214 



HOW TO WORK 



Sulphates of Potash and Soda, Muriate of Ammonia, Borax, Alum, 

 Sulphate of Copper, Biniodide of Mercury, and a variety of other 

 salts, can be readily obtained in microscopical crystals in this manner. 

 Mr. Glaisher has made some beautiful observations on snow-flakes. 

 Copies of his drawings are represented in pis. XLVI and -XL VI I, 

 figs. 285 to 303. Among organic crystalline substances may^be men- 

 tioned, Quinine, lodo-sulphate of Quinine, Salicine, Brucia, Oxalic 

 Acid, and Oxalates, particularly Oxalate of Ammonia. 



Different faces of the crystal, as it lies in the liquid, may be 

 brought into view by slightly moving the thin glass cover with a fine- 

 pointed instrument, such as a needle, while the preparation is in the 

 field of the microscope. With a little practice, crystals may in this 

 manner be made to rotate in the mother-liquor. Crystals which are 

 precipitated by the addition of some reagent, such as nitrate of urea 

 by nitric acid, must be examined in a little of the solution. The 

 addition of water would, in many instances, destroy them imme- 

 diately. Crystals of muriate of ammonia and creatine are represented 

 in pi. XL VII. Other crystals are seen in pis. XLII, XLVIII, and 

 XLIX. 



The influence of the crystals upon polarised light should be 

 examined, and in cases in which the nature of the crystal has not 

 been ascertained, its angles should be carefully measured, and 

 accurate drawings made. Their behaviour with chemical reagents is 

 next to be ascertained, and their solubility in water, alcohol, and other 

 fluids must be noted. For these experiments different portions must 

 be taken and separately tested in the manner referred to in p. 208. 



A drop of the solution may also be evaporated rapidly nearly to 

 dryness, and allowed to crystallise upon the slide without being 

 covered over, when the substance will often be found to assume a 

 variety of beautiful forms, such as crosslets, dendritic expansions, &<-., 

 which vary according to the rapidity with which the evaporation has 

 been conducted, and other circumstances. 



Mr. Thomas Davies has obtained some beautiful results by crys- 

 tallising mixed salts, some of which exhibit a re-arrangement of 

 crystalline form after fusion. A mixture of sulphate of copper and 

 sulphate of magnev'a, and sulphates of zinc and magnesia form good 

 examples. They must be examined with the aid of polarised light 

 and a selenite plate. See the copies of the photographs of the salts 

 in Mr. Davies's second paper in the Microscopical Journal for July, 

 1865, p. 205. 



By carefully crystallising a solution of sulphate of copper at 

 various degrees of temperature, Mr. R. Thomas, of Oxford, has 

 succeeded in obtaining a series of crystalline forms of a peculiar 



