228 APPENDIX. 



caused a fresh deposition of boracic acid upon them as 

 nuclei, which assumed the form of very delicate cilia, 

 spreading in all directions as from a centre. These 

 fringed circles shewed the analytic property in an admi- 

 rable manner, exhibiting four quadrants coloured alter- 

 nately with complementary colours of great vivacity. 



3. Another instance which is worthy of mention is the 

 oxalate of potash and chromium, a salt whose optical pro- 

 perties have been investigated by Sir David Brewster*. 

 If some gum arabic is added to a solution of this salt, and 

 a drop of it put between two plates of glass, it abandons 

 its usual mode of crystallization for another, which re- 

 sembles a microscopic vegetation composed of minute 

 prisms growing one out of another, and variously ar- 

 ranged in sprigs and branchlets ; while in other places 

 it assumes an undulating capillary form, much resem- 

 bling in miniature the tufts or locks of a species of 

 Conferva which is seen growing in pools of water or in 

 the sea. Now these objects are possessed of a high 

 analytic power, insomuch that, when a plate of sulphate 

 of lime is placed beneath them, they assume a colour of 

 great intensity and splendour, which is changed for the 

 complementary tint when the polarization of the incident 

 ray is reversed. 



4. Nitre. — If nitre and gum arabic are dissolved to- 

 gether in hot water, a drop of the solution put on a glass 

 plate yields very good analytic crystals. These have a 



* Philosophical Transactions for 1835, p. 91. 



