, an instrument for determining the purity of the air, or the quantity of pure and dephlogisticated or vital air contained in it, chiefly by means of its diminution on a mixture with nitrous air.

Instruments of this kind have been but lately made, and that in consequence of the experiments and discoveries of Dr. Priestley, for determining the salubrity of different kinds of air. That writer having discovered, that when nitrous air is mixed with any other air, their original bulk is diminished; and that the diminution is nearly, if not exactly, in proportion to its salubrity; he was hence led to adopt nitrous air as a true test of the purity of respirable air; and nothing more seemed to be necessary but an easy, expeditious, and accurate method of estimating the degree of diminution in different cases; and for this purpose, the Eudiometer was contrived; of which several kinds have been invented, the principal of which are the following.

I. The Eudiometer originally used by Dr. Priestley is a divided glass tube, into which, after having filled it with common water, and inverted it into the same, one measure or more of common air, and an equal quantity of nitrous air, are introduced by means of a small phial, which is called the measure; and thus the diminution of the volume of the mixture, which is seen at once by means of the graduations of the tube, instantly discovers the purity of the air required.

II. The discovery of Dr. Priestley was announced to the public in the year 1772; and several persons, both at home and abroad, presently availed themselves of it, by framing other more accurate instruments. The sirst of these was contrived by M. Landriani; an account of which is published in the 6th volume of Rosier's Journal, for the year 1775. It consists of a glass tube, fitted by grinding to a cylindrical vessel, to which are joined two glass cocks and a small bason; the whole being fitted to a wooden frame. In this instrument quicksilver is used instead of water; though that is attended with an inconvenience, because the nitrous air acts upon the metal, and renders the experiment ambiguous.

III. In 1777, Mr. Magellan published an account of three Eudiometers invented by himself, consisting of glass vessels of rather difficult construction, and trouble some use. Mr. Cavallo observes, that the construction of all the three is founded on a supposition, that the mixture of nitrous and atmospherical air, having continued for some time to diminish, afterwards increases again; which it seems is a mistake: neither do they give accurate or uniform results in any two experiments made with nitrous and common air of precisely the same quality.

IV. A preferable method of discovering the purity of the air by means of an Eudiometer, is recommended by M. Fontana, of very great accuracy. The instrument is originally nothing more than a divided glass tube, though the inventor afterwards added to it a complicated apparatus, perhaps of little or no use. The first simple Eudiometer consisted only of a glass tube, uniformly cylindrical, about 18 inches long, and 3-4ths of an inch diameter within side, the outside being marked with a diamond at such distances as are exactly filled by equal measures of elastic fluids: and when any parts of these divisions are required, the edge of a ruler, divided into inches and smaller parts, is held against the tube, so as that the first division of the ruler may coincide with one of the marks on the tube. The nitrous and atmospherical air are introduced into this tube, in order to be diminished, and thence the purity of the atmospheric air ascertained.

V. M. Saussure of Geneva has also invented an Eudiometer, which he thinks is more exact than any of those before described; the apparatus of which is as follows: 1. A cylindrical glass bottle, with a ground stopple, containing about 5 1/2 ounces, which serves as a receiver for mixing the two airs.—2. A small glass phial, to serve as a measure, and is about one-third the size of the receiver.—3. A small pair of scales that may weigh very exactly.—4. Several glass bottles, for containing the nitrous or other air to be used, and which may supply the place of the recipient when broken. The method of using it is as follows: The receiver is to be filled with water, closed exactly with its glass stopper, wiped dry on the outside, and then weighed very nicely. Being then immerged in a vessel of water, and held with the mouth downwards, the stopple is removed, and, by means of a funnel, two measures of common and one of nitrous air are introduced into it, one after another: these diminish as soon as they come into contact; in consequence of which the water enters the recipient in proportionable quantity. After being stopped and well shaken, to promote the diminution, the receiver is to be opened again under water; then stopped and shaken again, and so on for three times successively, after which the bottle is stopped for the last time under water, then taken out, wiped very clean and dry, and exactly weighed as before. It is plain that now, the bottle being filled partly with elastic fluid and partly with water, it must be lighter than when quite full of water; and the difference between those two weights, shows nearly what quantity of water would fill the space occupied by the diminished elastic fluid. Now, in making experiments with airs of different degrees of purity, the said difference will be greater when the diminution is less, or when the air is less pure, and vice versa; by which means the comparative purity between two different kinds of air is determined.

VI. But as this method, notwithstanding the encomiums bestowed on it by the inventor, is subject to several errors and inconveniences; to remedy all these, another instrument was invented by Mr. Cavallo; the descrip- | tion of which, being long, may be seen in his Treatise on the Nature and Properties of Air, pa. 344.

Other constructions of the Eudiometer have also been given by Mr. Cavendish and Mr. Scheele. For farther information, see Magellan's Letter to Dr. Priestley, containing the Description of a Glass Apparatus, &c, and of New Eudiometers &c, 1777, pa. 15 &c; Priestley's Exp. and Obs. on Air, vol. 3, preface and appendix; the methods of Dr. Ingenhousz in Philos. Trans. vol. 66, art. 15; see also the Philos. Trans. vol. 73; and Cavallo's Treatise on Air, pa. 274, 315, 316, 317, 328, 340, 344, and 834.

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Entry taken from A Mathematical and Philosophical Dictionary, by Charles Hutton, 1796.

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EULER (Leonard)