THE PROPERTIES

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of the dense earthy particles contained in vegetables. Inmaking charcoal, the vegetables are burned in such a man-ner, that they do not inflame; and then the combustiblematter unites itself intimately with the most dense earthyparticles. Charcoal can only be decomposed by fire; which,at the beginning of the ignition, separates the combustiblematter from the earthy particles; which are then termedcinders.

19. When charcoal is exposed in the open air to a suffi-cient degree of fire, its property is to burn and be con-sumed. It sometimes emits a little blue flame; but morefrequently sparkles, and becomes red, without emitting anyflame. If, in this state, it be agitated by the wind, itappears of a more lively and brilliant red in proportion tothe force of the wind, and is sooner reduced to cinders.From this property it enters into the composition of gun-powder.

20. Generally speaking, charcoal appears red only when agreater degree of fire is applied to it than will burn sulphur;the propagation of the ignited motion, and its total con-sumption, are effected more slowly than in sulphur. Theseeffects differ according to the quality of the charcoal: thatWhich is the lightest, or which contains least of the denseearthy particles, takes fire and is consumed the quickest.

21. The more the air is rarefied, (12.) the more diffi-cultly charcoal burns, and the greater degree of fire isrequisite to keep it red-hot.

22. Saltpetre is formed from a combination of the nitrousacid wish a fixed alkali. It is extracted principally fromanimal and vegetable substances, found in a putrescent state,mixed with stones, earths, and piaisters ; and is separatedfrom them by boiling them in a water impregnated witha fixed alkali. The salt chrystalizes in long filaments, lyingthe one upon the other. This is the only'method of puri-fying it; and must be repeated two or three times, accord-ing to the quantity of the heterogeneous matters. When,by this process, the saltpetre has been well purified, it hasalways the same properties, from whatsoever substances itmay have been extracted.

23. Its properties are, to dissolve in water, more fully inboiling than in cold water; and to liquefy, by a greaterdegree of heat than is required to liquefy sulphur. If theiieat be increased while the saltpetre is in fusion, it sublimes

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