EUDIO'METER, an instrument invented by Dr. Priestley, and originally employed by him in ascertaining the goodness of atmo spheric air obtained from various places and , under different circumstances. The use of the eudiometer, termed audiometry, has, since its original contrivance, been extended to all gaseous mixtures, but especially to determin ing the quantity of oxygen which they contain when resulting from the operations of ana lysis.
The principle upon which the use of the eudiometer depends, so far as atmospheric air and oxygen gas are concerned, is that of ex posing them to the action of some substance, whether solid, fluid, or gaseous, which, on account of its affinity for oxygen, combines with it and leaves the gas with which it is mixed unacted upon.
The eudiometer invented by Dr. Priestley was extremely simple. He filled a phial with water, and displaced the water with the gase ous mixture to be examined ; the volume of this being noted, it was transferred into an air-jar. An equal volume of nitric oxide was added to it, and they remained together a few minutes. When this part of the process was over, the gas was transferred to a graduated glass tube. After noting the volume of the gas, the result was expressed in measures and decimal parts ; thus, when equal volumes of common air and nitric oxide were mixed, and they afterwards occupied the space of one volume and two-tenths, Dr. Priestley, iu speaking of the air so tried, said the measures of the test were 1.2, or the standard of the air was 1.2.
Numerous attempts have been made to render the eudiometrical application exact and certain by Cavendish, Fontana, Ingenhouz, Sardiniani, Dalton, Gay Lussac, Henry, Thomson, Davy, and others. The eudiometer of Scheele was a graduated glass tube contain ing a certain volume of air, which was exposed to a mixture of sulphur and iron-filings made into a paste with water. De Marta, instead of using sulphur and iron, employed a solu tion of sulphuret of potassium prepared by dissolving sulphur in a solution of potash. Guyton employed sulphuret of potassium also in his eudiometer, but he used it in a solid state, and applied heat to expedite its action.
The eudiometer of Seguin is a glass tube, filled with and inverted in mercury ; a small piece of phosphorus is put under the open end of the tube, and by its lightness it im mediately rises to the top of it, where it is melted by the approach of red-hot iron. A measured portion of the gas to be examined is then passed into the tube ; the phosphorus inflames on each addition of the gas, and the mercury rises, owing to the condensation of the oxygen. The quantity of the residual gas is determined by transferring it into a gra duated tube, and the difference between the quantity submitted to experiment and that left after it indicates that of the oxygen ab sorbed. Berthollet also employed phosphorus in his eudiometer, but instead of heating it, as in the above-described method, he allowed combination to take place between it and the oxygen by slow combustion. Dr. Hope, Dr.
Henry, and Mr. Pepys, employed a eudio meter, in which the test liquid was either a solution of iron impregnated with nitric oxide, or a solution of sulphuret of potassium. Volta's method of determining the quantity of oxygen contained in gaseous mixtures is by means of combustion with a known volume of hydrogen gas ; for it having been ascer tained that when a mixture of oxygen and hydrogen gas is fired, one-third of the dimi nution is owing to the condensation of oxygen, we have only to observe the measure of the contraction of volume to ascertain that of the oxygen which was present. Various modes of effecting this have been devised by Volta, Mitscherlich, Dr. Ure, and others.
Dohereiner has suggested a eudiometrical process, founded on his curious discovery of the property which spongy platinum possesses of causing the combination of oxygen and hydrogen gases. In this eudiometer the combination occurs without explosion, and yields results of great accuracy. Dobereiner found that when the spongy platinum was mixed with certain substances, so as to pre vent its immediate and explosive action, it caused the oxygen and hydrogen to combine with moderate rapidity. Dr. Henry and Dr. Turner employed modifications of this process.