SORBY — STRUCTURE OF CRYSTALS. 469 



the use of transmitted light with very high magnifying powers, for 

 which purpose they should be from about yJo-th to 1 ^ th of an 

 inch thick. I here particularly draw attention to the necessity of 

 not using any polishing powder, because it enters into, and fills up, 

 cavities and flaws, and would easily give rise to factitious appear- 

 ances that might mislead most fatally. This must be carefully borne 

 in mind when the manner in which a section has been prepared is 

 not known ; for otherwise it might be worse than useless. The 

 surfaces of the rock or mineral should be dressed extremely fine, with 

 water, on a perfectly flat piece of very hard and smooth Water-of- 

 Ayr stone, when they usually become quite sufficiently polished. 

 One side of the section is of course fixed on glass with Canada 

 balsam before it is rubbed down very thin ; and when finished, thin 

 glass should be mounted over the upper surface with the same sub- 

 stance. This not only preserves the object from injury, but has the 

 most important eifect of rendering it far more clear and transparent. 

 In many cases, however, it is unnecessary to prepare such sections. 

 Portions of the mineral can be broken off sufficiently thin, and when 

 mounted under glass in Canada balsam can be most readily examined 

 with high magnifying powers. This method has not only the ad- 

 vantage of saving much time, but, there being no necessity for a 

 strong heat, the risk of expelling the fluid from the fluid-cavities is 

 obviated. For some purposes, however, thin sections are quite in- 

 dispensable ; and in mounting them care should be taken not to 

 employ a higher temperature than is absolutely necessary. 



For the examination of mounted fragments, I have found Messrs. 

 Smith and Beck's ^th-inch object-glass of very great value on 

 account of not approaching the object too closely ; and by using 

 along with this their second eye-piece with a micrometer, the size of 

 anything can be measured at once. This is the power I have chiefly 

 employed, and is about 400 linear ; the divisions of the micrometer 

 are -g-^ -^-th of an inch ; and thus cavities much less than that are 

 quite distinct, their contents are easily seen, and their dimensions 

 and that of the vacuity can be measured to within 5 * -g-th of an 

 inch. 



§ 2. Water contained in crystals. 



The difference between the water mechanically enclosed in the 

 fluid-cavities of a crystal, and that chemically combined with, the sub- 

 stance of which it is composed, is of course most complete. That 

 chemically combined is one of the essential constituents of the 

 mineral, cannot be seen with any kind of magnifying power, and is 

 probably not in the state of liquid water ; whereas that in the fluid- 

 cavities is altogether unessential to the existence of the substance, 

 and is in the form of a visible liquid, merely enclosed mechanically. 



When a mineral contains fluid-cavities, of course it does not neces- 

 sarily follow that the fluid is water ; and it is often difficult to ascer- 

 tain what it is when in the cavities. If, however, the mineral contains 

 no chemically combined water, it is easy to prove what it is when 

 out of the cavities. On applying a strong heat, the expansion of the 



