MICROSCOPIC OBJECTS. 93 



Fig. 185. Crystals of Oxalic Acid, x 40. 



As mentioned at fig. 184, some crystals have the power of 

 analyzing polarized light. Sulphate of magnesia, which 

 crystallizes somewhat similarly to oxalic acid, can therefore 

 scarcely be distinguished from it, either in regard to its 

 form or power of polarization. But by adding a drop from 

 the test-bottle of chloride of calcium, the sulphuric acid of 

 the sulphate of magnesia, having a greater affinity for the 

 lime than for the magnesia, combines with it, and forms tufts 

 of the needle-shaped crystals of sulphate of lime (see fig. 182) ; 

 while upon applying the chloride of calcium to the oxalic 

 acid, rectangular crystals of oxalate of lime are chiefly formed. 



The crystals are best seen when mounted in balsam. 



Fig. 186. Feathery Crystals of Boracic Acid, x 40. 



These crystals may be prepared as follows : To a hot 

 saturated solution of borax in water, add | part of sulphuric 

 acid ; as the liquid cools, crystals of boracic acid are deposited 

 at the bottom of the test-tube. The mother liquor must 

 then be poured off, and the crystals dried. If the feathery 

 form is wanted, they must then be dissolved in alcohol, a 

 drop evaporated on a glass slide, and castor-oil immediately 

 applied. Next cover with thin glass, and seal the cover as 

 usual. Even with these precautions the crystals will not 

 often keep long, as they are extremely deliquescent. 



If the solution of borax be treated with phosphoric acid, and 

 the water mixed with the crystals deposited from the mixture 

 be evaporated on the slide, minute disks will occasionally be 

 found. When mounted in Canada balsam and examined 

 under polarized light, crosses will be observed rotating round 

 the centres of these as the polarizer is moved. 



