MANUFACTURE OF COAL-GAS.

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it by cold or pressure. In this condition it is evolved from the leaves of plants under theinfluence of light, and constitutes about one-fifth of the bulk of our atmosphere. By far thelargest amount of oxygen however exists in combination with other elements; thus, eightout of every nine tons of water are pure oxygen, and it forms at least one-third of the totalweight of the mineral crust of our globe. It is therefore the most abundant of all elements.Oxygen gas is heavier than atmospheric air; 100 cubic inches at 00° Fahr. and 30 inchesbarometric pressure weighing 34'193 grains, whilst 100 cubic inches of the latter weigh only31'0117 grains. Since atmospheric air is taken as unity when comparing the density ofgases, the specific gravity of oxygen is 1 1026. It eminently supports combustion, all com-bustible bodies when introduced into it burning much more vividly than in common air;indeed the property of supporting combustion which common air possesses, is owing to thepresence of this gas.

The union of oxygen with other elements is always attended by an elevation of tempera-ture. When the union is very slow, as in the rusting of iron, the heat developed cannot beappreciated; but when combination is rapidly effected, it is attended with the phenomenonof light as well as heat, as in ordinary combustion.

It was formerly thought that the heat developed during combustion was always propor-tionate to the quantity of oxygen consumed ; but recent experiments have not confirmed thisopinion. We are indebted to Berthier for an expeditious process for ascertaining the valueof a combustible founded on this fact, lie mixes the substance to be tested with severaltimes its weight of oxide of lead (litharge), and exposes the mixture in a crucible to a heatsufficient to fuse it. Oxide of lead is composed of oxygen and lead, in the proportion of16 parts by weight of the former to 208 of the latter. At a red-heat the combustible matterseizes upon the oxygen and reduces the lead to a metallic state. This melts, and collects inthe form of a button at the bottom of the crucible. As we have shown that the proportionsin which bodies unite are always uniform, it is evident that for every portion of oxygen con-sumed a certain portion of lead will be reduced, so that the effective combustible matter inthe compound examined will be in due proportion to the weight of the button of lead. Thisprocess is not entirely free from objections, but sufficiently so for many practical purposes.

Oxygen is usually a constituent of Coal-Gas as supplied to the consumer, owing to un-avoidable leakages and the introduction of atmospheric air into the retorts and purifierswhen these vessels are opened. Its presence is highly injurious to the illuminating power ofthe gas, as will be presently noticed ; and as there is no practicable means of removing itfrom the gas, its introduction ought to be guarded against as much as possible.

Hudroyen .—This element, like oxygen, is in the free state a colourless, invisible, andinodorous gas, scarcely soluble in water. It is very rarely met with in nature uncombined;but has lately been detected in the free state in the gases which issue from volcanoes. Incombination it constitutes one-ninth of the total weight of the waters of our globe, and alsoenters largely into the composition of animals and vegetables, and of substances derived fromthem, as oils, peat, coal, and bitumen. Its amount in coal-gas is liable to much variation.

Hydrogen gas is generated in abundance nearly pure when steam is passed over iron, zinc,and several other metals in a fine state of division at a full red-heat. Mixed with carbonic

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