To prevent the action of fire on wood, marine salt, vitriol, and alum have all been used. Various ways of employing them have been adopted; but they do not absolutely prevent wood taking fire in an active heat. For the same purpose, (_Coll. Academ._ tome xi, p. 487,) a mixture of green vitriol, and quicklime is recommended, by which we form sulphate of lime and oxide of iron. The _Journal de Paris_ of 1781 contains various processes. At Vienna, saline substances are employed. The combustion of wood is the same, in all cases, in which oxygen is concerned; but the products in some particulars may vary. Hence saw-dust, when mixed with nitrate of potassa, and inflamed, will burn, and produce little or no smoke, because the combustion is rapid and perfect; but when employed with sulphur and nitre, it produces much smoke. Here the oxygen is furnished by the nitre, and carbonic acid gas is formed. The same thing takes place, when a mixture of saw-dust and nitre is used in artificial fire; and, according as the decomposition is more or less rapid, the combustion will be so likewise. The particular applications of saw-dust will be noticed hereafter. With respect to _lycopodium_ or puff ball and various species of agaric, or the medullary excrescences of trees, which are used in some preparations of artificial fire, we may observe, that the first is confined principally to theatrical fire-works, and the second to the preparation of spunk, or tinder, called also pyrotechnical sponge. See _Pyrotechnical Sponge_. As to the substance usually called _lightning wood_, found in the hollow of the stumps of trees, and sometimes on the surface, which, from having lost its compactness and other characters of ligneous fibre, is called _rotten_ wood, it is in fact the solid part of the wood in a state of decomposition, in consequence of which, it becomes a _solar phosphorus_. It appears to owe its phosphorescent property, i. e. its power of shining in the dark, to the previous absorption of light, and not, as some have suggested, to the presence of phosphorus, or the emission of any gaseous compound, which contains it. The process of animal putrefaction will produce such appearances, but, in this case, the cause is different. Turf or peat, a substance found, and employed as fuel, in some countries, and found in boggy situations, is partially decomposed vegetable matter, consisting of a congeries of fibres or roots. But black mould is the result of a decomposition of vegetable substances, in which the ligneous fibre is carbonized, and mixed with earth. The formation of mould, however, is owing more to the decay of leaves &c. (See _Coal_.) Dr. Shaw (_Travels to the Holy Land_) observes, that when they were either to boil or bake, camel's dung was their common fuel; which, after being exposed a day or two in the sun, catches fire like touch-wood; and burns as light as charcoal. _Sec. L. Of Linseed Oil._