OP ARTS AND SCIENCES. 47 



protoxide and sesquioxide radicals which the various varieties contain. 

 The limits usually assigned to this variation correspond to the ratios — 



K : R : Si= 1 : 2 : 3, and R : S : Si = 1 : 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 com- 

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



But this resemblance in chemical constitution only appears when we 

 compare the Biotite mica wnth 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 follow- 

 ing evidence: — 



First. As the above analysis shows the water is united in definite 

 and atomic proportions amounting to six molecules to every five mole- 

 cules 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 magnesian micas, 

 indicate that the mineral is an orthosilicate, the amount of basic radical, 

 exclusive of the water, is amply sufficient to saturate the atomicity of 

 the silicon. 



Thirdly. It was observed by Professor Brush — and his observa- 

 tions 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 molecular 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 combined 

 water is in the same condition in Jefferisite as in the zeolites, and in 

 many crystallized salts, — a condition which has long been known in 

 chemistry as water of crystallization. We shall not here attempt to 



