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. 



