This is the lightest of the gases, and in deed the lightest substance whose gravity can be ascertained by weighing. Its spe cific gravity varies considerably, accord ing to its state with regard to humidity. When it has been transmitted through water, or has remained for some time ex posed to it, it is about ten times lighter than atmospheric air ; when it has been received over quicksilver, and exposed to any substance which attracts water strong ly, as quicklime, it is nearly 13 times lighter, or atmospheric air being 1,000, hydrogen is 84. Its specific gravity in this state, water being 1000, is stated by Lavoisier at 0.0946.100 cubic inches weigh 2.613 grains. It is from this levity, that it was applied with success to the con struction of balloons ; a varnished silk or linen bag, filled with it, having a specific gravity so much less than atmospheric air, as not only to rise in the atmosphere, but also to elevate an additional weight.
The chemical property, by which hy drogen gas is most eminently distinguish ed, is its great inflammability. When an ignited body is approached to it, in con tact with the atmosphere, it is immediate ly kindled, and continues to burn while the air is admitted ; if previously mixed with atmospheric air, and a burning body approached to the mixture, or an electric spark sent through it, it instantly inflames with detonation ; and when it has been mixed with oxygen gas, the detonation is more violent. When burning at the ex tremity of a capillary tube, on bringing a wide tube over the flame, a singular phenomenon, accidentally observed by Dr. Higgins, is produced, that of sounds of various tones, which vary ion acuteness and strength, according to the width, the length of the tube, and the kind of sub stance of which it is formed, owing, ap parently, as Picket and De la Rive have explained it, to the vibrations excited in the matter of the tube by the rapid ex pansion and condensation of the watery vapour near and around the flame, and which, regulated and equalized by regu lar reflections from the sides of the tube, constitute a musical sound. (Nicholson's Journal, 8vo. vol. i. p. 129 ; ibid. vol. iv. p. 23).
Though hydrogen gas b e inflammable,it is incapable of supporting the combustion of other inflammables. If a burning body be quickly immersed in it, it is imme diately extinguished.
This gas is incapable of supporting animal life by respiration ; an animal im mersed in it is soon killed. At the same time, it does not appear to be so positive ly deleterious as the other noxious gases. Scheele long ago observed, that he was able to breathe it for twenty inspirations. (Treatise on Air and Fire, p.160.) Fon tana showed, what Scheele indeed had ob served, that if the lungs were previously emptied as much as possible of atmo spheric air, by a forcible expiration, it cannot be breathed so long, though still it did not appear to him to be positively deleterious, like some of the unrespirable gases, (Opuscules Physiques, p. 2.) Ro sier, even after expelling the air from the lungs, breathed hydrogen gas for several respirations; and Mr. Davy, in his experi ments on the respiration of the gases, re marked, that in one experiment, after a complete exhaustion of the lungs, he found great difficulty in breathing hydro gen for half a minute, though in a subse quent experiment, with the same prepara tion, he breathed it for near a minute.
The first six or seven inspirations pro duced no sensations whatever ; in half a minute, a sense of oppression was felt at the breast, which increased until the pain of suffocation interrupted the experiment. (Chemical Researches, p. 400. 466.) Hy drogen, therefore, is incapable of support ing life ; the respiration of it can be con tinued only for a short time, and animals confined in it soon die. It appears only to prove fatal, not by a positively noxious quality, but by excluding atmospheric air, the due supply of which, by respiration, is indispensable to life. Blood exposed to it acquires a deep black colour, and the gas suffers a diminution of volume Hydrogen is not, as several of the other gases are, noxious to vegetable life ; at the same time it appears to contribute little to the nourishment of plants, Dr. Priestley having found, that it still con tinued inflammable after a growing vege table had been confined in it for several months. It can apparently supply, to a certain extent, the place of light, in sup porting vegetation. Von Humboldt ob served, that some cryptogamic plants in mines, and of course secluded from light, were not pale, but of a green colour, such as they would have had from growing under exposure to the light of day; and he concluded, with sufficient probability, i that the agency of light had, in this case, been supplied by the hydrogen gas, which is evolved in greater or less abun dance in such situations.
Hydrogen gas is so sparingly soluble in water, that, when agitated with it, it suf fers no perceptible diminution of volume. When the water has been previously freed from atmospheric air, Mr. Henry found, that one hundred cubic inches take up 1.5 of the gas under a common atmosphe ric pressure ; under increased pressure, a larger quantity, equal to one-third of the volume of the water, is absorbed.
The affinities of hydrogen seem princi pally exerted towards inflammable bodies. It unites with sulphur, phosphorus, and carbon, and forms gaseous compounds ; it appears to be capable of dissolving even some of the metals, particularly, iron, zinc, and arsenic. United with nitrogen, it forms one of the alkalies, ammonia : with oxygen, water. It is also a constituent principle of the greater number of the ve getable and animal products.
Hydrogen gas may be regarded as a product of some natural operations. It is found collected often in mines, derived probably from the decomposition of water by metals ; it is known to the miners by the name of fire-damp, and is often the cause of accidents, from exploding on the approach of an ignited body. It is also extricated from stagnant water, and from marshy situations, from the slow decom position of vegetable and animal sub stances, holding, dissolved in it, carbon, and perhaps also phosphorus and nitro gen, and forming, as has been supposed with some probability, gases, which ren der the air of such places unhealthy. From its levity, it has been supposed, that the quantity of it thus produced at the surface of the earth will rise through the atmo sphere, and occupy the higher regions; and on its presence some of the pheno mena of meteorology, particularly the sudden appearance of some fiery meteors, have been supposed to depend. Its af finities have not been ascertained with any precision, as to their relative force.