Heated with hydrate of potassium it forms formate of potassium.

Hydrate of potassium +Carbonic oxide=Formate of potassium.

KHCO2.

The formation of these bodies, and its conversion into carbonic acid when burnt, distinguish carbonic oxide from other gases.

Combinations of Carbon with Hydrogen.

§ 163. A great many animal and vegetable substances which contain carbon and hydrogen, give up when heated a portion of those elements in union with one another-that is, as "hydrocarbons." The number of proportions in which carbon and hydrogen unite with one another, even when no other element is present (that is, the number of hydrocarbons), appears to be almost unlimited; no other two elements unite in such various proportions or give rise to such a variety of different compounds. Many scores of such bodies are known; some of them are solid, some liquid, and some gaseous. When organized matter is heated, the hydrocarbons formed are generally various.

§ 164. Olefiant gas, CH, also called "heavy carburetted hydrogen, or ethylen."-When alcohol is heated with some body having a strong affinity for water, such as chloride of zinc, ZnCl,, or sulphuric acid, the alcohol is decomposed into water and olefiant gas. Alcohol, which is spirits of wine from which the physically mixed water has been removed, still contains the elements of water; its empirical (§ 45) formula is CHO. If sulphuric acid and alcohol be heated together to a temperature of about 160° C., the following decomposition takes place :Alcohol Sulphuric acid EtherWater + Sulphuric acid. 2(C2H2O) + H2SO =2C2H ̧‚0+ H ̧0 + H2SO

6

=

:

The body 2C,H,, O is the liquid known as ether or "sulphuric ether." In other words, at the above temperature only one equivalent of water is separated from two equivalents of alcohol. At a higher temperature every molecule of alcohol is decomposed, giving up water :—

Alcohol Sulphuric acid = Olefiant gas + Water + Sulphuric acid. CH,O + H2SO = CH + H2O + H2SO

2

Besides the CH, there is also found at the same time some CO2 and SO2. Both these bodies being acids may be separated from the olefiant gas by passing the gaseous mixture through hydrate of potassium; any vapour of ether which generally accompanies it having been absorbed by means of strong cold sulphuric acid. The arrangement conveniently adopted to prepare the gas on a small scale is seen in fig. 61. Flask F contains a mixture of alcohol and sulphuric acid made into a paste with sand (to prevent frothing). The gas from F is first led, by means of the tubes B and A, through the "wash-bottle" G, containing strong sulphuric acid, into which the end of the

tube A dips. After giving up any vapour of ether in G, the gases pass by means of the tubes C and D into a solution of hydrate of potassium in the second wash-bottle, E. In E the SO, and CO are kept back, and the purified olefiant gas is finally delivered through tube H. It may be collected over water or mercury.

§ 165. Olefiant gas consists of two volumes of carbon vapour (each weighing 12) united to four volumes of hydrogen (each weighing 1), and condensed to two volumes. The density of the 2×12+4 gas is therefore or 14 (compared with air 0.965). 100 2

volumes of water at 0° C. dissolve about nine volumes of the gas. Olefiant gas may be liquefied by very great pressure. It is colourless, and has a sweet taste.

§ 166. When a light is applied to olefiant gas as it issues into the air, it takes fire and burns with a bright white light, owing to the carbon (which the gas contains in abundance) being heated white-hot by the combustion of the hydrogen (see Chapter XV., Flame). The gas used for illuminating-purposes, and which is obtained by heating coal, wood, oil, &c. in closed vessels (gas-retorts), owes its illuminating power in a great measure to the olefiant gas which it contains. Ordinary coal-gas contains about 4 per cent. by volume of olefiant gas.

§ 167. Olefiant gas unites directly with chlorine, bromine, and iodine, forming bodies of the composition

The first of these, CH,Cl2, is a colourless liquid, insoluble in water, and considered therefore by its discoverers to resemble oil. Hence the name, oil-producing or olefiant gas, which C,H retains. In order that olefiant gas and chlorine may unite, they are led in the moist state into a large globe of glass, which is exposed to the sunshine. The two gases then unite and form the liquid compound, which is purified by washing with water and rectification. Olefiant gas also unites with bromine when the gas is led into the liquid element. The result in this case is also a colourless liquid. When olefiant gas is led over iodine kept at a temperature of 60° C., combination ensues and a white crystalline solid is formed, which is insoluble in water, but soluble in alcohol. Bodies are known having the composition of olefiant gas, united with oxygen and hydrogen; but the gas does not unite directly with these elements.

