The mortars of the powder-mill, are hollow pieces of wood, capable of holding twenty pounds of paste, composed of the substances before mentioned, which are incorporated by means of the pestle. There are usually twenty-four mortars in each mill, where are made, each day, four hundred and eighty pounds of gunpowder; care being taken, to sprinkle the ingredients with water, from time to time, lest they should take fire. This precaution is absolutely necessary, and if attended to, would prevent many of the explosions, which, unhappily, take place, in the manufacture of powder. The friction must be great, and, therefore, the increase of temperature, occasioned in this manner, ought to be guarded against. This can only be done, by diminishing the time, or number of the blows, or by proportioning the weight of the pestle, and the frequent addition of water. The last is the most certain, and indeed, the water is in some respects, necessary to promote a more intimate mixture of the materials. The observations of M. David, on the use of water in the manufacture of powder, are certainly correct. The pestle is a piece of wood, ten feet high, and four and a half inches broad, armed at the bottom with a round piece of metal. It weighs about sixty pounds. Having mentioned one cause of the explosion of powder-mills, that of friction produced by the pestle, we find that it has been accounted for on another principle. The _Annales de Chimie_, tome xxxv, mentions some instances of spontaneous combustion in powder mills. It is well known, that charcoal has the property of absorbing several gases, and the observations of Rouppe and Berthollet, on this subject, are conclusive. It is also known, that charcoal, which contains hydrogen, when exposed to atmospheric air, will absorb oxygen, and form water; and during this combination, heat must be generated, by the emission of caloric from the oxygen gas. It is said, then, that in cases of spontaneous combustion, when nitre, sulphur, and charcoal, are mixed together, (unless water be added to prevent it), this effect will ensue, and fire be produced. We know, however, that percussion is one source of heat; and in truth, if that opinion be well founded, percussion itself may facilitate the union of hydrogen, with the oxygen of the air, and necessarily operate as a secondary cause of such explosions. Another opinion has been advanced by Bartholdi, to account for the spontaneous combustion in powder mills: namely, that charcoal sometimes contains phosphorus, combined with hydrogen, which, by the action of the pestle, is disengaged in the form of gas, and inflames, the moment it comes in contact with the air. Others again suppose, that it sometimes contains pyrophorus.