litude between a compound body and its constituents is anything but an exceptional case; in fact it is this change of properties that distinguishes chemical union from mere mechanical mixture.
Our tallow candle is composed of two invisible gases and a black solid, and is therefore a much more extraordinary compound than the white phosphoric acid. When a candle is burned, the products of the combustion are invisible gases; these gases can nevertheless be collected by the chemist, and are found to weigh more than the original candle. Coal, coke, wood, and other combustibles which are employed as fuel, likewise form gaseous compounds with the oxygen of the atmosphere. This is a very significant fact, for were the products of combustion invariably solid, like phosphoric acid, the world would long since have been buried in ashes.
We have examined the first of the so-called elements of the ancients, and have proved it to be a manifestation of intense chemical action between two or more bodies. Let us now proceed to consider the nature of Air.
"There exists a certain thing," says a philosopher of the sixteenth century, "which we do not perceive, and in the midst of which is plunged the whole universe of living beings. This thing comes from the stars, and we call it air. Fire, in order that it may burn, requires wood, but it also requires air. The air, then, is the life, for if air be wanting