"that 10-27ths of the diminution over water are oxygen, and "17-27ths nitrous gas. It is proper, as soon as the greater part of the diminution has ensued, to transfer the mixture through water into a graduated vessel, without using any "agitation. 66 "If pure nitrous gas be admitted to pure oxygen gas in a "narrow eudiometer tube, so that the oxygen gas is upper"most, the two unite very nearly in the same uniform pro"portion as above. If, on the other hand, the nitrous be "the upper gas, a much less quantity of it disappears, viz. "1.24 nitrous to one oxygen. If undiluted nitrous gas be "admitted to pure oxygen gas, in a wide vessel over water, "the whole effect takes place immediately; and one measure "of oxygen will condense 3.4 nitrous gas. 66 "To render this rule more intelligible, an example may "be necessary. Let 100 measures of common air be admitted to 100 measures of a mixture of nitrous gas, with an equal "volume of azotic or hydrogen gas. After standing a few "minutes in the eudiometer, there will be found 144 measures. "The loss 56 being divided by the common divisor, 2.7, gives "21 nearly for the oxygen gas present in 100 measures of "common air." To these directions I may add, that when atmospherical air is the subject of experiment, it is scarcely necessary to dilute the nitrous gas, with any other gas, previously to its use. If a number of experiments be made, it will be proper, in all cases, to let the gases remain together the same definite time (say 10 minutes) before noting the diminution; and it is needless to transfer them into another vessel. If the mixed gas, under examination, contain much more oxygen than is present in atmospherical air, then it is proper to dilute the nitrous gas with an equal bulk of nitrogen or hydrogen gas; and, in this case, the narrower the tube in which the experiment is made, the more accurate will be the result. Subsequent experience has convinced me that the method, proposed by Mr. Dalton, though sufficiently correct when applied to a mixture of the same, or nearly the same, standard as the atmosphere, cannot be relied on, when the proportion of oxygen is either considerably greater or less. In the former case, the process gives too great a diminution, sometimes indeed to such an extent as to indicate more oxygen gas, than the whole amount of what was submitted to experiment. When the air, on which we are operating, is of an inferior standard to the atmosphere, we do not learn its full proportion of oxygen. Notwithstanding these objections, however, the method has considerable value, since it may be applied to determine the proportion of oxygen in some mixed gases, to which other eudiometrical tests are not applicable; for example, to mixtures of hydro-carburet and oxygen gases. The application of nitrous gas to eudiometrical purposes, it has been observed by Gay Lussac, is susceptible of perfect accuracy, provided certain precautions be observed which he has pointed out, and which were suggested by his theoretical views of the constitution of these gases. A narrow tube he finds to be unfit for an eudiometer, his object being to form nitrous acid gas, which is but slowly absorbed by water. Instead therefore of a narrow tube, we must take a wide vessel, such as a small tumbler glass; and to 100 parts of atmospheric air, previously measured, we must add at once 100 measures of nitrous gas. A red fume will appear, which will soon be absorbed without agitation, and in half a minute, or a minute at most, the absorption will be complete. Pass the residuum into a graduated tube, and it will be found, almost invariably, that 84 measures have disappeared. Dividing this number by 4, we have 21 for the quantity of oxygen condensed. By a series of experiments on mixtures of oxygen and nitrogen gases in various proportions, Gay Lussac found that this eudiometrical process may be depended upon, whether the oxygen exceed or fall short considerably of the proportion contained in atmospheric air. (g) The generation of an acid, by the admixture of nitrous gas with common air or oxygen gas, may be shown by the following experiment. Paste a slip of litmus-paper within a glass jar, near the bottom; and into the jar, filled with and Inverted in water, admit as much nitrous gas, previously well l, as will displace the water below the level of the paper. The colour of the litmus will remain unchanged; but, on adding atmospheric air or oxygen gas, it will be immediately reddened. (h) The acid, thus produced, is either nitrous or hypo-nitrous, according to the circumstances of the experiment, the presence of water favouring the production of the latter, and its absence promoting that of nitrous acid.