442 Crystals for the Micro-Polariscope. 



crystals, which are interesting, not only from their intense 

 polarizing property, but from their formation being an exceed- 

 ingly delicate test for the presence of quina. The formation of 

 the large tabular crystals of this salt, which are sometimes 

 used as substitutes for tourmaline, requires very nice manipu- 

 lation and considerable care; but the rosette-like crystals of 

 small size for use as polariscope objects are easily made in the 

 following manner : — 



Take Sulphate of Quinine . . 12 grains. 



Acetic Acid . . . half an ounce. 



Proof Spirit . . . half an ounce. 



Tincture of Iodine . . 25 drops. 



The acetic acid is that of commerce, having a specific gravity 

 of 1044 or thereabouts. The tincture of iodine is made by 

 dissolving 40 grains of the metalloid in an ounce of rectified 

 spirit. Dissolve the sulphate of quinine in the acetic acid, 

 and add the proof spirit ; then heat to 130' Fahr. and drop in 

 the tincture of iodine. While hot, the liquid thus produced 

 has a dark sherry colour, but on cooling it deposits beautiful 

 spangle-like crystals, which look more like fragments of the 

 wing-cases of cantharides than anything else to which I can 

 compare them. A drop or two of the hot solution allowed to 

 cool upon the slide gives them at once, and the supernatant 

 liquid can be drained off, and the specimen dried under a bell 

 glass. The crystals are best preserved by covering them with 

 a thin layer of a solution of Canada balsam in ether, containing 

 a trace of iodine. 



There is perhaps no more really beautiful polariscope object 

 than a well-prepared slide of salicine; but none of the books 

 treating on the preparation of objects, with which I am ac- 

 quainted, give any details as to the method of obtaining those 

 exquisite radiating peacock-feather-like crystals, which one 

 meets with in cabinets, and which are sold by professional 

 mounters. The secret — if secret it be — is this : vou take a 

 solution, not too strong, of salicin in water, pour a few drops 

 on a clean slide, and apply a gentle heat so as to evaporate 

 the liquid. If too much heat be used, the salicin will be fused, 

 and perhaps burned ; but as the slide becomes dry, the heat 

 must be so managed that the film shall break up into little 

 circular spots, each radiating from its centre. No description 

 will, however, give more than the general method of proceedm-.'; 

 in this case ; success can only be obtained by experience ; and 

 after one or two trials, which will most probably be failures, no 

 difficulty will be found. It is, however, imperative in this, as 

 all other cases, that the slide be chemically clean. 



With the exceptions which I have now pointed out, it will 

 be found that all the substances enumerated in the list above 



