FIG. 184.

most perfectly by means of the compound blow-pipe of Dr. Hare. The two gases are collected in gasometers, or Indiarubber bags, Fig. 183, which are connected by flexible tubes with the brass jet, Fig. 184; the flow being increased by pressure on the bags, and controlled by stop-cocks. The gases are emitted together and burned at the orifice, a. When ignited, they give rise to a blue flame which is hardly visible, but which has intense heating power, and pro

Blow-pipe Jet.

a

duces the most remarkable effects. A steel watch-spring burns in it with a shower of scintillations. Substances

which do not fuse in the hottest blast-furnaces melt in this heat like wax, or dissipate in vapor.

504. The Lime-Ball-A little ball of lime, however, of the size of a pea, remains unaltered in the flame. It glows with a blinding brilliancy, producing what is known as the "Drummond light," or the "calcium light." It is cmployed as a substitute for the rays of the sun in the solar, or oxyhydrogen microscope, and is used in coast-surveys for night-signals. In all ordinary illuminations the principle is the same as that of the lime-light. The substances employed are compounds of carbon and hydrogen; the union of oxygen and hydrogen gives rise to heat, and the luminous carbon-particles at the same time set free in the heated space are the source of the light.

505. How the Candle burns.-The materials used for illumination, whether solids or liquids, are always converted into gas before burning. The candle first becomes a lamp, and then a gas-burner. When lighted, the heat radiates downward, so as to melt the material of the candle and form a hollow cup filled with the liquid combustible, Fig. 185, and thus the candle becomes an oil-burner. From this roservoir, the wick draws up the oil into the

FIG. 185.

flame. Here, in the midst of a high heat, and cut off from the air, it undergoes another change exactly as if it were inclosed and heated in a gas-maker's retort; it is converted into gas, and in this form finally burned. As the wick rises into the flame, it fills the interior with a sooty mass, and interferes with the combustion.

506. Structure of the Flame.-As the wick remains thus unconsumed in the interior of the flame, it is obvious there can be no fire there. If we lower a piece of glass or a wire gauze over a candle or gas flame, as in Fig. 186, we shall see an interior dark space surrounded by a ring of fire. This inner sphere is filled with dark unburned hydrocarbon vapors, which are inclosed by a shell of fire, or burning gas. If one end of a small glass tube be introduced into the candle-flame, as in Fig. 187, these interior gases will be conveyed away, and may be lighted at the other end.

Burning
Candle.

507. Order of the Combustion.— There is an order of combustion in the flame, which depends upon the order of affinities. In Fig. 188, a represents the nucleus of hydrocarbon vapor. If, now, oxygen from without had the same affinity for both its elements, they would be consumed together, with but little luminous effect. But the oxygen decomposes the gaseous compound, and, seizing upon the hydrogen first, surrounds a with the intensely heated space, b. At the same time the carbon-particles are set free, and, being heated white-hot, give out the motion of light. The cone b

FIG. 186.

The Flame hollow.
FIG. 187.

Gas from Flame.

FIG. 188.

is therefore the place of burning hydrogen and the seat of illumination. The incandescent carbon-particles, as they pass outward, meet with oxygen at c, and are converted into carbonic dioxide in the outer cone. To prove the constant presence of free carbon in the flame, it is only necessary to introduce into it any cold body, as a piece of porcelain, when carbon will be copiously deposited upon it as soot. Fig. 189 represents a cross-section of the flame and the arrangement of its parts; CH the unburned carbon and hydrogen, H the sphere of burning hydrogen across which the carbon-particles float, and lastly the sphere of burning carbon. It will be observed, by noting any common flame, that at the base it burns blue, and

00

FIG. 189.

H

CO2

CH

HO

Cross-Section of the

Shells of
Flame.

yields but little light. This is because the oxygen at this point is so abundant that it burns simultaneously both hydrogen and carbon. A candle-flame moved swiftly through the air gives a diminished light for the same reason.

508. Effect of Temperature on the Flame. The amount of light produced depends upon the intensity of the heat. Dr. Draper found that a body at 2,600° emitted almost 40

Flame.

FIG. 190.

Copper Coil.

A piece of fine

times as much light as at 1,900°. If by any means the temperature of the flame falls below a certain limit it is immediately extinguished. The flame of a candle may be put out by lowering over it a coil of cold copper wire, Fig. 190. wire gauze held across the flame of a candle cools the combustible gases below the point of ignition, so that they rise through the meshes in the form of smoke, Fig. 191. The gauze may become red-hot and still not allow the flame to pass, so rapidly is the heat conducted away by the wire.

Yet the cooled gases may be rekindled above, when the flame will go on burning as before, Fig. 192.

509. Safety-Lamp.-On this principle the safety-lamp is

FIG. 191.

constructed. The explosions of hydric carbide in coalmines caused immense destruction of life, and various arrangements had been fruitlessly contrived to prevent these terrible accidents when Sir Humphry Davy took hold of the subject. He commenced a series of researches upon flame in August, 1815, and with such success as to produce the perfected

lamp at the Royal Institution of London in the succeeding November. The safety-lamp consists simply of an ordinary oil-lamp inclosed in a cage of wire

FIG. 193.

space

FIG. 192.

Gas burns above.

gauze which permits the light to pass out, but prevents all exit of flame, Fig. 193. The within the gauze often becomes filled with flame, from the burning of the mixed gases which penetrate the net-work; but the isolation is so complete that the explosive mixture without is not fired. Fatal 'explosions still occasionally take place, but they are due to carelessness of the miners. As the intensity of light depends upon the rapid consumption of oxygen, there must be a free supply of air, and provision for the escape Safety-Lamp of combustion products.

DIVISION III.-ORGANIC CHEMISTRY.

CHAPTER XXV.

§ 1. Hydrocarbons and their Derivatives.

510. The Marsh-Gas Series (Paraffins).-When the unitweight of carbon combines with four unit-weights of hydrogen the result is the simplest known hydride of carbon, and since this is a saturated compound, or molecule, it is incapable of combining with chlorine, bromine, or any monad element, but it may exchange the whole or a

part of its hydrogen for an equivalent quantity of another element. It is from these hydrocarbons, or compounds having this composition, that more or less directly the