224s Objects for the Microscope. 



CHAPTER XYI. 



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 

 acquainted with the most common forms, if we use our 

 microscope 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 needles, in hexa- 

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

 thus the perfect knowledge of the laws and 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 ; 

 but the finest crystals are met with at Montmartre, near 

 Paris. 



The primary form is that of an oblique rectangular 

 prism, with ten rhomboidal faces, two of which are larger 

 than the rest. 



