Within the steel cylinder is suspended a bottle charged with a powerful acid, probably sulphuric acid--but the secrets of patents must not be revealed. The bottle can be instantaneously broken by a lever or weight, and the acid is precipitated into the cylinder, which is filled with an alkaline fluid--perhaps a solution of carbonate of soda. The mixture of these fluids rapidly produces large quantities of carbonic acid gas, which is a great enemy to fire. Moreover, water absorbs the gas easily; and when generated in the cylinder, the expansion of the gas causes a propelling power, varying from seventy to a hundred pounds per square inch. Consequently, a jet of water propelled by the gas shoots out a distance varying from thirty to fifty feet; and when it reaches the fire, the heat evaporates the water, and liberates the gas held in solution, which chokes the fire. This is the general principle of most chemical fire-engines. There are several varieties; but they are, no doubt, chiefly based on the rapid evolution of carbonic acid gas. If you find the principle difficult to understand, imagine a soda-water bottle bursting, or the contents spurting forth if the cork be suddenly removed, and you will not be so surprised at the stream jetting forth from an extinctor. Soda-water is, of course, aërated by being charged with carbonic acid gas. These chemical extinctors are of all sizes; they range from small bottles upward, to large double-tank machines, and drawn by horses. The small bottles contain the necessary materials, so arranged that, when the bottle is thrown down, the gas is generated and the fire choked. Both Germany and the United States make large use of chemical fire-engines, some of which are capable of giving a pressure of a hundred and forty pounds, and perhaps more, to the square inch. Cases filled with sulphur, saltpetre, and other chemicals are sometimes used, which, being ignited, send forth a choking vapour, stifling all fire in a confined space; again, other contrivances discharge ammoniacal gases and hydrochloric acid. Extinctors, or fire-annihilators, have been invented or introduced by several persons. Mr. T. Phillips was responsible for one in 1849, which generated steam and carbonic acid. Two or three persons seem to have had a hand in an apparatus developed by Mr. W. B. Dick about twenty years later, and patented April, 1869. This consisted of an iron cylinder furnished with tartaric acid, bicarbonate of soda and water, and generating the carbonic acid gas. The first inventor of this appliance was a Dr. Carlier, who suggested it, or something like it, a few years previously.