ANALYSIS OF GASES. The analysis of gases differs from that of solids and liquids in that it is more easy to measure than to weigh gases, and hence the results are usually given in per centages by volume. For many gases reagents are known which absorb the gas readily and com pletely. Thus, a mixture of carbon dioxide. ethy lene, oxygen, carbon monoxide, and nitrogen may be analyzed by bringing a measured volume into contact with caustic potash (which absorbs the carbon dioxide), then with fuming sulphuric acid (which absorbs the ethylene), them with an al kaline solution of pyrogallol (which absorbs the oxygen), then with a solution of cuprous chloride (which absorbs the carbon monoxide), and noting the contraction caused by each treat ment. The nitrogen remains behind unabsorbed. Hydrogen and marsh-gas are usually determined by comhustion with oxygen. Gases very soluble in water, such as sulphur dioxide, are absorbed in that liquid, and then the amount dissolved is determined by a volumetric process. Carbon dioxide in air offers a special case. As in nor
mal air only 3 parts in 10.000 are present, the ordinary process of measuring the volume before and after treatment with caustic potash requires special apparatus and great care to get good re sults. Usually a large volume is treated with a measured quantity of a solution of barium hy droxide of known strength, a portion of the barium hydroxide being thus converted into in soluble barium carbonate, and the rest estimated volumetrically.
When the highest degree of accuracy in gas analysis is required, the gases must be confined over mercury; further, only solid absorbents must be used, and careful corrections must be made for changes of pressure and temperature. When water is used to confine the gas, some in accuracy is introduced, since all gases are more or less soluble in water. in technical work, how ever, a very high degree of accuracy is hut rarely required. The technical analysis of gases has assumed great practical importance, owing to the extension of the use of gaseous fuels.