Crystals for the Micro-Polariscope. 437 



CRYSTALS FOR THE MICRO-POLARISCOPE. 



BY HAEEY NAPIER DRAPER, F.C.S. 



The polariscope is now so usual an adjunct to the microscope, 

 and the beautiful phenomena of which it gives us glimpses are 

 so commonly subjects of intellectual observation, that I feel no 

 apology to be needed as an introduction to a chapter on polari- 

 scope crystals. Before, however, I proceed to give a few 

 practical hints and directions to those who may be desirous of 

 making a collection of these interesting objects, I may state 

 that I was first led to experiment in this direction by the fol- 

 lowing considerations. It occurred to me that the characters 

 which crystalline bodies exhibit when viewed by polarized 

 light might be much more employed as aids to chemical analy- 

 sis than has yet been shown; that, for example, the very 

 marked difference which distinguishes the alkaloid of the willow 

 and poplar — salicine — from that of Peruvian bark — quinine — 

 might be found to exist to an equal extent between crystalline 

 inorganic bodies. There are many of these latter which have, 

 like the alkaloids instanced, certain points of resemblance, both 

 as regards colour and crystalline form, and to which therefore 

 it would be advantageous to be able to apply so easy a test. 

 The salts, sulphate of zinc, and sulphate of magnesium, will 

 serve as examples to illustrate what I mean. If these sub- 

 stances, crystallized, as they are ordinarily found in commerce, 

 be placed side by side, the most practised eye will fail to dis- 

 cover any difference which would denote the composition of 

 either. Nor will the microscopical examination of the crystals 

 be of any avail, for the sulphates of zinc and of magnesium 

 form crystals belonging to the same system, and which ex- 

 actly resemble each other. In a case like this, did it prove 

 that, examined by polarized light, the two salts presented cha- 

 racteristic differences, the NicoPs prisms would give valuable 

 aid. Unfortunately, however, this is not the case, and so far 

 as the micro-polariscope is concerned, a slide of Epsom salt 

 might be labelled sulphate of zinc, or one of sulphate of zinc 

 Epsom salt, without the least chance of the error being dis- 

 covered. 



Although, however, the failure of my attempt to extend 

 the usefulness of this method of observation was signal, the 

 success which attended some experiments, made with the object 

 of devising some method of obtaining constantly good results 

 in these minute crystallizations, was encouraging. This, 

 although to the uninitiated it may appear a very simple matter, 

 is, if the ordinary plan of procedure be adopted, really very 



