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Air experiment

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An attempt to analyse the Air by a great variety of chymio-statical experiments.

Stephen Hales (1677–1761) is perhaps best known as the inventor of what later became known as the ‘pneumatic trough’, an instrument which enabled chemists to collect and isolate gases emitted during a chemical reaction. Worth’s copy of Hales’ Vegetable Staticks, published a year before this discovery, shows Hales’ keen interest in experiments on gases given off by vegetables during fermentation. Hales is regarded as the founding father of ‘pneumatic chemistry’ but he was keenly aware of the debt he owed to both Robert Boyle and Isaac Newton, viewing his experiments as a continuation of the ground-breaking work of Boyle on the physical properties of air. These two men, along with John Friend at Oxford, were the principal influences on Hales’ chemistry. As Allan and Schofield argue (1980), Hales’ study of air was to be ‘the starting point for eighteenth-century pneumatic chemistry’. The following illustration depicts Hales’ method of separating gaseous products of chemical reactions. It is followed by Hales’ explanation of the experiment.

Hales air experiment
Stephen Hales, Vegetable Staticks (London, 1727), p. 160.

‘A specimen of an attempt to analyse the Air by a great variety of chymio-statical Experiments, which shew in how great a proportion Air is wrought into the composition of animal, vegetable, and mineral Substances, and withal how readily it resumes its former elastick state, when in the dissolution of those Substances it is disingaged from them.…. The excellent Mr. Boyle made many Experiments on the Air, and among other discoveries, found that a good quantity of Air was producible from Vegetables, by putting Grapes, Plums, Gooseberries, Cherries, Pease, and several other sorts of fruits and grains into exhausted and unexhausted receivers, where they continued for several days emitting great quantities of Air.

Being desirous to make some further researches into this matter, and to find what proportion of this Air I could obtain out of the different substances, in which it was lodged and incorporated, I made the following chymio-statical Experiments: For as, whatever advance has here been made in the knowledge of the nature of Vegetables, has been owing to statical Experiments, so since nature, in all here operations, acts conformably to those mechanick laws, which were established at her first insitution; it is therefore reasonable to conclude, that the likeliest way to enquire, by chymical operations, into the nature of a fluid, too fine to be the object of our sight, must be by finding out some means to estimate what influence the usual methods of analysing the animal, vegetable and mineral kingdoms, has on that subtile fluid; and this I effected by affixing to retorts and boltheads hydrostatical gages in the following manner, viz.

In order to make an estimate of the quantity of Air, which arose from any body by distillation or fusion, I first put the matter which I intended to distill into the small retort r (Fig. 33) and then at a cemented fast to it the glass vessel a b, which was very capacious at b, with a hole in the bottom. I bound bladder over the cement which was made of tobacco-pipe clay and bean flower, well mixed with some hair, tying over all four small sticks, which served as splinters to strengthen the joynt; sometimes, instead of the glass vessel a b, I made use of a large bolthead, which had a round hole cut, with a red hot iron ring at the bottom of it; through which hole was put one leg of an inverted syphon, which reached up as far as z. Matters being thus prepared, holding the retort uppermost, I immersed the bolt-head into a large vessel of water, to a  the top of the bolthead; as the water rushed in at the bottom of the bolthead, the Air was driven out thro’ the syphon: When the bolthead was full of water to z, then I closed the outward orifice of the syphon with the end of my finger, and at the same time drew the other leg of it out of the bolthead, by which means the water continued up to z, and could not subside. Then I placed under the bolthead, while it was in the water, the vessel x x, which done, I lifted the vessel x x with the bolthead in it out of the water and typed a waxed thread at z  to mark the height of the water: And then approached the retort gradually to the fire, taking care to screen the whole bolthead from the heat of the fire.

The descent of the water in the bolthead shewed the sums of the expansion of the Air, and of the matter which was distilling: The expansion of the Air alone, when the lower part of the retort was beginning to be red hot, was at a medium, nearly equal to the capacity of the retorts, so that it then took up a double space; and in a white and almost melting heat, the Air took up a triple space or something more: For which reason the least retorts are best for these Experiments. The expansion of the distilling bodies was sometimes very little, and sometimes many times greater than that of the Air in the retort, according to their different natures.

When the matter was sufficiently distilled, the retort &c was gradually removed from the fire, and when cool enough, was carried into another room, where there was no fire. When all was thoroughly cold, either the following day, or sometimes 3 or 4 days after, I marked the surface of the water y, when it then stood; if the surface of the water was below z, then the empty space between y and z shewed how much Air was generated, or raised from a fix’d to an elastick state, by the action of the fire in distillation: But if y the surface of the water was above z, the space between z and y, which was filled with water, shewed the quantity of Air which had been absorbed in the operation, i.e. was changed from a repelling elastick to a fix’d state, by the strong attraction of other particles, which I therefore call absorbing.

