No correct ideas of combustion were attained until the time of Lavoisier.[2] This great French savant gave precision and accuracy to the investigations of chemical science by the introduction of the balance. He disproved the theory that “water is the ultimate principle of all things,” and prepared the way for a clear apprehension of the truth that matter, though constantly changing its form, is never destroyed. He also announced the correct theory of combustion. Until this time scientists had held what was called the “Phlogiston[3] Theory.” We can but smile at the absurdity of this belief, and yet no hypothesis was ever taught more positively, or maintained more tenaciously. It declared, in brief, that when substances burned, they parted with a certain material called phlogiston. When, at length, its advocates were asked to explain the fact, discovered by Dr. Priestly,[5] that quicksilver, when burned, weighed more than before, they were forced to put forward the ridiculous statement that phlogiston possessed the property of “buoyancy” so that when it was contained in a body its weight was lessened; which was as wise as the brilliant supposition that a person can lift himself over a fence by tugging at his boot straps. After a fierce struggle they were forced to confess that they had placed “the cart before the horse.” The truth was precisely opposite to their statement. Substances when they burn take up something instead of giving it off. That something is oxygen, and a body when burned, if it can be weighed, will be found to weigh as much more as the added weight of the oxygen which has united with it. Example: Iron-rust is iron, plus oxygen. [Illustration: MAGNESIUM RIBBON BURNING, AND PRODUCING MAGNESIC OXIDE (MgO).[4]] THE TRUE EXPLANATION. We shall here confine ourselves to the consideration of the heat and light produced by chemical action. It will be remembered that by this term (chemical action) is meant the process of uniting two or more different elements to form a compound different from either. We usually consider air essential to combustion, but this is not necessarily the fact. Gold foil or powdered antimony, dropped into a jar of chlorine, spontaneously ignites. Even in the interior of the earth, heat must be produced by the uniting of any elements that have an affinity for each other. [Illustration: BORACIC ACID IMPARTS A GREEN COLOR TO THE FLAME OF ALCOHOL.[6]] The most common agent of combustion is oxygen. Of this interesting gas some description has been given in a preceding article. It is the fruitful source of almost all of our artificial heat. The fallen tree in the forest is slowly consumed by it, not less surely than the flaming wood and coal in our stoves. The human body is a furnace. In the minute corpuscles[7] of the blood, carbon is uniting with oxygen as certainly as are the particles of carbon in the flame of our lamps.