Dr. Irvine refers all cases of combustion to a change in the capacity of bodies for caloric; which depends on the nature of the products: if they have a greater capacity, no flame ensues, and the caloric remains more or less quiescent; if they possess a less capacity, flame is the consequence. There are exceptions to this doctrine.
In the emission of caloric, Dr. Black supposes, that it is given out, in consequence of the resulting attraction of the new compound for caloric being less than that of its ingredients, when separate. M. Curadou (Journal de Physique, 1809) observes, that, in preparing the artificial stone, one-half of which is composed of water, by mixing one part of sulphuric acid with two parts of clay, and a sufficient quantity of water, a higher temperature is produced than that of boiling water. In this instance, we find that, in the formation of sulphate of alumina, which envelopes the silica, the water is solidified, as in many other cases, and, while it forms a solid substance, the caloric of fluidity is liberated. The heat, he remarks, is sometimes so great as to set fire to inflammable substances.
Sec. LI. Of the Indian White Fire.
This preparation (feu blanc Indien of the French) is described in the Archives des Découvertes, &c. vol. ii, p. 300. It appears, that it was kept secret in France, and was used by the French astronomers for signals.
In 1807, M. de Zach published some account of it, in his Astronomical and Geographical correspondence.
The case, in which the composition is put, is ten inches in diameter and four in height; but may be of any size, according to the quantity of the composition to be burnt, and the degree of light required. It was seen 40 miles at sea. General Ray lighted, on the English coast, a case of this fire, which was seen very distinctly on the French coast.
Composition of White Fire.
| Saltpetre | 24 parts. |
| Sulphur | 7 —— |
| Red arsenic | 2 —— |
This powder lights without explosion, and illuminates with great brilliancy. Care must be taken not to breathe the Arsenical vapours, which are produced by the combustion.