Among the means of producing heat that of compression is well known. The common condensing syringe, for inflaming spunk or touch paper, is on this principle.
This syringe is now made very portable, not more than six inches in length and about three-eighths of an inch in diameter. The end of the piston, which fits tight in the cylinder, has a small cavity, in which the spunk is put, so that, when the piston is suddenly compressed, the air is condensed, and a temperature produced, sufficient to inflame it. The air, in the cylinder, is condensed in the ratio of about one to forty. The calculations on the degree of compression, which atmospheric air must undergo to produce fire by this kind of percussion, with observations on the subject, may be seen in M. Biot, (Traité de Physique Experimentale, &c. tome ii, p. 17), with other remarks concerning the sources of caloric.
To account for certain phenomena in the atmosphere, some of which are accompanied with detonations, Mr. Nicholson (Chemical Dictionary, article Air, atmospherical), conceives that the lower atmosphere consists chiefly of oxygen and nitrogen, together with moisture, and the occasional vapours or exhalations of bodies. The upper atmosphere seems to be composed of a large proportion of hydrogen, a fluid of so much less specific gravity than any other, that it must naturally ascend to the highest place; where, being occasionally set on fire by electricity, it appears to be the cause of the aurora borealis and fire-balls. It may easily be understood, that this will only happen on the confines of the respective masses of common atmospherical air, and of the inflammable air; that the combustion will extend progressively, though rapidly, in flashings from the place where it commences; and that, when, by any means, a stream of inflammable air, in its progress towards the upper atmosphere, is set on fire at one end, its ignition may be much more rapid than what happens higher up, where oxygen is wanting; and at the same time more definite in its figure and progression, so as to form the appearance of a fire-ball.
Detonations frequently accompany combustion. There are many interesting experiments on this subject, some of which we will notice in this place, viz.
Experiment 1. If a small portion of fulminating powder be placed on a fire-shovel over a hot fire, it will become brown, then melt, and swell up, and finally explode. See [Fulminating powder.]
Experiment 2. Iron filings and sulphur, made into a paste with water, and buried in the ground for a few hours, will unite, decompose the water, and inflame; throwing up the earth with violence and noise. See [Artificial Volcano.]
Experiment 3. If nitrate of copper be spread on tin foil and wetted, and the foil immediately wrapped up, scintillations of fire will follow, accompanied with slight detonations.
Experiment 4. Five or six grains of sulphuret of antimony, with half its weight of chlorate of potassa, when struck with a hammer will cause a loud detonation.
Experiment 5. Two grains of chlorate of potassa, and one grain of flowers of sulphur, when rubbed together, will produce a detonating noise; and the same mixture, struck with a hammer, will give a loud report. See [Chlorate of Potassa.]
Experiment 6. One grain of phosphorus and two grains of chlorate of potassa, struck in the same manner, will produce a violent explosion. See [Phosphorus.]