* The nature of the product may be shown, in a general way, as follows: Into a jar, filled with and inverted in mercury, pass a small quantity of a solution of pure potash; and, afterward, measures of oxygen and nitrous gases, separately, and in proper proportion. On removing the solution from the jar, exposing it for some time to the atmosphere, and afterward evaporating it, crystals of nitrate of potash will be formed, a salt which is ascertained to be formed of potash and nitric acid. () Nitrous gas is decomposed by exposure to almost all bodies that attract oxygen. Thus, iron filings decompose it, and become oxydized, affording a proof of the presence of oxygen in this gas. During this process, water, ammonia, and nitrous oxide, in the proportion of one volume from two of nitrous gas, are generated. Sulphuret of potash, &c., have a similar effect. Sulphuret of barytes gives one half its volume of nitrogen. Mixed with sulphurous acid, nitrous gas is decomposed, and this acid is changed into the sulphuric, but not unless water is present. Nitrous gas does not, with hydrogen gas, afford a mixture that can be exploded by the electric spark; but with ammoniacal gas it may be fired in a Volta's eudiometer over mercury. The oxygen of the nitrous gas unites with the hydrogen of the ammonia, and the nitrogen of both gases is set at liberty. Bodies, that have a still more powerful affinity for oxygen, decompose nitrous gas into its ultimate elements. Charcoal ignited in 100 measures, gives 50 measures of nitrogen gas, and 50 of carbonic acid. Arsenic, zinc, or potassium, when heated in it, evolve half its volume of nitrogen. Nitrous gas should consist, therefore, of 1 volume of oxygen + 1 volume • Dalton, in Thomson's Annals, x. 39. of nitrogen, neither of which elements is in a state of condensation. We may therefore consider nitrous gas as constituted of one atom of nitrogen = 14, and two-atoms of oxygen = 16, and its representative number will be 30. Its composition then is No distinct information is obtained respecting the constitution of nitrous gas by the long continued action of electricity. One half the azote, according to Mr. Dalton, is liberated, and the remainder unites with the evolved oxygen and composes nitrous acid (System, p. 334.) (k) Nitrous gas and chlorine, when both perfectly dry, have no action whatsoever on each other; but, if water be present, there is an immediate decomposition, its hydrogen combining with the chlorine to form muriatic acid, and its oxygen with the nitrous gas to form nitrous acid. (1) Nitrous gas is absorbed by the green sulphate and muriate of iron, which do not absorb nitrogen gas. To ascertain, therefore, how much nitrogen gas a given quantity of nitrous gas contains, let it be agitated in a graduated tube with one of these solutions. This analysis is necessary, previously to deducing, from its effects on atmospheric air, the proportion of oxygen gas; for we must subtract from the residuum the quantity of nitrogen introduced by the nitrous gas. From the important use which is now made in eudiometry of this solution of nitrous gas in sulphate of iron, it may be proper to describe the mode of its preparation. Dissolve as much of the green sulphate of iron in water as the water will take up, or dissolve iron filings in sulphuric acid, diluted with five or six parts of water, leaving an excess of the iron, in order to ensure the perfect saturation of the acid. Fill a wide-mouthed bottle with this solution, invert it * For an account of these salts, see chap. ix. sect. 16. in a cupful of the same, and into the inverted bottle receive the nitrous gas, as it is generated by the proper materials, shaking the inverted bottle frequently. The colour of this solution will be changed to black, and the production of gas and the agitation are to be continued, till the absorption can be carried no farther. The impregnated solution should be preserved in a number of small bottles, not holding more than an ounce or two each. The most commodious method of applying this solution, is by means of Dr. Hope's eudiometer, already described. Of Hypo-nitrous Acid. When 400 measures of nitrous gas and 100 measures of oxygen (in which, taken together, the nitrogen and oxygen are to each other by measure as 100 to 150) are mixed togcther over a solution of potash confined by mercury, we obtain 100 measures of a compound, called by Gay Lussac pernitrous acid.* Mr. Dalton, who obtained it several years ago, and then considered it as nitrous acid, proposed to call it sub-nitrous acid; but the name hypo-nitrous seems to me more conformable to the analogy of similar acids having sulphur and phosphorus for their basis. This new compound is so far hypothetical, that it has never yet been exhibited in a separate form; for when a stronger acid is added, to expel it from the potash, it is resolved into nitrous gas and nitrous acid. Hypo-nitrous acid is, also, frequently generated, when nitrous and oxygen gases, or nitrous gas and common air, are mingled together in cudiometrical processes. At the same time nitrous and nitric acids are produced, in proportions to the hypo-nitrous and to each other, which are modified by the circumstances of the experiment.‡ Calculating from the proportions of its elements and their state of condensation, 100 cubic inches of hypo-nitrous acid * Ann. de Chim. et Phys. i. 400. Dalton, Thomson's Annals, x. 83. + Thomson's Annals, vol. ix. |