VOL. LXXIV.] PHILOSOPHICAL TRANSACTIONS. 499 



becomes dephlogisticated, or, in other words, receives an increase of dephlogis- 

 ticated air ; which shows that the change in the acid is not owing to the sun's 

 rays communicating phlogiston to it, but to their enabling it to absorb phlogis- 

 ton from the water contained in it, and so to produce dephlogisticated air. Mr. 

 Scheele also found, that the dark colour acquired by luna cornea on exposure to 

 the light, is owing to part of the silver being revived ; and that gold, dissolved 

 in aqua regia and deprived by distillation of the nitrous and superfluous marine 

 acid, is revived by the same means ; and there is the utmost reason to think, that, 

 in both cases, the revival of the metal is owing to its absorbing phlogiston from 

 the water. 



Vegetables seem to consist almost entirely of fixed and phlogisticated air, united 

 to a large proportion of phlogiston and some water ; since by burning in the open 

 air, in which their phlogiston unites to the dephlogisticated part of the atmos- 

 phere and forms water, they seem to be reduced almost entirely to water and 

 those two kinds of air. Now plants growing in water without earth, can re- 

 ceive nourishment only from the water and air, and must therefore in all proba- 

 bility absorb their phlogiston from the water. It is known also that plants grow- 

 ing in the dark do not thrive well, and grow in a very different manner from 

 what they do when exposed to the light. 



From what has been said, it seems likely that the use of light, in promoting 

 the growth of plants and the production of dephlogisticated air from them, is, 

 that it enables them to absorb phlogiston from the water. To this it may per- 

 haps be objected, that though plants do not thrive well in the dark, yet they do 

 grow, and should therefore, according to this hypothesis, absorb water from the 

 atmosphere, and yield dephlogisticated air, which they have not been found to 

 do. But we have no proof that they grew at all in any of those cases in which 

 they were found not to yield dephlogisticated air ; for though they will grow in 

 the dark, yet their vegetative powers may perhaps at first be entirely checked by 

 it, especially considering the unnatural situation in which they must be placed 

 in such experiments. Perhaps too plants growing in the dark may be able to 

 absorb phlogiston from water not much impregnated with dephlogisticated air, 

 but not from water strongly impregnated with it ; and consequently, when kept 

 under water in the dark, may perhaps at first yield some dephlogisticated air, 

 which, instead of rising to the surface, may be absorbed by the water, and be- 

 fore the water is so much impregnated as to suffer any to escape, the plant may 

 cease to vegetate, unless the water is changed. Unless therefore it could be 



trous acid of the same strength, and the fumes are colourless. This is called dephlogisticated spirit 

 of nitre, as it appears to be really deprived of phlogiston by the process. The manner of preparing 

 it, as well as its property of regaining its yellow colour by exposure to the light, is mentioned by Mr. 

 Scheele in the Stockholm Memoirs, 1774. — Orig. 



3 S 2 



