Excerpts from Tractatus Quinque Medico-Physici (1674), translated as Medico-Physical Works (Oxford, 1926)
But although the spirit of nitre does not proceed altogether from the air, still we must believe that some part of it originates from the air. For, since some part of the nitre is derived from the air, as has been shown above, while the fixed salt, of which nitre in part consists, proceeds from the earth, the remainder of the nitre, that is to say, its acid and fiery spirit, must be derived, in part at least, from the air. But in order that the aerial part of the spirit of nitre may be better understood, we must briefly premise the following.
First, it is, I think, to be admitted that something aerial, whatever it may be, is necessary to the production of any flame--a fact which the experiments of Boyle have placed beyond doubt, since it is established by these experiments that a lighted lamp goes out much sooner in a glass that contains no air than it does in the same when filled with air--a clear proof that the flame enclosed in the glass goes out, not so much because it is choked, as some have supposed, by its own soot, as because it is deprived of its aerial food. For since there is more room for receiving the smoke in the empty glass than in the glass that is full of air, the lamp would go out in the latter sooner than in the former, if its extinction were due to the smoke. Besides, no sulphureous matter, if placed in a glass from which the air has been pumped, can be kindled either by ignited charcoal or iron, or by the solar rays collected by means of a burning-glass; so that there can be no doubt whatever that certain aerial particles are quite indispensable to the production of fire, and, indeed, it is our opinion that these are mainly instrumental in the production of fire, and that the shape of the flame is mainly dependent upon these, thrown into extremely brisk motion, as will be explained at greater length below. But it is not to be supposed that the air itself, but only that its more active and subtle part is the igneo-aerial food, since a lamp enclosed in a glass goes out when there is still an ample enough supply of air in it, for neither is it to be believed that the particles of air which existed in the said glass are annihilated by the burning of the lamp, nor yet that they are dissipated, since they are unable to penetrate the glass. Further, it is impossible that these igneo-aerial particles are any perfect nitre, as is generally supposed--for it was already pointed out that not the very nitre as a whole, but only a certain part of it, resides in the air.
Finally, the nitro-aerial particles in the flame produced by solar rays collected by a burning-glass are particularly bright. This celestial flame appears to be due merely to the nitro-aerial particles of the atmosphere set in fiery motion by the action and intense impulse of light. And this we must suppose is the reason that antimony, when calcined by the solar beams, is fixed and made diaphoretic, just as if it were changed into Bezoardicum minerale by spirit of nitre poured upon it and drawn off again and again. Indeed, it is probable that it is the nitro-aerial particles with which that spirit abounds, and in some motion of which the solar rays consist, that fix antimony and render it diaphoretic. It favours this view that antimony acquires a diaphoretic virtue, not only from the spirit of nitre and the solar rays, but also from the flame of nitre in which nitro-aerial particles are more densely collected. Nor should it be overlooked that antimony, calcined by the solar rays, is considerably increased in weight, as has been ascertained by experiment. Indeed, we can scarcely imagine any other source for this increase of the antimony than the nitro-aerial and igneous particles fixed in it during calcination.
I am aware that it is the common opinion that the diaphoretic virtue of antimony is due to the loss of its extraneous and combustible sulphur in its calcination. But I am not sure that this view is quite consistent with truth. For it is well known that if antimony and nitre are mixed and thrown into a heated crucible, a very impetuous flame will arise from them, since the sulphur of the antimony ignites the nitre mixed with it. If, however, the antimony has detonated (as the chemists phrase it) with about a double quantity of nitre, then nitre mixed with it will no longer produce a flame, since the combustible sulphur of the antimony has been entirely removed in the first detonation. And still the antimony has not yet acquired the diaphoretic virtue. Hence, for its further fixation, charcoal or some sulphureous matter should be put from time to time into the crucible in which the antimony, along with the nitre last added to it, has been fused, so that the nitre may ignite and the antimony be fixed by its long-enduring flame. Clearly, then, the fixation of antimony appears to be caused, not so much by the removal of its extraneous sulphur, as by the fixation in it of the nitro-aerial particles in which the flame of nitre abounds.
