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Thomson’s theory of combustion.

The formation of combustibles in plants obviously requires thepresence and agency of light. The leaves of plants emit oxigengas when exposed to the sun's rays, but never in the shade, orin the dark, (page 120).

Besides vegetation, we are acquainted with two other methodsof unburning products, or of converting them into products andcombustibles, by exposing them, in certain circumstances, to theagency of fire or of electricity. The oxides of lead, mercury,tic. when heated to redness, are decomposed, oxigen gas is emit-ted, and the pure metal remains behind, (page 187). In thiscase the necessary caloric and light must be furnished by thefire; a circumstance which explains why such reductions alwaysrequire a red heat. When carbonic acid is made to pass repeatedlyover red-hot charcoal, it combines with a portion of charcoal,and is converted into gazeous oxid of carbon. If this gas be acombustible oxid, the base of the carbonic acid and its oxigenmust have been supplied with light and caloric from the fire;but if it be a partial combustible, it is merely a compound ofcarbonic acid and charcoal: which of the two it is, remains stillto be ascertained.

Electricity decomposes water, and converts it into oxigen gasand hidrogen gas; it must therefore supply the heat and the lightwhich these bodies lost when converted into a product.

These facts, together with the exact correspondence of thetheory given above with the phenomena of combustion, renderit so probable, that Dr. Thomson has ventured to propose it asan additional step towards a full explanation of the theory of com-bustion. Every additional experiment has served to confirm itmore and more. It even throws light upon the curious experi-ments of the accension of metals with sulphur, (page i49) whichsucceed, as stated already, in vacuo, under mercury, in nitro-gen gas, kc. ^

Dr. Thomson has noticed that the same emission of caloric andlight, or of fire, takes place when melted sulphur is made to