EUDIOMETRY. The measurement of the quantity of oxygen contained in atmospheric air, or, indeed, in any as in which it is not intimately com bined, is named eudiometry, and the instrument by which it is performed is named the eudiometer. There are two modes of effecting this ; either by pre senting to the oxygen any substance having a strong affinity for oxygen, or by exploding it with hydrogen in a strong glass vessel by means of the electric spark, and in either case estimating the quantity by the decrease in the bulk of the gas. In the first method, the substances usually employed to absorb the oxygen, are either phosphorus, sulphuret of potash, or nitrous gas, which latter substance, when used according to the directions of M. Gay Lussac, seems preferable to any other ; these directions are as follows :—Take a very wide tube or tumbler, for example, invert it in water, and having introduced into it 100 parts of the air to be examined, pass into it 100 parts of nitrous gas. There is instantly exhibited a red vapour, which is nitrous acid gas, and which being very soluble in water, disappears speedily without agitation, and after a minute at most the absorption is complete ; then transfer the residuum into a graduated tube, and it will be found that the absorption is almost uniformly 84 parts, provided atmospheric air was used ; and as nitrous acid (the resulting compound) consists of three volumes of nitrous gas and one volume of oxygen, one-fourth of the absorption, equal 21 parts, indicates the quantity per cent. of oxygen. M. Gay Lussac shows by numerous experi ments, the accuracy of the above process in varied circumstances. There is this great advantage attending it ; that the proportion of oxygen gas being estimated by an absorption four times greater than its own volume, the errors of experiment are reduced to one-fourth. The analysis of combustible gases, and the supporters of combustion, reciprocally by explosion with the electric spark, furnishes, when it can be applied, one of the most elegant and speedy methods of chemical research ; but is attended with some danger, from the liability of the tube to burst, if a close tube is used, or with a risk of failure from the ejection of the mercury when the tube is merely sealed by that fluid. This has given rise to several modifications of the apparatus, most of which are somewhat complex and costly; but that invented by Dr. Ure, and communicated
by him to the Royal Society of Edinburgh, is at once simple, cheap, and effective. It consists of a glass syphon, having an interior diameter of from two-tenths to four-tenths of an inch. Its legs are nearly of an equal length, each being from six to nine inches long. One end is open and slightly funnelled ; the other end is hermetically sealed, and has inserted near it by the blow-pipe two platina wires. The outer end of one wire is incurvated across, so as nearly to touch the open end of the tube ; the outer end of the other wire is formed into a small hook, to allow a little spherical button to be attached to it when the electric spark is to be transmitted. To use it, the whole syphon must be filled with water or mercury, then plunge the open leg into a pneumatic trough, and introduce into it any convenient quantity of the gases from a glass measure tube, containing them previously mixed in determinate proportions. Then, applying a finger to the orifice of the syphon, remove it from the trough, and transfer the gases into the sealed leg, by holding the syphon leg uppermost. Then bring the mercury to a level in both tubes, by adding or displacing a portion, and note carefully the volume of gas in the sealed leg, which should be graduated to one hundredth parts of a cubic inch. Then applying again the fore finger to the orifice, so as also to touch the end of the platina wire, bring the pendant ball or button to the electric machine, and transmit the spark. After the explosion, on gradually sliding the finger to one side, and admitting the air, the mercury will rise in the sealed leg more or less above that in the other ; then pour in mercury till the equilibrium be restored, and read off, as before, the bulk of the remaining gas, and the difference of the two volumes will denote the true quantity of oxygen, without requiring any reduction or allowances. So perfectly is the shock of the explosion deadened by the elasticity of the air confined between the finger and the surface of the mercury in the open end of the tube, that nothing but a slight push or pressure at the tip of the finger is felt, even when the included gas is in considerable quantity and of a highly explosive nature; and the projection of the mercury or displacement of the gas is obviously impossible.