GAS

GAS, the name given by Van Helmont, and after him by other chemists, to those elastic fluids extricated from different terrestrial substances, and which are not condensable by cold. Of these a good number have been observed by Van Helmont: such as, the gas ventosum, or atmospheric air; the gas sylvestre, or fluid extricated during fermentations and effervescences, called by later authors, fixed, salutious, and fixable air; gas pingue, or the fluid expelled from inflammable substances by heat; gas flammereum, or the fluid produced in the deflagration of nitre. To these other authors have added inflammable air or gas, nitrous-acid gas, marine gas, alkaline gas, &c.

An account of the most remarkable properties of all these different fluids is given under the articles AIR, CHEMISTRY, DAMPS, &c. Of their composition very little is known with certainty. Dr Priestley some time ago discovered a method of procuring very pure atmospheric air from a mixture of nitrous acid with red lead, or with any dephlogisticated earth, as related under the article AIR, n^o 44. Hence he concluded, that the nitrous acid, and likewise earth, entered into the composition of the air we breathe. The proofs of this, however, from subsequent experiments, seem to be but slightly founded. It is certain, that part of the earth, which in Dr Priestley's first experiments was thought to enter into the composition of his dephlogisticated air, was afterwards found to separate from it, and to have been elevated merely by the force of the air extricating itself from the terrestrial substance. With respect to the acid, the case is still more dubious. It was found that dephlogisticated air might be procured from red-lead and oil of vitriol; to which purpose the following experiment is recorded in the Appendix to the Chemical Dictionary. " Forty-eight pennyweights of red-lead were put into a long-necked retort, the contents of which were ten cubic inches; and upon this red-lead, 24 pennyweights of oil of vitriol were poured. The nose of the retort was then immersed under water, and over it an inverted jar filled with water was placed. The mixture of oil of vitriol and red-lead became very hot, and ten cubic inches of air were soon thrown into the jar without the application of external heat. Upon applying the flame of a lamp to the bottom of the retort, bubbles of air passed copiously into the jars, which were successively changed, that the air received at different times of the operation might be examined. The quantity of air which had been expelled from the above mixture of red-lead and vitriolic acid, was found to be 36 cubic inches, after the proper allowances for the air contained in the retort had been made; and this air was found to have all the properties of that procured by Dr Priestley from nitrous acid and red lead."

From some late experiments made by Dr Priestley him-

Gas. himself, it appears, that very pure air may be obtained by means of the vitriolic acid, or indeed without any acid at all. In the course of his experiments, the doctor found, that dephlogisticated air might be obtained from the green, blue, and white vitriols. Suspecting, however, the purity of these vitriols which were prepared by others, he prepared some green vitriol himself by dissolving clean iron-filings in the vitriolic acid diluted with water. Distilling the matter in a retort, he had the same results as before; the dephlogisticated air which came over last, being very turbid, and exceedingly pure. — He now suspected the purity of his oil of vitriol, which at present is generally procured from sulphur with the addition of nitre. He therefore next employed the vitriolic acid prepared in Neumann's manner, in which no nitre is used: but dephlogisticated air was still produced from the combination of iron filings with this purer acid. And lest the mixture of these two substances might be suspected of having attracted pure air, in consequence of their exposure to the atmosphere during their combination, he conducted the experiment in the following scrupulous manner. He dissolved five pennyweights one grain of iron in a sufficient quantity of pure oil of vitriol which had been carefully prepared for this purpose by Mr Winch, so as to be free from any admixture of the nitrous acid. The distillation was performed in the same retort in which the solution had been made, and in the continuation of the same process; so that all communication with the external air was most effectually prevented. Conducting the process with these attentions, and distilling the matter to dryness, the succeeding products were, first, the common air a little phlogisticated; then a little fixed air, and much vitriolic-acid air; and lastly, a considerable quantity of dephlogisticated air. The residuum still weighed more than the iron filings employed; and had the heat been increased, more air might perhaps have been procured.

Adding fresh oil of vitriol to this residuum, and treating it as before, but in a gun-barrel, a still larger quantity of dephlogisticated air was produced; so that the oil of vitriol appeared capable of generating dephlogisticated air as often as it was mixed with iron, as well as the nitrous acid when mixed with red-lead, &c. in his former experiments.

On putting an ounce of manganese into a small retort, with a very long narrow neck, and exposing it to a red sand-heat, 40 ounce-measures of air were expelled in different portions. Part of this, in every portion, was fixed air, and at first almost wholly so: but four-fifths of the last produce was the purest dephlogisticated air. From an ounce of calaminaris without addition, 316 ounce-measures of gas were expelled by a red heat: the whole of this, however, was fixed air, except about four ounce-measures, which were nearly as good as common air.

