1098 MICKOCRYSTALLISATION, ETC. 



the development of colour. The substances marked d are distin- 

 guished by possessing the curious property termed pleocliroisin, 

 which was first noticed by Dr. Wbllaston and carefully investigated 

 by Sir D. Brewster. This property, to which was previously applied 

 the misnomer dichroism, consists in the exhibition by these crystals 

 of colours varying with the direction in which they are examined ; 

 thus, the cube-shaped crystals of magnesium platinocyanide reflect 

 light of a deep red colour from two parallel faces, whilst' light of a 

 vivid beetle-green is reflected from the other four faces. Pleochroism 

 is only exhibited by doubly refracting substances, and is caused by 

 the fact that the two plane polarised rays into which a ray passing 

 into the crystal is decomposed, are absorbed selectively that is to 

 say, the crystalline medium absorbs light of certain colours from the 

 one polarised ray, whilst absorbing quite differently coloured com- 

 ponents from the second ray. Pleochroic substances are most easily 



FIG. 816. Spiral crystallisation of copper sulphate. 



recognised by the fact that they change in colour when rotated 011 

 the microscope stage in plane polarised light namely, when only one 

 Nicol prism is interposed between the eye and the lamp. It not 

 unfrequently happens that a remarkably beautiful specimen of 

 crystallisation develops itself which the observer desires to keep 

 for display. In order to do this successfully, it is necessary to 

 exclude the air ; and Mr. "Warrington recommends castor oil as the 

 best preservative. A small quantity of this should be poured on 

 the crystallised surface, a gentle warmth applied, and a thin glass 

 cover then laid upon the drop and gradually pressed down ; and 

 after the superfluous oil has been removed from the margin a coat 

 of gold-size or other varnish is to be applied. Although most of the 

 objects furnished by vegetable and animal structures, which are 

 advantageously shown by polarised light, have been already noticed 

 in their appropriate places, it will be useful here to recapitulate the 

 principal, with some additions. 



