oxygen) ; and j'e, the sesquioxide, or peroxide of the same metal, in which. 

 three atoms of oxygen are present. 



