The Romance of Modern Chemistry

PRODUCTION OF LIGHT AND HEAT one or two points which have been mentioned already. In the first place, it shows what fundamental changes those substances undergo which take part in a chemical action; we start with a piece of metallic ribbon and the invisible air, and there is left behind a soft, white, powdery mass of magnesia. In the second place, the intense light observed when magnesium burns is due to the presence of little particles of infusible magnesia, which are rendered incandescent by the great heat of the chemical action. Again, it is easy to show that just as the carbon dioxide and water produced by the combustion of a candle are heavier than the candle, so the white powder produced by burning a piece of magnesium ribbon weighs more than the ribbon. The discovery that the products of combustion are heavier than the combustible substance was really a very important one in the history of chemistry ; for up to about 120 years ago it was generally supposed that a combustible substance contained something called phlogiston, which came out of the substance when it was burned. It was the famous French chemist Lavoisier who finally overthrew this theory, and emphasised the fact that instead of losing anything when it was burned, a combustible substance actually became heavier. The meaning of the term “ combustion ” has been ex- tended in the foregoing paragraphs so that the burning of coal and the burning of magnesium are brought under the same category. We may now extend the term still further to cover many chemical processes, which, although they do not very obviously produce light and heat, yet depend essentially on the same chemical phenomenon, namely, the combination of some substance with the oxygen of the atmosphere. These are cases of slow com- 115