SORBY STRUCTURE OF CRYSTALS. 455 



Cavities above yo^th °f an mcn m diameter, very large. 

 „ from j-Q-Qth to x ^ th, moderately large. 

 „ from 1 q^o th to x oooo th, moderately small. 

 ,, less than yxrcro-g-th, ver y small. 



In some respects it is best to mount crystals formed in the wet 

 way, in as shallow glass cells as will hold them, in a concentrated 

 cold solution of the salt itself; for then, never being dried, none of 

 the cavities can lose their fluid, which is not the case if they are 

 mounted in varnish or Canada balsam. Independent of that, some 

 facts are seen to greater advantage when crystals are mounted in 

 clear varnish in cells which may be made out of cardboard with a 

 wadding-puuch ; and I therefore usually adopt both methods. The 

 magnifying powers generally required are from 50 to 400 linear ; 

 and when the crystals possess strong double refraction, a polarizer or 

 analyser should be used, so adjusted that only one image of the 

 cavities is visible ; or else they appear indistinct. 



I have in nearly all cases lithographed the accompanying figures 

 (Pis. XVI.-XIX.) as seen with one adjustment of the focus, but 

 have slightly shaded the enclosed crystals, although they are some- 

 times quite colourless, in order to distinguish them from irregula- 

 rities on the surface of the cavities. The figures with a dotted 

 outline represent portions of crystals, whereas all the rest are entire 

 crystals or detached cavities, as if the surrounding paper were the 

 clear substance of the larger crystals in which they occur. 



Few substances could be more suitable, as characteristic types, 

 than the chlorides of sodium and potassium, sulphate and bichromate 

 of potash, alum, and sulphate of zinc. 



When a solution of chloride of potassium is allowed to evaporate 

 spontaneously, the character and form of the crystals vary most 

 remarkably, according as the weather is cold or warm, moist or dry. 

 In cool damp weather, when evaporation goes on slowly, sometimes 

 all the crystals are similar to fig. 1, having a square patch towards 

 the centre, which is white by reflected, but black by transmitted 

 light, whilst the rest of the crystal is clear and transparent. When 

 much more highly magnified, it is seen that the opacity of the 

 central portion is due to vast numbers of minute cavities, the amount 

 of which varies in bands parallel to the sides of the crystal, and also 

 in such a manner as to give rise to the peculiar cross seen in the 

 figure. These cavities are full of the liquid from which the crystals 

 were deposited ; and this having a smaller power of refraction than 

 the substance of the crystal, the contents reflect and strongly refract 

 the light, and, like the bubbles of air in the water of a cataract, give 

 a white appearance as seen by reflexion, or darkness and opacity 

 when viewed by transmitted light. 



The manner in which these cavities are formed is well illustrated 

 by fig. 2, where the unshaded portion represents part of the extreme 

 edge of a crystal of common salt, and the shaded the concentrated 

 solution from which it crystallized, entering into a deep notch formed 

 by the irregular growth of the crystal. If, on the further growth 



