OF ARTS AND SCIENCES. 37 



of crystallization. I also expect to show that the several minerals 

 referred to above are members of a family of hydrous silicates closely 

 allied and parallel to the well-known family of anhydrous silicates 

 called the micas, and that their molecules differ from those of the mag- 

 nesiau micas chiefly in containing a definite number of molecules of 

 water ; that is, water of crystallization. I shall call this family of 

 minerals " the vermiculites," using the original name, as " mica" is now 

 employed, to designate a class ; and I sh;dl call the three species (or 

 varieties ?) of this family Jefferisite, Culsageeite, and Hallite ; which 

 correspond, as I expect to show, to the two varieties of Biotite and to 

 Phlogopite respectively. It will appear that the original vermiculite 

 has the same composition as the variety from the Culsagee mine. 

 Finally, attention will also be asked to some unexpected discoveries to 

 which the optical examination of these minerals has led. 



Water of Crystallization and Water of Constitution. — In the salts 

 of the stronger acids, the distinction between water of constitution 

 (basic water, as it is sometimes called) and water of crystallization is, 

 as a rule, well marked. Water of crystallization generally escapes at a 

 comparatively low temperature, and the loss is attended with a com- 

 plete disintegration of the crystals, which usually fall to powder. 

 Water of constitution is held more firmly, and the salt must be heated 

 to a much higher temperature before it is expelled. Moreover, 

 athough the loss of basic water may greatly alter the external aspect 

 of the body, yet there is not the same complete breaking up of the 

 structui'e as before. To illustrate these points, it is only necessary 

 to refer to the well-known reactions of the common rhombic sodic 

 phosphate when heated. 



It is generally believed that the water of crystallization exists as 

 such in the salt, the molecules of water forming a part of the structure 

 of the crystals ; and the facts to be presented in this paper tend to sup- 

 port this view. But our modern theories assume that the so-called 

 water of constitution, which may be also driven oflT when the salt is 

 sufficiently heated, is formed under the influence of the heat from the 

 atoms of hydroxyl JI- - , which are united to the acid radical of the 

 compound. But in every simple salt, whatever may be our theories 

 in regard to the mode of combination, the weight of the water of 

 constitution, as well as that of the water of crystallization, must bear to 

 the weight of the salt a definite proportion, which can easily be calcu- 

 lated from the symbol ; and the ratio between the amount of hydrogen 

 and the amounts of the other radicals must be that of the atomic weights 

 of these radicals or of their multiples. In any given class of salts, 