When I would measure the quantity of this new generated Air, I separated the bolt-head from the retort, and putting a cork into the small end of the bolthead, I inverted it, and poured in water to z. Then from another vessel (in which I had a known quantity of water by weight) I poured in water to y; so the quantity of water which was wanting, upon weighing this vessel again, was equal to the bulk of the new generated Air. I chose to measure the quantities of Air, and the matter from whence it arose, by one common measure of cubick inches, estimated from the specifick gravities of the several substances, that thereby the proportion of one to the other might the more readily be seen.

I made use of the following means to measure the great quantities of Air, which were either raised and generated, or absorbed by the fermentation arising from the mixture of variety of solid and fluid substances, whereby I could easily estimate the surprising effects of fermentaion on the air, viz.

I put into the bolthead b (Fig. 34.) the ingredients, and then run the long neck of the bolthead into the deep cylindrical glass a y, and inclined the inverted glass a y, and bolthead almost horizontally in a large vessel of water, that the water might run into the glass a y; when it was almost up to a the top of the bolthead, I then immersed the bottom of the bolthead, and lower y of the cylindrical glass under water, raising at the same time, the end a uppermost. Then before I took them out of the water, I set the bolthead and lower part of the cylindrical glass a y into the earthen vessel x x full of water, and having lifted all out of the great vessel of water, I marked the surface  z of the water in the glass a y.

If the ingredients in the bolthead, upon fermenting generated Air, then the water would fall from z to y, and the empty space z y was equal to the bulk of the quantity of Air generated: But if the ingredients upon fermentation did absorbe or fix the active particles of Air, then the surface of the water would ascend from z to n, and the space z n, which was filled with water, was equal to the bulk of Air, which was absorbed by the ingredients, or by the fume arising from them: When the quantities of Air, either generated or absorbed, were very great, then I made use of large chymical receivers instead of the glass a y: but if these quantities were very small, then instead of the bolthead and deep cylindrical glass a y, I made use of a small cylindrical glass, or a common beer glass inverted, and place under it a Viol or Jelly glass, taking care that the water did not come at the ingredients in them, which was easily prevented by drawing the water up under the inverted glass to what height I pleased by means of a syphon; I measured the bulk of the spaces z y or z n, by pouring in a known quantity of water, as in the foregoing Experiment, and making an allowance for the bulk of the neck of the bolthead, within the space z y….’

Boyle’s Air Pump

As Hales makes clear, the starting point for his investigations into air were the works of Robert Boyle. Crosland (2002) rightly argues that Boyle’s famous air pump experiment falls more naturally into the broader realm of natural philosophical research than chemistry proper, Boyle being far more interested in the compressibility of air and its ability to expand than any strictly chemical concerns. However, his interest in measuring gases was an important precondition for later work such as that of Hales. Perhaps his most famous experiment on air was his celebrated air pump, described in Worth’s copy of Boyle’s collected works, printed in London in in 1725.

Boyle’s air pump.
Robert Boyle, The philosophical works (London, 1725), 3 vols, vol 2, plate VI.

The air-pump described.

