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 ; that it is the radicle of hydrocyanic acid and cyanic acid; and that prussic acid is a compound of carbon, hydrogen, and nitrogen. Cyanogen, it may be mentioned, plays an important part in many organic reactions and compounds.

Gay-Lussac observed that the action of chlorine on hydrocyanic acid replaced the hydrogen forming cyanogen chloride, or acide chlorcyanique of his day; and he found that when beeswax was bleached by means of chlorine, it lost hydrogen and took up an equal volume of chlorine. At a royal ball at the Tuileries, in the reign of Charles X. (brother of Louis XVI.), the guests were "much annoyed by the irritating vapours which came from the wax candles to illuminate the apartments." This was due to the fact that the manufacturer had used chlorine to bleach the wax, and the irritating vapours contained hydrochloric acid.

In 1816 "Gay-Lussac made the remarkable observation that when a crystal of common potash alum is hung up in a saturated solution of ammonia alum it grows exactly as if it had been placed in the solution from which it was originally obtained. From this fact he drew the conclusion that the molecules of these two alums possess the same form." This was the beginning of Mitscherlich's law of isomorphism, which was discovered in 1810.

Gay-Lussac, by igniting potassium carbonate in iodine vapour, or chlorine gas, obtained an evolution of oxygen