Let a moistened bladder be stretched over the circular orifice of any vessel and tied to it just as the skin of a drum is stretched; then let a small bell-jar in which a little animal, say a mouse, has been put, be accurately applied to the said bladder by placing a weight upon the jar lest the animal inside should upset it (as is shown in Plate V., Fig. 2). When things have been arranged in this manner it will in a short time be seen that the jar is firmly fixed to the bladder; and the bladder also, at the place where it lies under the jar, is forced upwards into the cavity of the glass just as if the jar had been applied with a flame enclosed in it. And this will take place while the animal is still breathing. Nay, if the jar be grasped by the hand and raised, the bladder, along with the vessel, will still adhere firmly to it unless the vessel is very heavy. And indeed a little animal placed in a cupping-glass which is to be fixed to the skin can supply to a small extent the place of the flame. And from this it is clear that the elastic power of the air enclosed in the aforesaid jar has been diminished by the breathing of the animal, so that it is no longer able to resist the pressure of the surrounding air.
But in order that this matter may be better understood, let me submit yet another experiment to the same effect--an experiment moreover from which it will be easy to perceive in what proportion the air is diminished as to its volume when deprived of vital particles by the breathing of the animal. Thus, let a small animal placed on a suitable support be enclosed in an inverted glass, or better, let the animal be put into a suitable cage and suspended in a glass jar just as the vessel is suspended in Plate V., Fig. 4. Then let the inverted glass be sunk a little into the water so that the water enclosed in the glass may stand at the same level as the water outside, as may be done by means of the bent syphon already described. When this is done let the water outside be drawn off a little in order that the height of the water within may be better observed. And let it be indicated by papers attached here and there to the sides of the glass. And so you will soon see the water sensibly rising into the cavity of the glass, although the heat produced by the presence of the animal in the glass, and also the breath proceeding from it, might be expected rather to produce an opposite effect.
But we can perceive in the following way the extent to which the air enclosed in the glass undergoes contraction before it becomes unsuitable for sustaining animal life. For let the space in the glass occupied by the air when the animal was at first placed in it and also the space occupied by the same air when the water has risen in the glass after the suffocation of the animal be measured, as can be done by pouring water into those spaces so as to fill them and measuring it--but warning should be given here in passing that when these spaces are thus measured everything should remain in the glass the same as before. And now let it be ascertained by calculation how much the first space is greater than the second. For to that extent the air is lessened as to its elastic force and volume by the breathing of the animal. And in fact I have ascertained from experiments with various animals that the air is reduced in volume by about one-fourteenth by the breathing of the animals. But care should be taken in making this experiment that the animal be placed only a little above the surface of the water, for a reason to be afterwards given.
From what has been said it is quite certain that animals in breathing draw from the air certain vital particles which are also elastic. So that there should be no doubt at all now that an aerial something absolutely necessary to life enters the blood of animals by means of respiration. And indeed if the necessity for breathing arose, as some have imagined, merely from this that the mass of the blood should be churned and divided into the most minute parts by the movement of the lungs, there would certainly be no reason why an animal, enclosed in a glass vessel in the manner described, should die so soon, because the air there avails as much after the death of the animal as before to inflate the lungs and consequently to churn the mass of the blood. For as that air is impelled by the pressure of nearly the whole atmosphere, there is nothing to hinder it from being urged into the dilated thorax of the animal, and on this the inflation of the lungs depends, as we have shown elsewhere.
There is now no reason therefore for denying the entrance of air into the blood because on account of the dulness of our senses the vessels by which it enters cannot be seen. For other ducts which serve to convey thicker liquids are not seen by the eye until their different capillaries, after a passage of some length, unite in a noticeable canal. For what keenness of vision has ever beheld the sources of the lymphatic or lacteal vessels or even of the veins? How much less may one discern these aerial ducts which must be very short and extremely small, for these ducts do not, like the others, run any considerable distance and at last join one another, but merely pass separately by a very short and obscure route through the membranes of the lungs; for that the aerial particles should be mixed with the blood in the minutest and most intimate way, it is necessary that they enter the blood by vessels or rather pores almost infinite in number, distributed, here and there, through the whole mass of the lungs. And yet in the lungs, when boiled and dissected, an almost infinite number of openings resembling most minute points are seen by the aid of the microscope. But whether these points are the mouths of capillary tracheae, or of vessels opening into the blood, I cannot state with certainty.