‘The first is a glass A, with a large mouth, a cover thereto, and a stop-cock fitted to the neck below. This would contain 30 quarts of water. BC, the mouth of it, is about four inches in diameter, and surrounded with a glass lip, almost an inch high, for the cover to rest on; wherein DE, is a brass ring, to cover, and be cemented on to the lip BC. To the internal orifice of this ring, a glass stopple is fitted, to keep out the external air. In the middle of this cover is a hole HI, half and inch in diameter, incircled with a ring, or socket; to which is adapted a brass stopple K, to be turn’d round, without admitting the least air. In the lower-end of this, is a hole 8, to admit a string, 8, 9, 10; which also passes thro’ a small brass ring L, fixed to the bottom of the stopple FG, to move what is contain’d in the exhausted vessel, or receiver. That the stop-cock N, in the first figure, might perfectly exclude the air, we fasten’d a thin tin-plate, M T V W, to the shank of the cock X, all along the neck of the receiver, with a cement made of pitch, rosin, and wood ashes, poured hot into the cavity of the plate; and to prevent the cement from running in at the orifice Z, of the shank X, it was stopt with a cork fix’d to a string, that it might be drawn out at the upper orifice of the receiver; and then the neck of the glass, being made warm, was pressed into the cement, which thus fill’d the interstices betwixt the tin-plate and the receiver, and betwixt the receiver and the shank of the cock.The lower part of our engine consists of a sucking-pump, supported by a wooden frame, with three legs 111, so contrived, that, for the freer motion of the hand, one side of it may stand perpendicular; and a-cross the middle of the frame we nail’d a piece of board 222, to which the principal part of the pump is fixed. The pump consists of an exact strong concave cylinder of brass, fourteen inches long, its cavity three inches in diameter; to which a sucker, 4455, is adapted, made up of two parts; one of which 44, is less in diameter than the cavity of the cylinder, with a thick piece of tann’d leather nail’d on it, whereby it excludes the air. The other part, a thick iron plate 55, is firmly join’d to the middle of the former, and is a little longer than the cylinder; one edge of it being smooth, and the other indented, to receive the teeth of a small iron-nut αβγ, fixed by two staples to the underside of the board naild a-cross 22, on which the cylinder stands; and it is turn’d by the handle 7.
The last part of the pump is the valve R, a hole at the top of the cylinder, and taper towards the cavity; to this is fitted a brass-plug, to be taken out as occasion requires. The engine being thus contrived, some oil must be pour’d in at the top of the receiver upon the stop-cock, to fill up the interstices of its parts, and that the key S, may turn with the greater ease. A quantity of oil, also, must be left in the cylinder, to prevent the air from getting betwixt that the sucker; for the like reasons, some must, likewise, be appl’d to the valve.

And here ‘tis proper to observe, that when we used oil, or water, separately, for this purpose, and they have not answered the end, a mixture of the two has afterwards proved effectual. And, that the air may not enter betwixt the brass-cover and the ring, ‘twill be convenient to lay some diachylon-plaister on their edges with a hot iron. That no air, also, may remain in the upper part of the cylinder, the handle is to be turn’d till the sucker rises to the top; and then, the valve being shut, it is to be drawn down to the bottom; by which means, the air being driven out of the cylinder, and a succession from without prevented, the cavity of the cylinder must be empty of air; so that, when the stop-cock is turn’d to afford a communication betwixt the receiver and the cylinder, part of the air before lodged in the receiver, will be drawn down into the cylinder; which, by turning back the key, is kept from entering the receiver again, and may, by unstopping the valve, and forcing up the sucker, be driven into the open air; and so, by repeated exsuctions out of the receiver, and expulsions out of the cylinder, the vessel may be exhausted as the experiment requires.’

Boyle, Robert, The philosophical works of the Honourable Robert Boyle Esq; abridged, methodized, and disposed under the general heads of physics, statics, pneumatics, Natural History, Chymistry, and Medicine…By Peter Shaw, M.D. (London, 1725), ii, pp 408-9.

None of the experiments described here should be attempted.


Allan, D. G. C. and Schofield, R. E. (1980), Stephen Hales: Scientist and philanthropist (London).

Allan, D. G. C. (2004), ‘Hales, Stephen (1677–1761)’, Oxford Dictionary of National Biography, Oxford University Press.

Bensaude-Vincent, Bernadette and Stengers, Isabelle (1996), A History of Chemistry Harvard University Press).

Boyle, Robert (1725), The philosophical works of the Honourable Robert Boyle Esq; abridged, methodized, and disposed under the general heads of physics, statics, pneumatics, Natural History, Chymistry, and Medicine…By Peter Shaw, M.D.


Crosland, Maurice (2002), ‘ “Slippery Substances”: Some practical and conceptual problems in the understanding of gases in the pre-Lavoisier era’ in Frederick L. Holmes and Trevor H. Levere, Instruments and Experimentation in the History of Chemistry (MIT Press), pp 79-104.

Hales, Stephen (1727), Vegetable staticks: or, an account of some statical experiments on the sap in vegetables: being an essay towards a natural history of vegetation. Also, a specimen of an attempt to analyse the air, By a great Variety of Chymio-Statical Experiments; Which were read at several Meetings before the Royal Society. By Steph. Hales, B. D. F. R. S. Rector of Farringdon, Hampshire, and Minister of Teddington, Middlesex (London).

Holmes, Frederick L. (2000), ‘The “Revolution in Chemistry and Physics”: Overthrow of a Reigning Paradigm or Competition between Contemporary Research Programs?’, Isis 91, no. 4, pp 735-53.

Levere, Trevor H. (2002), ‘Measuring Gases and Measuring Goodness’, in Frederick L. Holmes and Trevor H. Levere, Instruments and Experimentation in the History of Chemistry (MIT Press), pp 105-136.

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