NOTES ON THE IDENTIFICATION, ETC. 211 



either with, or without, the addition of a selenite plate. Here 

 again, the duration of evaporation has a marked effect, also the 

 strength of the solution. If the substance is deposited in a thin 

 film, it may be altogether invisible without polarised light. Thick 

 crystals frequently produce colour without the selenite, and those 

 that are very thick may depolarise without any colouration. This 

 being borne in mind, no difficulty is experienced in practice, as it 

 is easy to compare with an alkaloid of known purity crystallised 

 under the same conditions. 



In the accompanying plates, I have endeavoured to give a 

 representation of various substances crystallised under the best 

 conditions, with the name of the solvent and the linear magnifica- 

 tion. The letter B signifies a black field (ordinary polarised light) 

 and Fa violet field produced by the selenite. In many cases, I 

 have found it difficult, if not impossible, to give a faithful drawing, 

 but that is of slight importance, since anyone who makes use of 

 this method would naturally prepare his own slides for comparison. 



Bark. 



Quinine deposited from alcohol is granular. 



Quinine Disulphate crystallises from alcohol in a network of 

 fine needles. 



Quinine Disulphate mixed with a little iodosulphate is a gor- 

 geous object, either with or without the selenite. It appears 

 like an assortment of peacocks' feathers with the crests towards 

 the centre. 



Quinine Disulphate mixed with quinidine sulphate forms little 

 feathery crystals totally distinct from either of the salts crystal- 

 lised alone. 



A mixture of quinidine, cinchonidine, and cinchonine will not 

 crystallise at all from alcohol, but dries up to a gummy mass. 



Quinidine crystallises from etherial alcohol in stellate groups of 

 monoclinic prisms, giving red centres and green at the ends 

 (Fig. I). 



Quinidine Sulphate has an entirely different form, each crystal 

 assuming an independent hue (Fig. 3). 



Cinchonidine crystallises from alcohol in globular tufts of needles 



