MICROSCOPIC OBJECTS. 92 



Fig. 183. Crystals of Chlorate of Potash, x 40. 



One of the best of the polarizing crystals is chlorate of 

 potash; but other salts of potash are almost equally interesting 

 (see fig. 184). 



To obtain a constant form of crystal from any salt requires 

 a certain amount of practice. The methods vary. Some 

 crystallize best from a saturated solution in alcohol, others 

 in warm or cold water; some, like salicine, require to be 

 fused, &c. ; but for micro-chemical analysis a saturated solu- 

 tion in cold water is perhaps best, as being least liable to in- 

 terfere with the test used. 



The crystals are drawn as seen under polarized light. 



Fig. 184. Crystals of Bichromate of Potash, x 40. 



From a hot saturated solution of this salt in water, beau- 

 tiful feathery crystals are formed, which, together with many 

 other crystals, have the property of analyzing polarized 

 light (see fig. 185). To show this power well, the crystals 

 of this and of the other salts, such as the nitrate of potash, 

 sulphate of magnesia, &c,, are best mounted in balsam. 



Although many crystals can scarcely be mounted at all on 

 account of their deliquescence, still the majority may be 

 mounted in castor-oil, glycerine, or Canada balsam, and 

 many others may be mounted dry. 



