sulphuric acid; when the gas is evolving rapidly a further quantity of a mixture of 1 part alcohol and 2 parts acid is run slowly in, so as not to cool the contents of the flask sufficiently to retard the evolution of the gas. The reaction involved may be expressed as follows:

The sulphuric acid simply plays the part of a dehydrating agent, and brings about the reaction by its affinity for the water.

Properties.-Ethene, or ethylene, is a colorless gas, having a peculiar, suffocating, ethereal odor. Its specific gravity is 0.9688. At a temperature of 10. 10° and a pressure of 51 atmospheres it liquefies, and boils at -103° under ordinary atmospheric pressure. It is soluble in 8 parts of water. Ethylene is an easily combustible gas, burning with a very luminous flame and evolving much smoke. It forms a mixture with 3 volumes of oxygen, which explodes violently on the application of flame. It forms an oily liquid with an equal volume of chlorine, which is known as the "oil of the Dutch chemists." From this the name of

"olefiant gas" is derived.

ACETYLENE.

ETHINE.

Molecular Weight, 25.8.

Formula, C2H2. Preparation.-Acetylene may be prepared by the direct union of carbon and hydrogen. This is accomplished when the carbon electrodes of a powerful battery approach each other in an atmosphere of hydrogen. It also results whenever there is imperfect combustion of substances rich in carbon and hydrogen. When calcium carbide is brought into contact with water, acetylene is evolved according to the following reaction:

Properties.-Acetylene is a colorless gas, with a peculiar, disagreeable odor. It is condensed to a liquid at 1° and under a pressure of 48 atmospheres. It is moderately soluble in water. Acetylene is combustible, burning with a bright but smoky flame. It is burned for illumination. See also page 618.

ILLUMINATING GAS.

COAL GAS.

History.-Gas was first used for illuminating purposes in the house of William Murdock, at Redruth, in Cornwall, England. He distilled coal in an iron retort, and lighted his house with the

gas which he thus manufactured. This occurred about 1792. Gas was used for lighting the streets of London in 1812, and those of Paris in 1815.

Preparation. Illuminating gas is prepared by the destructive distillation of bituminous coal, although occasionally some other substances rich in carbon, like rosin, are used.

The process is accomplished in iron or fire-clay retorts, ten to twelve feet in length and two or three feet in diameter. Usually five of these retorts are placed in a furnace together. The temperature employed approaches low redness.

The products of this dry distillation are: (1) coke, (2) water, (3) tar, (4) ammonium carbonate, (5) liquid hydrocarbons, (6) hydrogen sulphide, (7) carbon dioxide, (8) carbon monoxide, (9) sulphur dioxide, (10) hydrogen, (11) marsh gas, (12) olefiant gas, and some other gaseous hydrocarbons.

Of these, 8, 10, 11, and 12 are useful for combustion and illumination; I is valuable for fuel; 3, 4, and 5 have other uses, to be considered later. The volatile products are first passed into the hydraulic main, where the water and tar are separated. From thence the gas goes into condensers, in which more tarry and oily liquids are removed. It then passes into the scrubbers, which consist of one or more columns of coke over which a spray of water trickles; this serves to wash the gas and remove ammonia. Next come the purifiers, in which the gas comes in contact with dry calcium hydrate or ferric hydrate, which serves to remove the sulphur compounds and carbon dioxide. From the purifiers the gas goes into the gasometers for storage and distribution.

The average yield per ton of coal is from ten to twelve thousand cubic feet.

Coal gas is considerably lighter than air, its specific gravity varying from 0.65 to 0.75.

The value of gas for illuminating purposes is determined by comparing the light from a jet burning five feet per hour with the light from a sperm candle consuming one hundred and twenty grains per hour. Such a gas flame should be equal to thirteen or fourteen candles.

CARBON AND OXYGEN.

There are two well-known compounds of these two elements:

Carbon monoxide, CO.

Carbon dioxide, CO2.

This compound is formed when coal burns with an incomplete supply of air. It may be easily recognized in the combustion of anthracite coal by its peculiar blue flame. Almost identical with this formation of it is the one by passing carbon dioxide over red-hot charcoal :

CO2+ C = 2CO.

The carbon may be replaced by iron and the same result attained. Carbon monoxide is most easily prepared by heating oxalic acid and concentrated sulphuric acid together:

The sulphuric acid acts by its affinity for water, and so causes a breaking down of the oxalic acid molecule.

The mixed gases are passed through a strong solution of sodium hydrate to absorb the carbon dioxide.

