SELENITE. 235 



CHAPTER XY. 



SLIDES OF CRYSTALLIZATION. 



THESE are beautiful polariscope objects, and extremely 

 useful to the young student as first lessons in crystallo- 

 graphy, and incentives to experimental knowledge of the 

 various forms of mineral substances. Crystals are con- 

 stantly met with in the examination of both animal and 

 vegetable tissues ; it is therefore necessary to become ac- 

 quainted with the most common forms, if we use our micro- 

 scope understandingly* 



In the cuticle of onion we find crystallized oxalate of 

 lime ; in rhubarb also, but varied in form, as it is combined 

 with tartaric, citric, or malic acid. Every crystallizable 

 mineral substance has a definite form of crystallization, and 

 often many accidental or secondary forms* Carbonate of 

 lime-^a substance well known as forming chalk^ marble, &c., 

 and abundant in animal structure is found in hundreds of 

 secondary forms ; in groups of radiating needlesj in hexa- 

 gons, in rhombohedral forms, as in the shell of the Oyster ; 

 thus the perfect knowledge of the laws arid accidents of 

 crystallization is a deep study ; in fact, it is to mineralogy 

 what mathematics is to common arithmetic, and cannot be 

 entered upon in a mere catalogue of slides. 



The following preparations are recommended for beauty 

 and utility, when examined with polarized light ; a plate of 

 selenite is frequently indispensable for the^display of colour 

 and accurate observation of outline. 



SELENITE 



is itself a form of crystallization ; native crystallized hy- 

 drated sulphate of lime, called also satin gypsum or quarry- 

 glass. It is found in the quarries on Shotover-hill, Oxford ; 



