The author next proceeds to consider the influence of rarefaction, produced by heat, upon combustion and explosion. A volume of air at 212° is expanded to 2·25 volumes. At a dull red heat its probable temperature then is 1032°, provided it expand equably for equal increments of heat.
M. Grotthus has concluded that expansion by heat destroys the explosive powers of gases, but Sir H. Davy found that two parts of oxygen and one of hydrogen expanded to 2·5, its original bulk, detonated at a red heat, and in another experiment, even at a lower temperature; whence it appears, that detonating gaseous mixtures have their inflammability rather increased than diminished, by expansion by heat. In prosecuting these inquiries, the author discovered that a mixture of oxygen and hydrogen produced water at a temperature below visible redness, and without explosion or even any luminous appearance; and at a temperature a little above the boiling point of quicksilver, charcoal converts oxygen into carbonic acid without any of the ordinary phenomena of combustion.
The third section relates to the effect of the mixture of different gases upon explosion and combustion. When 1 part of a mixture of oxygen and hydrogen, in the proportions that form water, is mixed with 8 parts of pure hydrogen, the electric spark does not inflame the mixture; and its combustion is similarly prevented by 9 parts of oxygen, 11 of nitrous oxide, 1 of carburetted hydrogen, 2 of sulphuretted hydrogen, 0·5 of olefiant gas, 2 of muriatic acid gas, and five sixths of silicated fluoric acid gas. It therefore appears that other causes, besides density and capacity for heat, interfered in these phenomena; for nitrous oxide, which is one third denser than oxygen, and which has a greater capacity for heat, has lower powers of preventing explosion; and hydrogen, though fifteen times lighter than oxygen, has a higher power of preventing explosion; and olefiant gas, in this respect, precedes the others in an infinitely higher ratio than could have been expected either from its density or capacity.
The author concludes this paper with some general observations, and practical inferences founded upon the previous detail of facts. Flame may be regarded as gaseous matter, of a temperature above that which is capable of giving to solids a white heat; for heated air, though not luminous, will communicate that high temperature to solid bodies. When we attempt to pass flame through fine wire gauze, the metal so far cools the gaseous matter that it is no longer luminous. The power of metallic and other tissues to prevent the combustion of explosive gaseous mixtures, will depend upon the heat required for their combustion, as compared with that acquired by the tissue; and the flame of those bodies which are most readily inflammable, and of those which produce most heat in combustion, will pass through a wire gauze capable of intercepting those flames that produce little heat; so that the flames of different substances will pass through wire gauze at different temperatures. For instance, a tissue that has 100 apertures in the square inch will intercept the flame of alcohol, but not that of hydrogen; and a tissue which would
