Crystals for the Micro-Polariscope. 441 



Citric Acid, Tartaric Acid, and Sugar of Milk niay all be 

 crystallized from warm solutions; but as crystals do not at 

 once form, the slides must, after removal from the solutions, 

 be laid aside, protected by a bell-glass for some hours until the 

 viscid, varnish-like layer, with which they are covered, changes 

 from the amorphous to the crystalline form. 



Napthaline forms, as I have found, one of the most beautiful 

 objects in the entire range of polariscope crystals. Crystalli- 

 zation from a solution is not, however, successful ; it must be 

 sublimed upon the slide. The most simple plan of proceeding- 

 is as follows : — Put two or three grains of napthaline in a thin 

 watch glass, and cover it with an inverted pill-box, in the bottom 

 of which a hole, half-an-inch in diameter, has been punched. 

 Place the centre of the slide over this hole, and apply a very 

 gentle heat, by means of a spirit lamp, to the under side of the 

 watch glass. The napthaline is very volatile, and its vapour 

 will condense on the cold slide in very beautiful crystals. 



Asparagine is readily soluble in boiling water, and the solu- 

 tion deposits well-formed crystals, which are very pretty ob- 

 jects, but it is by evaporating a solution of asparagine rapidly 

 on the glass, so as to form confused crusts of small irregular 

 crystals, that the most beautiful slides are obtained. 



Oxalate of Chromium and Potash may be easily prepared, 

 as has already been described in the Intellectual Obseevee,* 

 by dissolving together in hot water one part of bichromate of 

 potash, two parts of binoxalate of potash, and two parts of 

 •oxalic acid. 



Acetate of Copper crystallizes beautifully from its solution in 

 acetic acid. The ordinary verdigris of the shops is dissolved 

 in this liquid by heat, the solution filtered while still hot and 

 used immediately. 



Narcotine and Santonine dissolve so slightly in water, that 

 •crystals cannot well be obtained from their aqueous solutions. 

 Both are, however, readily soluble in chloroform, forming 

 solutions which give crystals by spontaneous evaporation with 

 great facility. 



For Iodide of Quinine and Vanillin alcohol is the best solvent. 

 "Vanillin is the odorous principle of the well-known seed-pod of 

 Vanilla planifolia, and its beautiful acicular crystals may be 

 often found completely investing good specimens of the bean. 

 By washing them off with as small a quantity as possible of 

 strong alcohol, a convenient solution will be at once obtained, 

 -or the crystals may be brushed directly off — a specimen which 

 has not been spoiled by handling — on to the slide. 



Iodo-sulphate of Quinine, more commonly known as Hera- 

 pathite, from the name of its discoverer, forms beautiful 



* Yol. i., p. 401. 