In a similar manner formic acid, or a formate, may be decomposed by sulphuric acid; in this case, however, no carbon dioxide is formed:

An economical method and one that will yield the gas rapidly consists in heating finely powdered potassium ferrocyanide with eight or ten times its weight of strong sulphuric acid :

As soon as the reaction commences the heat must be removed and the vessel cooled, if necessary, in order to prevent too rapid evolution of the gas. The water necessary in the above reaction is derived from the water of crystallization in the potassium ferrocyanide and from the small quantity in the commercial sulphuric acid.

Properties.-Carbon monoxide is a colorless gas, with a peculiar odor. It is very slightly soluble in water and has no effect on litmus paper. It readily burns in air or oxygen with a peculiar pale-blue flame, the product of its combustion being carbon dioxide:

When added to half its volume of oxygen, a mixture is formed which explodes on the application of flame.

The action of carbon monoxide on the human system is that of a narcotic poison. A small amount in the air is sufficient to produce giddiness and headache, followed by insensibility. Its peculiar action appears to be due to the formation of a compound with the hæmoglobin of the blood, the latter acquiring a purplish color.

History. Carbon dioxide was discovered by Paracelsus about 1520, and was further investigated by Van Helmuth in the early part of the seventeenth century. Its chemical nature was first made out by Lavoisier.

Occurrence.-Carbon dioxide occurs free in the atmosphere to the extent of three or four volumes in ten thousand. It is found in many mineral waters, and in the craters of some extinct volcanoes. Combined with various bases as carbonates it is still more abundantly distributed. The principal one of these compounds is calcium carbonate, which, as marble, limestone, and chalk, is one of the most abundant of minerals.

Preparation.-Carbonates and bicarbonates when heated yield the gas according to the following:

CaO + COg

2NaHCO3 = Na2CO3 + H2O + CO2.

When small quantities are desired it is rapidly and conveniently made from marble and hydrochloric acid :

CaCO3 + 2HCl

= CaCl + H2O + CO2.

The same result may be attained by the use of sulphuric acid, but the calcium sulphate which is formed is quite insoluble, and therefore difficult to remove from the flask.

This

When a current of air is passed over red-hot carbon the product is carbon dioxide, provided the air be kept in excess. and the method by heating a carbonate are used for furnishing the gas in the manufacture of carbonates on the large scale.

Properties.-Carbon dioxide is a colorless gas with a faint odor and weak acid taste. It is 1.52 times heavier than air and 21.945 times heavier than hydrogen. I volume of water dissolves i volume of the gas at ordinary atmospheric pressure. This solubility of the gas is increased 1 volume for every increase of 1 atmosphere in pressure.

A solution containing about 5 volumes of the gas in water constitutes the so-called "soda-water" which is dispensed in the pharmacies of this country. This solution is prepared by generating the gas from marble dust and sulphuric acid, in closed copper generators, and, after purifying the gas by passing it through water containing sodium bicarbonate, it is passed into water in steel cylinders, which are constantly agitated in order to facilitate the absorption. Only porcelain-lined steel cylinders and blocktin pipes should be used. Lead and copper are dissolved by the solution. It was formerly the custom to store the solution in copper cylinders lined with tin. In such vessels the soda-water frequently becomes contaminated with copper. The water should give no coloration with ammonium hydrate or sulphide, and no precipitate with potassium ferrocyanide, thus indicating the absence of copper.

The gas is more soluble in alcohol than in water.

Under a pressure of 35 atmospheres at a temperature of o° carbon dioxide is condensed to a colorless, mobile liquid. This liquid has a specific gravity of 0.923 at 0°, and 0.782 at 20°. It is not miscible with water, and has no action on dry litmus paper. When liquid carbon dioxide is suddenly released from pressure, such an intense degree of cold is produced as to cause a portion of the liquid to solidify. This solidification may also be accomplished by cooling the liquid to -65°. This solid carbon dioxide is capable of producing an intense degree of cold; to accomplish this the solid is mixed with ether and placed under the receiver of an air pump. On account of the rapid vaporization of the oxide a temperature of -110° is produced. This solid may be handled with safety, as it is always surrounded by a thin layer of the gas; when, however, it is pressed into contact with the skin it will cause a blister.

Carbon dioxide is neither combustible nor a supporter of combustion. This property may be illustrated by lowering a lighted taper into a vessel of it, or, since it is heavier than air, by pouring it down an inclined board on which are placed a number of lighted candles. Under the name of "choke damp" it someimes collects in old wells, and is detected by lowering a lighted candle, which will be extinguished if the gas is present. This choke damp results from the explosion of marsh gas in mines, and frequently fatally supplements the destruction caused by the latter.

When present in the atmosphere to the extent of 1 per cent.