Q() JtKMAllKS ON COMBUSTION. 



left that the colour of the light emitted during combuftioft 

 varies, and that this variation ufually depends, not upon the 

 fupporter, but upon the combuftible. Thus carbonic acid 

 burns with a blue flange, carbonated hidrogen with a white, 

 and charcoal with a red ; fulphur with a b!ue or violet, zinc 

 with a greenifli white, and phofphorus with a white. 

 Natural forma- The formation of combuftibles in plants obviouily requires 

 *"!!" ° ( c ?T buf " *be prefence and agency of light ; for when plants vegetate in 

 porters : Vegc- the dark, their carbon is not increafed, nor is any oily or refin- 

 tation. ous matter formed in them. The leaves of plants emit oxigen 



gas when expofed to the fun's rays, but never in the fhade, or 

 in the dark. Senebier has demonftrated that this emiffion is 

 occasioned by the decompofition of carbonic acid. This acid, 

 which is a product- of combuftion, is decompofed by the leaves 

 of plants affifted by funfhine, and converted into oxigen gas and 

 charcoal, a. fupporter and a combuftible. This proccfs is exactly 

 the reverfe of combuftion, and mult therefore reftore the fub- 

 ftances which had been loft during combuftion ; that is to fay, 

 caloric and light. But the fun's rays confift of thefe two bo- 

 dies. Thus we fee why plants require funfhine. A part of 

 vegetation confiits in decompofing, or unburning, produces, 

 and converting them into fupporters and combuftibles 5 but 

 for fuch a converfion caloric and light are abfolutely necelTary . 

 The fame effefts Befides vegetation, we are acquainted with two other mc- 

 y "*" thods of unburning products, or of converting them into pro- 



ducts and combuftible ; by expofing them, in certain circum- 

 ftances, to the agency of fire or of electricity. The oxides of 

 gold, filver, and mercury, when heated to rednefs, are decom- 

 pofed, oxigen gas is emitted, and the pure metal remains be- 

 hind. In this cafe the necefTary caloric and light muft be fur- 

 nifiied by the fire ; a circumftance which explains why fuch 

 reductions always require a red heat. When carbonic acid is 

 made to pafs repeatedly over red-hot charcoal, it combines 

 with a portion of charcoal, and is converted into carbonic ox- 

 ide gas. If this gas be a combuftible oxide, the bafe of the 

 carbonic acid and its oxigen muft have been fupplied with 

 light and caloric from the fire ; but if it be a. partial combuftible, 

 it is merely a compound of carbonic acid and charcoal: which 

 of the two it is, remains ftill to be afcertained. Electricity de- 

 compofes water, and converts it into oxigen gas and hidrogen 

 gas ; it muft therefore fupply the heat and the light which 



thefe bodies loft when converted into a producl. 



Thefe 



