AND ON THE COLOURS EXHIBITED BY CERTAIN FLAMES. 453 



green has already been noticed. The tincture extracted from 

 the petals of the common red peony (Pceonia officinalis)^ by 

 a weak solution of carbonate of potash, is a yet more striking 

 example of the same phenomenon, to wliich we may add the 

 dark-coloured liquid formed by impregnating a solution of sul- 

 phat of iron with nitrous gas, and various mixtures of red and 

 blue, or green media. In a green solution of manganesiate of 

 potash (Chameleon mineral)^ the maxima correspond likewise 

 to the extreme red, and the green ; but the absorbing power on 

 these two rays being nearly equal, the change of tint does not 

 take place. In an aqueous infusion of logwood, the type has, 

 in like manner, two maxima, corresponding to the red and 

 green ; but the former is always predominant, and the only 

 change of colour it undergoes by a change of thickness, is from 

 a ruddy purple to a deep-red. 



11. Among blue media we may in like manner distinguish, 

 such as absorb the spectrum with an energy regularly increas- 

 ing, from the more to the less refrangible extremity (whose 

 type in consequence is as in Fig. 8.), and those whose types 

 have other maxima, as in Fig. 2. Blue solutions of copper are 

 in the former case ; and the best example, perhaps, is the beau- 

 tiful blue liquid produced by dissolving carbonate of copper in 

 carbonate of ammonia, or supersaturating any cupreous salt 

 with the ammoniacal carbonate. The extreme violet ray seems 

 capable of penetrating through almost any thickness of this 

 medium ; and this property, joined to the unalterable nature 

 of the solution, and the facility of its preparation, render it a 

 valuable resource in optical experiments ; and a tube of some 

 inches in length, closed at the ends with glass-plates, and filled 

 with this solution, is the instrument I always employ in expe- 

 riments 



