129. Oxygen must be Heated before it will combine with ordinary Combustibles. The jet of coal-gas has no disposition to burn until a lighted taper is applied to it. The oxygen of the air is at all times in contact with wood and coal, yet they do not burn unless they are first kindled. When even as inflammable a gas as hydrogen is mixed with oxygen, it does not burn unless ignited with a taper or an electric spark. "At the ordinary temperature of the air its chemical affinities are dormant; and, although endowed with forces which are irresistible when in action, it awaits the necessary conditions to call them forth. One of the grandest works of ancient art which have come down to us, is the colossal statue of the Farnese Hercules. The hero of ancient mythology is represented in an erect form, leaning on his club, and ready for action; but at the moment every one of the well-developed muscles of his ponderous frame is fully relaxed, and the figure is a perfect ideal of repose, yet a wonderful embodiment of power. Here in this antique we have most perfectly typified the passive condition of oxygen, the hero of the chemical elements. Raise now the temperature to a red heat, and in a moment all is changed. The dormant energies of its mighty powers are aroused, and it rushes into combination with all combustible matter, surrounded by those glorious manifestations of light and heat which every conflagration presents." COOKE.

130. Combustion is Self-sustaining. It is not necessary to arouse any large amount of oxygen to activity in order to insure the continuance of the combustion. When, for instance, a lighted match is held to the wick of a candle, it excites but a few molecules of oxygen to activity. These few rush into combination with the elements of the candle, and by so doing develop sufficient. heat to awaken the activity of more oxygen, which in

turn enters into combination and develops more heat. In this way a supply of active oxygen is maintained until the candle is consumed.

The slowness of combustion depends upon the fact, that the oxygen is mixed with the inert nitrogen in such proportions as generally to "restrain the awakened energies of the fire-element within the narrow limits which man appoints."

131. The Point of Ignition. — Different substances begin to burn at very different temperatures. This is well illustrated in the kindling of a coal-fire. Shavings are put into the grate first, then kindling-wood, then charcoal, and finally hard coal. The shavings are lighted by means of a match. The match is a bit of dry, soft wood, one end of which is covered with sulphur and tipped with phosphorus. It is a well-known fact, that when two bodies are rubbed together, heat is developed. On striking the match, sufficient heat is developed by the friction to ignite the phosphorus, which takes fire at a temperature of about 150° Fahrenheit. The phosphorus in burning develops heat enough to ignite the sulphur, which burns at a temperature of about 500°. The burning sulphur develops heat enough to ignite the wood of the match; the match, to ignite the shavings; the shavings, the kindling-wood; the kindling-wood, the charcoal; and the charcoal, the hard coal, which requires the temperature of a full white heat to set it on fire.

132. The Products of Combustion are not always Gaseous. In the burning of coal-gas and of a candle, the products are wholly gaseous, and in the burning of wood they are mainly gaseous; but when metals, as copper and iron, burn in oxygen, the products of their combustion are solid.

133. Magnesium and some other Metals will burn in the Air. - A magnesium wire will burn brightly in the

air (96). Calcium, also, burns readily in the air (113), as aluminium (97) does, if pulverized and strongly heated.

It will be noticed that all these elements are rare in their free state. Many of the rare metals, as potassium, sodium, magnesium, and calcium, are as combustible as carbon; but they differ from carbon in giving rise to solid products while burning.

134. Oxygen is not the only Supporter of Combustion. — If a piece of gold-leaf or Dutch foil be dropped into a jar of chlorine, it vanishes in a flash of light (53). Here the burning consists in a combination with chlorine. Tin and copper foil and pulverized antimony will also burn in chlorine. Many metals will burn in the vapor of sulphur; the burning being then a combination of the metal with sulphur.

135. The Materials of the Earth's Crust are chiefly Chemical Compounds. We have seen that when marble is acted upon by muriatic acid (42) we obtain carbonic acid, CO2, and calcic chloride, CaCl. The calcium, carbon, and oxygen of these compounds must come from the marble, or calcic carbonate, CaCO3.

We find, then, in marble two very combustible substances, calcium and carbon, combined with oxygen. Limestone has the same composition. Calcium, then, which is a very great rarity in a free state, is a very abundant element in nature.

It has been found that the rocks and solid matter of the earth are made up chiefly of such combustible elements as potassium, calcium, magnesium, aluminium, and carbon, combined with oxygen. Many of the rarest and most costly metals, then, are the most abundant in nature. The fact that they are so rare in a free state is due to their extreme combustibility. It is very difficult to separate them from oxygen, and no less difficult to keep them separate.

136. The Present Materials of the Earth are Products of Combustion. We see, then, that water, the most abundant of liquids, is made up of the very combustible elements, hydrogen and oxygen; while the solid materials of the earth are composed chiefly of the very combustible elements, potassium, magnesium, calcium, aluminium, carbon, and silicon, in combination with oxygen. These materials are the products of combustion. There must have been a time when these elements existed together in a free state. Then, by some means unknown to us, the mass took fire; and the conflagration raged until all the materials were consumed. There was more oxygen than was needed for the combustion; and this is now found in the air in a free state. Oxygen was the most abundant of all the elements, since it alone makes up half the weight of the solid earth, eight-ninths the weight of the water, and one-fifth the weight of the atmosphere.

The diagram below (from Cooke) illustrates what has been said of the composition of the earth. It will be seen, that, of the 65 elements, 13 alone make up at least .99 of its known mass, while the other 52 do not constitute altogether more than .01.

SLOW COMBUSTION.

137. Partially Active Condition of Oxygen.—We have seen (11, 12) that, besides its active and passive states, oxygen exists in a third, or intermediate, condition, in which it is partially active. In this form it plays a very important part in nature. It acts silently and slowly, taking months, and even years, to accomplish its work; but the results "far surpass in true grandeur those dazzling displays of power which the fire-element manifests when fully aroused."

138. Decay. - When wood, or any other organic substance (that is, any compound of vegetable or animal origin) is exposed to moist air, it decays. It first becomes rotten, and then slowly disappears. This decay consists in a gradual union of the substance with oxygen; in other words, a slow combustion. A log of wood which rots in the forest undergoes the very same change as one which is burnt on the hearth; the sole difference being, that, while the latter burns up in a few hours, the former is consumed only after the lapse of many years.

Wood, like other vegetable compounds, consists mainly of carbon, hydrogen, and oxygen. When it burns on the hearth, its carbon and hydrogen combine with the oxygen of the air, forming carbonic acid and water, which pass off in the smoke. The hydrogen is more combustible than the carbon, and therefore burns first, leaving the carbon in the form of glowing coals. These are consumed in their turn, gradually smouldering away, until nothing is left but a little ashes.

Quite the same process is going on with the decaying log in the forest. In decay, as in burning, the hydrogen of the wood unites with the atmospheric oxygen sooner than the carbon does; hence, in this stage of its decay, the wood becomes darker, and more like charcoal. At