4

§ 168. Olefiant gas is absorbed by fuming sulphuric acid (a mixture of H2SO, and SO,). This property and the above-mentioned reactions with chlorine serve to distinguish it from most other gaseous hydrocarbons. Olefiant gas is also gradually absorbed by sulphuric acid alone, H.SO. On diluting and distilling the solution, the olefiant gas resumes water and is reconverted into alcohol.

§ 169. Marsh-gas, CH, or "light carburetted hydrogen."

Vegetable matter, when undergoing spontaneous decomposition in the moist state, gives off a portion of its carbon in union with hydrogen in the form of CH,. The bubbles which rise on disturbing stagnant ponds on whose bottom vegetable matter is rotting, consist almost entirely of marsh-gas, whence the name which the gas bears. The great masses of vegetable matter which have been buried in the earth and which form coal, have given up a portion of their carbon as marsh-gas. This gas is often retained in a state of compression in the seams of coal; and the latter, when broken by the miner, give out the gas in large quantity. This gas, as it is formed in the coal-mine, is known by the name of "firedamp." Like the preceding gas, marsh-gas or firedamp burns when mixed with oxygen and heated. Hence it sometimes happens that the flame of the miner's lamp ignites the mixture of atmospheric air and firedamp in the mine, causing disastrous explosions (see Chapter XV., Flame). The products of the combustion are, of course, carbonic acid and water; the former is often produced in such large quantity as to suffocate those who have escaped the first explosion of the firedamp. The carbonic acid so formed is known as chokedamp."

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§ 170. Marsh-gas is prepared in a state of purity by heating acetates with alkalies. Acetic acid, which is the acid of vinegar, is a compound of carbon, oxygen, and hydrogen. It is derived from alcohol by the replacement of a portion of the hydrogen of the latter by oxygen. This is, in fact, the change which takes place when wine, beer, &c. turn sour in the process of vinegarmaking.

The acetic acid, H,C,O,, may have its first equivalent of hydrogen replaced by a metal, and forms thereby an acetate; thus KH,CO is the acetate of potassium. When acetate of potassium is heated with hydrate of potassium, it is decomposed into carbonate of potassium and marsh-gas; thus:

§ 171. Marsh-gas is colourless and tasteless; it results from the union of 1 vol. of carbon vapour weighing 12, with 4 vols. of hydrogen, each weighing 1, condensed to 2 vols. The specific gravity or density of marsh-gas is therefore 8 (compared with air 0.5517). When heated in the air, marsh-gas burns with a bluish flame of feeble luminosity; if previously heated, its light is much more brilliant and becomes yellow. Marsh-gas is one of the chief constituents of coal-gas, which contains about 40 per cent. of it.

§ 172. There is no good test for marsh-gas. It is usually detected and estimated in gaseous mixtures by determining the quantities of carbonic acid and water which are formed when a given volume of the gas is completely burnt.

Compound of Carbon with Nitrogen.

§ 173. Cyanogen, CN.-This body cannot be here closely examined, although its compounds are numerous and important. The two elements, carbon and nitrogen, when united in the above proportion, form a group which behaves very much as a single element, and closely resembles, chemically, chlorine, bromine, and iodine (see Chapter IX.). For this reason a distinct name and symbol is generally given to it.

Cyanogen = CN= Cy.

§ 174. When animal matter, rich in nitrogen, as leather, horn, &c., is heated red-hot with carbonate of potassium and metallic iron, a complicated reaction takes place, resulting in the formation, amongst other products, of a compound of potassium, iron, and cyanogen, which is obtained on treating the cold mass with water and crystallizing. This body has the composition

4KCy, FeCy2 ; 6H2O or K ̧FeCу, 6H ̧0,

2

and is known in commerce as the "yellow prussiate of potash,” or ferrocyanide of potassium. It is manufactured on a large scale, being the source of "prussian blue," and is extensively used in dyeing. The water which the crystallized salt contains may be removed by heat.