with very desirable accuracy. An ingenious and simple method of doing this has been suggested by Mr. Davy. See Nich. Jour. N. S. iv. 32. Comp. with Thomson's Chemistry, i. 341. second edition. See also EXPANSION.

5. Methods of separating the different gasses from each other.-As experiments often produce two, three, or more species of gass, it is necessary to be able to separate these from each other, that we may ascertain the quantity and species of each. Suppose that under the jar, standing on the shelf of a pneumatic trough, is contained a quantity of different gasses mixed together, and standing over mercury: we begin with marking with slips of paper the height at which the mercury stands within the glass; and then introduce about a cubical inch of water into the jar, which will swim over the surface of the mercury. If the mixture of gass contains any muriatic or sulphuric acid gass, a rapid and considerable absorption will instantly take place, from the strong tendency these two gasses have, especially the former, to combine with or be absorbed by water. If the water produces only a slight absorption of gass, hardly equal to its own bulk, we conclude that the mixture contains neither muriatic acid, sulphuric acid, nor ammoniacal gass; but that it contains carbonic acid gass, of which water absorbs only about its own bulk. To ascertain this conjecture, introduce some solution of caustic alkali, and the carbonic acid will be gradually absorbed in the course of a few hours: it combines with the alkali, and the remaining gass is left almost perfectly free from any sensible residuum of carbonic acid gas. After each experiment of this kind, we must carefully mark the height at which the mercury stands within the jar, by slips of paper pasted on, and varnished over when dry, that they may not be rubbed off when placed in the water apparatus. It is likewise necessary to register the difference between the surface of the mercury in the jar, and the height of the barometer and thermometer.

When all the gass or gasses absorbable by water are absorbed, water is admitted into the jar to displace the mercury; and the mercury in the cistern is covered with water to the depth of an inch or two. After this, the jar is to be transported by the flat dish before-mentioned, into the water apparatus; and the quantity of gas remaining is to be ascertained by changing it into a graduated jar. Small trials of it are then to be made by experiments in little jars, to ascertain nearly the nature of the gass in question. For instance, into a small jar full of the gass, a lighted taper is introduced; if the taper is not immediately -extinguished, we conclude the gass to contain oxygen gass; and in proportion to the brightness of the flame we may judge if it contain less or more of oxygen gass than atmospheric air contains. If, on the contrary, the taper be instantly extinguished, we have strong reason to presume that the residuum is chiefly composed of azotic gass. If, upon the approach of the taper, the gass takes fire and burns quickly at the surface with a white flame, we conclude it to be pure hydrogen gass; if this flame is blue, we judge it consists of carbonated or carburetted hydrogen gass; and if it takes fire with a sudden deflagration, that it is a mixture of oxygen and hydrogen gass. If, again, upon mixing a portion of the residuum with oxygen gass red fumes are produced, we conclude that it contains nitrous gas. These preliminary trials give some general knowledge of the proper VOL. V.

ties of the gass, and the nature of the mixture, but are not sufficient to determine the proportions and quantities of the several gasses of which it is composed. For this purpose all the methods of analysis must be employed; and to direct these properly, it is of great use to have a previous approximation by the above methods. Suppose, for instance, we know that the residuum consists of oxygen and azotic gass mixed together, put a determinate quantity, as 100 parts, into a graduated tube, introduce a solution of sulphuret of potash, and leave it in contact with the gass; in a few days the oxygen will be absorbed, and the azotic gass left pure. A more expeditious method is pointed out in the article EUDIOMETRY. If hydrogen gass be present, a quantity of the gasseous mixture is introduced into Volta's FuDIOMETER (see that word), along with a known proportion of oxygen gass; these are deflagrated together by means of the electrical spark; fresh portions of oxygen gass are successively added till deflagration ceases, and till the greatest possible diminution is produced. By this process water is formed, and is immediately united to the water of the apparatus; but if the hydrogen gass contain carbon, carbonic acid is formed at the same time, which is not absorbed so quickly; the quantity of this is readily ascertained by assisting its absorption by agitation. If the mixture contain nitrous gass, by the addition of oxygen gass, with which it combines into nitric acid, we can very nearly ascertain its quantity from the diminution produced by this union.

These general remarks are made to convey an idea of this kind of investigation, and to shew the leading principles and rationale of the mode of operation: it is not pretended that they will explain every possible case, which would require a work of large and indefinite extent. A long expe rience is necessary to become familiar with the analysis of gasses. In many cases, from the difficulty of overcoming the strong affinities by which several of the gasses are combined with each other, and of determining when the separation is complete, we must vary our experiments in every possible point of view; adding new agents to the combination, and keeping out others; and thus continuing our trials, till we are certain of the truth and exactitude of our conclusion.

To the foregoing observations on the means of separating the gasses, for which we are principally indebted to Lavoisier, we must add, that since the publication of that chemist's valuable elementary treatise, various instruments have been contrived for attaining more readily and completely the object of which we are treating, and for receiving and retaining the gasses produced. The earliest of these, which indeed was known to Lavoisier, though he lived not to obtain a perfect set, was the celebrated apparatus of Woulfe, a description of which may be seen under his name. An improved apparatus for preserving separate the gasseous products evolved in many processes was invented by Mr. Pepys, jun. and is represented in Pl. 78. Fig. 1. A, the retort joined to a tubulated receiver B. C. the adapter ground into the neck of the receiver, and furnished with a glass valve made in the same manner as those used in the improved Nooth's apparatus. D. Woulfe's joined to the adapter to receive the unabsorbed gass; and E, a bent tube to carry the gass that may still pass unabsorbed either into a pneumatic apparatus, or into a chimney.

P