COMBUSTION.

9

a new compound, which is a gas or kind of air, is pro-duced. If we put a piece of salt into water, it willget less and less, and at last will disappear altogether,having wholly dissolved; but the salt is not destroyed,it is only dissolved in the water.

22. Now we may compare the burning of a candleto dissolving a piece of salt ; for all the solid matterof the candle remains diffused throughout the air,after it is burnt, just as the salt remains dissolved inthe water: but with this difference—the salt is dissolvedin the water, but not combined with it. The elementsof the tallow are dissolved in the air, but they havecombined with a quantity of oxygen, because they havea strong affinity or attraction for it. If the solution ofsalt is left some time in a warm place, the waterevaporates, and we get the salt again, unchanged; butin the case of the candle, its elements have combinedwith oxygen, and they cannot be separated again fromit except by the action of something, which having amore powerful attraction for the oxygen than it has forthe elements of the candle, causes it to relinquish them.

23. There are substances which have sufficientattraction for the oxygen to effect this, though,indeed, we cannot get back the tallow, but only itselements, or the simple substances of which it is com-posed. What has been said with regard to the burningof a candle is equally applicable to the burning of wood,coal, or in fact any combustible matter. In all casesthey burn in consequence of their affinity for the