In making some experiments on vegetation, the doctor discovered, that dephlogisticated air was in some cases produced naturally. Having observed bubbles of air that seemed to issue spontaneously from the roots of several plants growing in water, he was at first led to suspect that this air had percolated through the plant; which had probably seized upon and retained the phlogiston of the air, and then emitted the purer

part. He found this conjecture verified with regard to the purity of the air; for, on examining some of it, he found that one measure of it, and one of nitrous air, occupied the space only of one measure. — Soon after, he found that the plants had no share in the production of this air; for, on taking them out of the vials, the remaining water continued to emit air as plentifully as when they were growing in it. He observed too, that the vials and other vessels in which this pure air had been emitted from the water had their bottoms and sides more or less covered with a green matter, from which the air evidently seemed to proceed. It appeared to him, however, that this green matter could neither be of an animal nor vegetable nature; but that it was a substance sui generis; and that neither the external air nor animalcules could have any share in the formation of it: for it was produced in vials closely corked, and in the middle vessel of Mr Parker's apparatus*. But from some experiments made by others, it appears that the green matter will not be deposited in vials closely corked, unless some air is included; and the quantity of the deposit bears some proportion to that of the air left in the vial. In open vials completely filled, and inverted in water, the water contained in the vials has an intermediate communication with the atmosphere, and the process goes on as described above: but if that communication is stopped from the beginning, by inverting the vials in quicksilver (a fluid impermeable to air), no green matter or pure air is produced.

On filling a number of vials with different kinds of water, as river-water, rain-water, pump-water, which contained a considerable quantity of fixed air, he found that no green matter was produced in any of them, except in those which contained the pump-water. Afterwards, however, he found that both the green matter and pure air was produced in great plenty from water strongly impregnated with fixed air. — One measure of the purest air he ever obtained in this way, when mixed with two measures of nitrous air, occupied the space only of 0.44 of a measure; "which (says Dr Priestley) is quite as pure as dephlogisticated air is, at a medium."

The most remarkable circumstance in this production of air is the instrumentality of the sun's light independent of his mere heat. Concerning this, the doctor has the following observations:

"Whatever air is naturally contained in water, or in substances dissolved in water, as calcareous matter, &c. becomes, after long standing, but especially when exposed to the sun, depurated, so as at length to become absolutely dephlogisticated; and that this air, being continually emitted by all water exposed to the action of the sun's rays, must contribute to the melioration of the state of the atmosphere in general.

"When water has been long kept in the shade, it has not generally yielded any other kind of air than it would have yielded at first; and though, when kept in an open vessel, the air has been better, it has never been so good as when exposed a much shorter time to the sun.

"No degree of warmth will supply the place of the sun's light: and though, when the water is once prepared by exposure to the sun, warmth will suffice to expel that air; yet, in this case, the air has never

Gas been so pure as that which has been yielded spontaneously, without additional heat. The reason of this may be, that, besides the air already depurated, and on that account ready to quit its union with the water, heat expels, together with it, the air that was phlogisticated, and held in a closer union with the water; which air, the action of light, whatever that is, would in time have depurated also.

Gascoigne. "The quantity of air yielded by water spontaneously, far exceeds that which can be expelled from it by heat. If the water naturally contains fixed air, yet, in consequence of this exposure to the sun's light, it is all dissipated, and the natural residuum of it becomes pure dephlogisticated air. For no fixed air at all, but the purest dephlogisticated air is at length procured from it; and water impregnated with fixed air yields, after this exposure, the greatest quantity of dephlogisticated air." From some experiments made by Dr Dobson of Liverpool, and Mr Becket of Bristol, it appears that air purer than the common atmosphere can be extracted from sea-water, and the water of the hot well at Bristol.

A new species of inflammable gas has been discovered by Dr Ingenhouz. This is procured by putting a single drop of vitriolic ether into an inflammable air-pistol, containing about ten cubic inches: it communicates to the common air contained in the pistol, a strong explosive force. It is very remarkable, that the gravity of this inflammable gas exceeds that extracted from iron, in the proportion of 150 to 25. It is even heavier than common air, in the proportion of 150 to 138: so that, if too great a quantity of it contained in the air-pistol, and the consequent exclusion of too much of the common air, prevent it from taking fire, it will fall out, on holding the pistol a few seconds inverted, with its mouth open; and, in consequence of the entrance of a proper quantity of common air in its room, the explosion will take place.—Another very remarkable circumstance is, that though ether itself is so very volatile, and evaporates so quickly; yet this elastic vapour, generated from it, will remain some hours in an open glass, without such diminution from evaporation, or its mixing with the atmosphere, as to destroy its inflammable quality.

From these experiments we cannot conclude any thing with certainty. They only evince, in one case, the transmutation of the gas sylvestre of Helmont, our fixed air, into atmospheric air. With regard to this last, it seems also to be pretty plain, that, in some cases, the element of fire, in others that of light, enters largely into its composition. But whether these elements are, in such cases, combined with any part of the terrestrial matter, or whether they are only new-modelled by some different arrangement of their parts, must be determined by future experiments.