Among the substances subject to spontaneous combustion, pulverized or finely-powdered charcoal is one of the most remarkable. During the last thirty years no fewer than four cases of the spontaneous inflammation of powdered charcoal have taken place in France. When charcoal is triturated in tuns with bronze bruisers, it is reduced into the state of the finest powder. In this condition it has the appearance of an unctuous fluid, and it occupies a space three times less than it does in rods of about six inches long. In this state of extreme division it absorbs air much more readily than it does when in rods. This absorption, which is so slow as to require several days for its completion, is accompanied with a disengagement of heat which rises from 340° to 360° nearly of Fahrenheit, and which is the true cause of the spontaneous inflammation. The inflammation commences near the centre of the mass, at the depth of five or six inches beneath its surface, and at this spot the temperature is always higher than at any other. Black charcoal, strongly distilled, heats and inflames more easily than the orange, or that which is little distilled, or than the charcoal made in boilers. The most inflammable charcoal must have a mass of at least 66lbs. avoirdupois, in order that it may be susceptible of spontaneous inflammation. With the other less inflammable varieties, the inflammation takes place only in larger masses. The inflammation of powdered charcoal is more active in proportion to the shortness of the interval between its carbonization and trituration. The free admission of air to the surface of the charcoal is also indispensable to its spontaneous combustion. Colonel Aubert, to whom we owe these interesting results, likewise found that when sulphur and saltpetre are added to the charcoal, it loses its power of inflaming spontaneously. But as there is still an absorption of air and a generation of heat, he is of opinion that it would not be prudent to leave these mixtures in too large masses after trituration.[34] [34] See _Edinburgh Journal of Science_, New Series, No. viii. p. 274. A species of spontaneous combustion, perfectly analogous to that now described, but produced almost instantaneously, was discovered by Professor Dobereiner of Jena in 1824. He found that when a jet of hydrogen gas was thrown upon recently prepared spongy platinum, the metal became almost instantly red hot, and set fire to gas. In this case the minutely divided platinum acted upon the hydrogen gas in the same manner as the minutely divided charcoal acted upon common air. Heat and combustion were produced by the absorption of both gases, though in the one case the effect was instantaneous, and in the other was the result of a prolonged absorption. [Illustration: _Fig. 78._]