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III. OXYGEN.

This substance exists in the state of an aëriform fluid, at every known temperature, and under every degree of pressure to which it has been yet subjected. It is transparent, colourless, and without taste or smell. It is heavier than air in the proportion of 1102.6 to 1000; and hence, while 100 cubic inches of air are held to weigh 31.0117 grains, at the temperature of 60° F. or 154 C. and under a barometrical pressure of 30 inches, 100 cubic inches of oxygen gas, under the same conditions, weigh 34.19349942. It combines with all the other bodies regarded as simple, with the exception of fluorine, with which it has not yet been made to combine. Its combination with other bodies either takes place slowly, or rapidly, and with the sensible evolution of heat and light; and bodies which burn in common air, burn, when ignited in oxygen gas, with increased splendour. It is necessary to the respiration of animals, and exists in the air of the atmosphere in a proportion suited to the wants of living creatures, and is dissolved by water in the quantity necessary to support the respiration of the innumerable creatures that inhabit the waters of the globe. It may be breathed in the unmixed state, but its action too greatly excites the system, and death after a time ensues. If a considerable animal, as a rabbit or a dog, is placed in this air, no effect is at first observed; but after a time, as an hour, the circulation and breathing become rapid, exhaustion at length ensues, then insensibility, and in 12 hours or less, death; and, on examining the dead creature, the blood is seen to be of a bright red in every part of the body, the heart frequently beating after breathing has ceased; and on the other hand, no animal is found to live in an atmosphere deprived of this element.

Oxygen, in combining with the other bodies termed simple, forms two classes of compounds, the first termed oxides, the second acids; and sulphur observes the same laws of combination.

No experiments, direct or indirect, have yet shewn, that oxygen has ever been decomposed, or formed by the union of other elements. In its actions and modes of combination, it exhibits a parallelism with sulphur, chlorine, and even cyanogen. The only known bodies whose atomic weight will allow us to resolve them into oxygen, are hydrogen and carbon. We may suppose that a molecule of each combines and forms a molecule of the root H C, and that this root, combining with an equivalent of hydrogen, forms H2 C, which we may employ as an expression for oxygen. A further molecule of carbon, on the assumptions made in the table, constitutes nitrogen H2 C2, so that a molecule of nitrogen, on this supposition, differs from one of oxygen by a molecule of carbon.

If we suppose nitrogen to be a compound body, we must, by a parity of reasoning, suppose oxygen to be so; for there is no such difference in the physical or chemical characters of the two bodies, as can allow us to assume that the one is derivative and the other simple.

IV. NITROGEN.

This substance, constituting about of the volume of the 충 atmosphere, entering largely into the composition of organic bodies, and forming a part of innumerable substances existing in the mineral kingdom, is an element everywhere diffused. It exists at all common temperatures as an aëriform fluid, without colour, taste, or odour, and has not yet been condensed into the liquid state, under any degree of cold and pressure. Its specific gravity is .9722, or according to some .976. It is eminently irrespirable, and no animal can live in it beyond the briefest period. It was thence termed azote, from a privative, and gwn life, in contradistinction to the respirable portion of the atmosphere, then termed vital air. It does not support combustion, and all burning bodies immersed in it are in an instant extinguished. It is not itself inflammable, but it combines with oxygen under conditions favourable to the union, that is in the nascent state, for, when it has passed into gas, it is very difficult to effect the combination. It manifests no acid reaction, and so does not affect the colour of plants, nor combine with the alkalies.

On comparing this substance with oxide of carbon, we find a similarity of characters sufficient to justify the opinion that they are of a similar nature. This latter body exists likewise at all known temperatures, as an aëriform fluid, without colour, taste, or odour, and has not been condensed into the liquid state, under any degree of cold and pressure. Its specific gravity is the same as that of nitrogen, namely, .9722. It is wholly irrespirable, producing, when breathed, almost instant insensibility. It does not support combustion, but, when ignited, burns with a faint blue flame. It thus

differs from nitrogen, in its facility of combining with oxygen, but this is a difference common to other bodies, as the metals in their different states. All the other characters of the two bodies are extremely alike. They are both eminently neutral with respect to other substances. They both combine with chlorine, although this is regarded an anomaly in the case of an oxide, yet it is no anomaly if nitrogen be likewise an oxide of carbon.

An opinion has long been entertained by chemists, that nitrogen is a compound body, and both Berzelius and Davy have eagerly supported this opinion. But as nitrogen has resisted the attempts to decompose it, chemists have been contented to acquiesce in their own dictum, that we must regard it as simple, because we have not been able to prove it compound. The juster argument would be, that we must regard it as compound, although we have not been able to prove it so by the agents we have employed.

But although nitrogen may not have been directly decomposed, there is reason to believe that in numerous unheeded experiments of the laboratory, its compound nature has been again and again shewn. When bitartrate of potassa is distilled, carbonate of ammonia is produced, the nitrogen of the ammonia being beyond a question derived from the carbon and oxygen of the tartrate.

Some years ago I commenced a series of experiments for the purpose of proving directly the composition of nitrogen. But being interrupted in these experiments, and not having since had an opportunity of verifying the results, I shall not here found upon them at all. I am contented for the present to shew by the arguments which induction supplies, that nitrogen is a compound body, and composed of the same elements as the oxide of carbon.

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But it does not follow that nitrogen is the same body as oxide of carbon. It is manifest that the elements of these

bodies are related to one another by a different degree of affinity, for while the elements of the oxide may be separated by many bodies, those of the other remain fixed under the most powerful agents of decomposition. Further, the two bodies are in a different relation to heat, for the specific heat of nitrogen is more than that of carbonic oxide. We are not, therefore, to suppose that these two bodies must present the same analogies in their own actions, or in the actions of their compounds, which we assume them to possess in their composition. Thus, we are not to infer that carbonic acid, CO + O, must be similar in its characters to nitrous oxide N+0. In carbonic acid the elements of the root are loosely combined, if we may so speak, and the second equivalent of oxygen must have a relation to the root different from that of the equivalent of oxygen to the root in nitrous oxide. While I maintain, with all the confidence that I can place in inductive reasoning, that nitrogen is isomeric with carbonic oxide, in the number and ratio of its elements, I do not maintain that nitrogen is, in all respects, the same body as carbonic oxide.

Hydrogen, or the molecules of hydrogen, it has been seen, may be supposed to combine with carbon, and form oxygen, and oxygen to combine with carbon, or the molecules of carbon, and form nitrogen. But when we arrive at nitrogen in the descending order, there are two groups of bodies almost equally related to it, namely sulphur, phosphorus, and the bodies allied to them, and chlorine and its analogues. It will be convenient to consider, in the first place, the latter class of bodies, which have many characters in common with the sulphur group, and may be regarded as proceeding from a common root. These bodies are fluorine, chlorine, bromine, and iodine.

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