Hence it is manifest that air is deprived of its elastic force by the breathing of animals very much in the same way as by the burning of flame. And indeed we must believe that animals and fire draw particles of the same kind from the air, as is further confirmed by the following experiment.
For let any animal be enclosed in a glass vessel along with a lamp so that the entrance of air from without is prevented, which is easily done if the orifice of the inverted glass be immersed in water in the manner already described. When this is done we shall soon see the lamp go out and the animal will not long survive the fatal torch. For I have ascertained by experiment that an animal enclosed in a glass vessel along with a lamp will not breathe much longer than half the time it would otherwise have lived.
Nor is there any reason for supposing that the animal is suffocated by the smoke of the lamp, for scarcely any smoke will emanate from it if spirit of wine is used, and indeed the animal will live in the glass for some time after the extinction of the lamp--that is, after the fumes have entirely disappeared--so that it is by no means to be supposed that it has been suffocated by the fumes of the lamp. But since the air enclosed in the glass is in part deprived of its nitro-aerial particles by the burning of the lamp, as has already been pointed out, it cannot support long the breathing of the animal, hence not only the lamp but also the animal soon expires for want of nitro-aerial particles.
But the reason why an animal can live for some time after the extinction of the lamp seems to be this. It is only by a continuous and moreover an abundant and rapid stream of nitro-aerial particles that a lamp is sustained. Consequently if the succession of nitro-aerial particles be but for a moment interrupted, or if they are not supplied in due abundance, the flame will immediately sink down and expire. Hence as soon as the nitro-aerial particles begin to come but sparsely and slowly to the flame it presently goes out. But a smaller ration of aerial nourishment and that introduced at intervals will suffice for animals; so that an animal can be sustained by the aerial particles remaining after the extinction of the flame. It supports this view that the movement of the subsiding lungs conduces not a little to draw in the aerial particles if any remain in the said glass and to carry them into the blood of the breathing animal. Hence it results that the animal does not die until the aerial particles have been entirely exhausted. And hence it is that the air in which an animal is suffocated is contracted in volume by more than twice as much as that in which a lamp goes out, as was formerly pointed out.
Further, having suspended combustible matter in a glass vessel beside an animal, I tried to ignite it by means of a burning-glass after the animal was suffocated, and that that might if possible succeed, I protected from the breath of the animal the side of the glass through which the solar rays were to be transmitted, by means of a piece of paper fixed to it as already described. But the experiment did not succeed. I shall not, however, make any certain pronouncement in regard to this, because wintry weather and a sky almost constantly wrapped in clouds prevented me from repeating the experiment. It is probable, however, that air which is unsuitable for supporting life is also incapable of producing flame, since a greater quantity of aerial particles is needed for the burning of a lamp than for sustaining life. But it is to be noted here that although flame and life are sustained by the same particles it is not on that account to be supposed that the mass of the blood is really on fire, as will be shown in the next chapter.
But assuredly difficulties by no means slight occur in connection with what has been said. For, in the first place, how should it be that an animal or a lamp enclosed in these glass vessels is unable to survive while a sufficient abundance of air is contained in them? For the water underneath ascends into a part only of these glasses and the remaining space is filled with air, and that air although diminished in volume is yet able to resist the pressure of the surrounding air.
Further, in what way shall we suppose that the air in the aforesaid glasses loses its elastic force? For we must believe that air is contained in them in undiminished quantity after the extinction of the flame and the death of the animal. For the aerial particles are not annihilated by the burning of the flame or the breathing of the animal. Nor are they driven out of the glass, for neither air nor any other elastic matter mixed with it is able to penetrate glass, as we have indicated above; for otherwise the pressure of the air in the glass could not be removed or diminished by any suction, inasmuch as the air or the elastic matter would immediately enter the glass from which the air had been exhausted and fill the space left by the air, especially since the pressure of the surrounding air assists towards their entrance.