254 Prof. J. P. Cooke on the Vermiculites. 



potassium and basic hydrogen in place of more than one half 

 of the magnesium of the Jefferisite. It should, however, be re- 

 membered in this connexion that the Biotites present a very wide 

 variation in the ratio between the amounts of the protoxide and 

 sesquioxide radicals which the various varieties contain. The 

 limits usually assigned to this variation correspond to the ratios 



II VI II VI 



R:R:Si = l:2:3, and K : R : Si=l : 1 : 2; 

 and the Pargas mica, with the ratio 2:3:5, falls between these 

 limits ; but the Culsagee variety of vermiculite corresponds to 

 the more common class of Biotites, which have the ratio 1:1:2. 



But this resemblance in chemical constitution only appears 

 when we compare the Biotite mica with the anhydrous Jefferisite ; 

 while it is the crystallized hydrous Jefferisite which so closely 

 resembles the magnesian micas in its crystallographic relations ; 

 and the question now arises, What is the condition of the large 

 amount of water (12| per cent.) which the crystallized mineral 

 contains ? 



To aid us in forming a conclusion on this point, we have the 

 following evidence : — 



First. As the above analysis shows, the water is united in de- 

 finite and atomic proportions amounting to six molecules to every 

 five molecules of silicon in the molecules of the mineral — that is, 

 sufficient to convert all the silicon into a hydrate, assuming that 

 the five silicon atoms in this hydrate are joined to each other by 

 the smallest possible number of bonds. 



Secondly. While both the crystallographic and the chemical 

 relations of Jefferisite to the other vermiculites, and to the mag- 

 nesian micas, indicate that the mineral is an orthosilicate, the 

 amount of basic radical, exclusive of the water, is amply suffi- 

 cient to saturate the atomicity of the silicon. 



Thirdly. It was observed by Professor Brush (and his obser- 

 vations have been fully confirmed by ourselves) that the water 

 is given off at a comparatively low temperature — about 300° C. ; 

 and, as every mineralogist knows, this dehydration is attended 

 with that remarkable exfoliation which is characteristic of the 

 vermiculites, and indicates a complete disintegration of the mo- 

 lecular structure. This exfoliation is wholly different from the 

 phenomena which the so-called hydrous micas present under 

 like conditions. In these last (which, as we suppose, contain 

 hydrogen as a part of the basic radical of their molecules) a very 

 high temperature is required to expel the water, and the loss is 

 attended by no such marked change of volume and disintegration. 



The conclusion that we draw from these facts is that the com- 

 bined water is in the same condition in Jefferisite as in the zeo- 

 lites and in many crystallized salts — a condition which has 



