904 



of atmospheric air, water, and ammonia, yield coloured substances. 

 This series of coloured products is usually comprehended, more for 

 convenience' sake than on account of chemical identity, under the 

 generic terra orceine." The whole subject of the chemistry of these 

 bodies is at present in a most unsatisfactory condition, demanding 

 fresh investigation and research, in illustration of which the author 

 exhibited tables of the colorific and colouring principles, so far as 

 they are at present known, showing their chemical formulae, and the 

 authority therefor, and various relative information. " It is highly 

 probable that, when the chemistry of the lichens has been more fully 

 studied, and the whole subject of their colour-educts and products 

 better understood, we shall begin to reduce the present confused mass 

 of complex substances, and find the same principles more extensively 

 diffused through different lichen species." Dr. Lindsay entered 

 somewhat minutely on the chemical reactions of the better-known 

 colorific and colouring principles, and their derivatives, so far, at 

 least, as these throw any light on the production and transmutation 

 of the red or purple colours extracted from what may be termed, par 

 excellence, the dye-lichens. After a few remarks on the chemical 

 constitution of orchil and litmus, as given by Kane, Gelis, Pareira, 

 and others, he discussed the subject of decolorization of weak infu- 

 sions of orchil and litmus by exclusion of atmospheric air, and by 

 various deoxidizing agents, and the various theories as to the causa- 

 tion of this phenomenon. " I have repeatedly had occasion to notice 

 that, when weak infusions of these substances are excluded for some 

 time from atmospheric air, in a bottle with a tightly- fitting cork, they 

 gradually lose colour, but rapidly regain it on re-exposure. It is 

 curious that both orchil and litmus are what are called transient or 

 false colours, i. e., they slowly lose their bloom and tint by long 

 exposure to the atmosphere. The colouring matter therefore appears 

 to be decolorized, both by exposure to and exclusion from the air, — 

 phenomena, apparently, of very opposite characters. The cause of 

 the latter phenomenon has never, so far as I am aware, been quite 

 satisfactorily explained ; but it has been variously supposed to be due — 



"1. To the mere negation of oxygen. 



*' -2. To the development, in the liquids, of various substances 

 capable of exerting a decolorizing influence on the colouring matter. 



" 3. To deoxidation of the colouring matter by substances which 

 have a great tendency to become oxidized or peroxidized ; e. g.^ 

 hydrogen in the case of decolorization by sulphuretted hydrogen, 

 nascent hydrogen, and the protoxides of iron and tin, &c. 



