the time when wanted, in a suitably-constructed metallic vessel, and discharged into the fire under pressure.

492. Carbonic Disulphide, CS,.-This is a very volatile, colorless liquid, of about 1.27 specific gravity, which boils at 118.5° F. It has a peculiar, very unpleasant odor and pungent taste. It has never been frozen, and is used in thermometers which are to measure very intense degrees of cold. It is highly inflammable, burning with a blue flame, and yielding carbonic dioxide and hydric sulphate. It dissolves sulphur, phosphorus, and iodine, and is dissolved in ether, but not in water. It is produced by bringing vapor of sulphur into contact with red-hot charcoal, the compound vapor being condensed in cold vessels. From its high dispersive power over light, it is used to fill hollow prisms of glass for spectroscopic observations. It is also applied in chemical manufactures, to a variety of purposes.

493. Cyanogen, C,N.—This substance may be best procured by heating mercuric cyanide (HgC,N,) in a glass tube or retort.

FIG. 182.

Burning Cyanogen.

The compound is decomposed, the cyanogen being evolved as a colorless gas, which may be ignited, burning with a beautiful blue flame, edged with purple, Fig. 182. Cyanogen is a transparent, colorless gas, poisonous if respired, and with a strong odor. It is very soluble in water, and hence must be collected in the pneumatic trough over mercury. It is reduced to a colorless, limpid liquid by a pressure of four atmospheres, and freezes into a transparent crystalline solid at 30° C.

494. Hydric Cyanide, HCN (Prussic Acid). This substance is found, in very small quantities, in the leaves, bark, blossoms, and fruit of the peach, cherry, sloe, and many other plants. It is best obtained by the decomposition of various metallic cyanides with a strong acid, and subjecting the mixture to distilla

tion. Hydric cyanide, when pure, is at common temperatures a colorless liquid, which solidifies at 15° C., and boils at 26° C. It has the peculiar odor of bitter almonds or peach-blossoms. It is exceedingly poisonous, one drop producing instant insensibility, and almost instant death. The inhalation of the vapor should, therefore, be most carefully guarded against. The prussic acid of the shops is a more or less dilute solution of hydric cyanide, in water. The hydrogen of hydric cyanide is replaceable by simple or compound monad radicles, giving rise to a scrics of compounds termed cyanides.

495. Potassic Cyanide, KCN.-This compound is formed when potassium is heated in cyanogen gas, or hydric cyanide vapor, but it is obtained on a large scale by the decomposition of potassic ferrocyanide (yellow prussiate of potash) (K.FeC.N.), by heat. It is a white crystalline body, very soluble in water, and exceedingly poisonous. It is much used in the arts in electroplating and gilding, and in the laboratory as a reagcnt.

§2. Combustion.

496. Combustion a Chemical Process.

Combustion, in its popular sense, is that form of chemical action which is accompanied by the disengagement of heat and light, and which usually takes place between the oxygen of the air and certain organic bodies, as wood, coal, oil, etc. The chemist, however, gives to the term a wider meaning, which includes all those forms of chemical action which result in the combination of bodies, with one or all of the constituents of a surrounding gaseous atmosphere; thus the violent burning of iron in oxygen, or its slow rusting in the air, the rapid consumption of wood in the furnace, or its gradual decay, are all alike, to him, cases of combustion. The nature of the gaseous atmosphere also makes no difference, and phosphorus or arsenic burning in chlorine gas, hydrogen, or iron in sulphur-vapor, are instances of

this form of chemical action, as well as the corresponding changes taking place in air, or oxygen gas. Bodies were formerly divided into combustibles and supporters of combustion. Oxygen was held to be the universal supporter of combustion, while hydrogen, carbon, and iron, which burn in it, were called combustibles. But if the atmosphere were hydrogen, the so-called supporters of combustion would burn in it equally well, and the fact is, the action is mutual and of the same kind on the part of both.

497. Rapid Combustion.—The beginning of rapid combustion is termed ignition. In order to induce ignition a certain elevation of temperature is required, and the maintenance of this temperature is essential to the continuance of the combustion. After a substance is once kindled, the heat given off by the rapid chemical action is usually more than sufficient to maintain the combustion until the burning body is consumed. The temperature at which rapid combustion may take place differs with different bodies. Thus, in atmospheric air, phosphorus ignites at 150° F., sulphur at 480° F., while the hydrocarbons require a temperature of nearly 1000° F. to kindle them. The stability of the order of Nature depends upon the gradation of the affinities between atmospheric oxygen and the hydrogen and carbon of organic bodies. These are only brought into action at high temperatures. Did these bodies, like phosphorus, ignite at a much lower degree, conflagrations, which are now comparatively rare, would become universal.

498. Explosive Combustion.-When two gaseous bodies, combustible in each other, are mixed and ignited, an explosion ensues. This is because the constituents of the gaseous mixture are so intimately blended that the heat evolved by the particles first ignited passes to those adjoining, and so on through the entire mass, with such velocity as to cause an almost instantaneous completion of the process. The products of this form of combus

tion are always in the form of highly-expanded gases or vapors, the intense rarefaction of which gives rise to a vacuum, and the particles of air rushing in to fill this, in colliding with each other produce the noise of the explosion.

499. Slow Combustion.-Oxygen, as well as other elements, frequently enters into slow combination at ordinary temperatures and without perceptible heat, as in the rusting of iron in the air. The cause of decay in vegetable and animal substances is the slow action of oxygen. This slow combustion is termed eremacausis. Heat, however, always accompanies this slow form of combustion. ounce of iron rusted in air, or burnt in oxygen, produces the same amount of heat, but in the former case it requires years for its development, and in the latter only as many minutes. Sometimes, under favorable circumstances, .the combination becomes so rapid that the accumulated heat produces ignition, causing the phenomenon called spontaneous combustion. This is most liable to occur with porous substances which expose a large surface to the air. The tow or cotton used for wiping the oil from machinery, and then laid away in heaps, often ignites in this manner, especially if exposed to the sun.

500. Heat of Combustion.-The complete burning of a combustible body requires the consumption of the same quantity of oxygen, whether the process goes on rapidly or slowly, and, in either case, the amount of heat set free is the same. Therefore, the intensity of the heat depends upon the rapidity of the combustion. Heat would be

liberated from the burning of a pound of coal in ten minutes, six times as fast as if its combustion occupied an hour. The burning in air of one pound of wood-charcoal will raise from the freezing to the boiling point 73 lbs. of water; one pound of mineral coal will correspondingly heat 60 lbs. of water; and one pound of dry wood will raise 35 lbs. of water through the same number of degrees. These are the highest results by careful experiments; in practice we ob

tain a much lower effect, both on account of imperfect combustion, and from the fact that a large proportion of the heated air escapes through the chimney, before it has given off as great an amount of heat as it is capable of producing.

501. Cause of the Heat.- It has been explained that chemical action produces heat by conversion of the motion of chemical atoms into neat-vibrations. We have atoms separated and powerfully attracted, like lifted weights; they rush together, collision arrests motion, and their force is given out as heat. It is the clash or impact of the atoms of oxygen against the elements of burning bodies, which gives us the heat and light of combustion. By figuring to ourselves the atoms shot across the molecular spaces with intense force, and thus parting with their motion, we have an explanation of the source of heat in combustion, which is in harmony with our latest knowledge of the nature of heat, and of its other modes of production, while in no other way is it possible to explain its chemical origin.

502. Nature of Flame.-Flame is produced by the combustion of gases. Substances which burn with flame are either gases already, or they contain a gas which is set free by the heat of combustion. But flame does not necessarily produce light. In the burning of pure oxygen and hydrogen, there is intense flame, but so little light that it can hardly be seen. If, into this non-luminous flame, we sift a little charcoal-dust, the particles of solid carbon are instantly heated to incandescence, and there is a bright

FIG. 183.

flash of light. The conditions of illumination are, therefore, first, an intense heat; and, second, a solid placed in the midst of it, which remains fixed, and gives out the light.

503. The Compound Blow-pipe. -These conditions are fulfilled