has to sustain this pressure; and it will be found by calculation, that the whole surface of a middle-sized person, will have to support from 15 to 20 tons of pressure all acting inwards, and having no other mechanical tendency than that of squeezing or compressing the materials of which the body is composed into a less

compass.

The above is a statement of facts, all of which I believe to be incontrovertible. But a very difficult question arises out of them, How is it that the animal frame is utterly insensible of the whole, or of any part of this enormous pressure upon it? In ordinary we feel no pressure on the surface of our bodies, either external or internal; neither when the barometer is stationary nor when it is in a most fluctuating state. I have never met with a satisfactory answer to this question, and I doubt whether such a one has ever been given; yet it must be allowed to be one of importance, both as it affects the physiology of the animal and vegetable kingdoms. Having had occasion for a few weeks past to ruminate on this subject, some new views have occurred to me; and it is the object of the present essay to unfold them, in order to elucidate the phenomena arising from aerial pressure on the animal economy more especially.

It is pretty well known that the specific gravity of living men in general, is less than that of water. Mr Robertson, formerly librarian to the Royal Society, procured an apparatus for the purpose of determining the specific gravity of the human body. He chose ten men promiscuously for the purpose. Of these, three were found very nearly of the same weight as water, one being a little heavier, and the other two a little lighter than water; two others were found only about .8 the weight of water; but the other five were of intermediate specific gravities. The average of the ten was, height 5 feet 63 inches; weight, 146 lbs. ; specific gravity, 891; bulk, 2.618 cubic feet. From this I think we may safely infer that the body of a full grown living man, when plunged over head in water, will be found upon the average to be nearly .9, the weight of an equal bulk of water.

It is remarkable that all the component parts of the animal frame, at least of the human subject, are severally specifically heavier than the whole body, with the exception of air. Bone, muscular flesh, blood, membrane, &c. are all heavier than water;

animal fat is perhaps the lightest of the components, but even this is heavier specifically than the whole man upon the average. Bone from the leg of a calf I found to be 1.24 specific gravity. The lean of beef (raw) I found 1.045 specific gravity. Blood is from 1.03 to 1.05 specific gravity according to circumstances. On the whole, the solid and liquid parts of the body, examined after life is extinct, would appear on an average to be somewhere about 5 per cent. heavier than water.

That part of the volume of man which is exclusively occupied by air, and which may therefore be considered as adding nothing materially to the weight of the body, consists of the air-tubes and air-cells of the lungs, the trachea or windpipe, the mouth and other appendages. It is not easy to ascertain the medium volume of air in the lungs of any individual. Messrs Allen and Pepys found the air remaining in the lungs of a man after death somewhat exceeded 100 cubic inches. I found formerly that after a full inspiration I could blow out 200 cubic inches of air from my lungs, but was then quite exhausted. My ordinary inspirations and expirations amounted each to about 30 cubic inches *.

Judging from the above facts and considerations, I should be disposed to conclude that the medium volume of air in the lungs of a middle-sized person would not be less, but rather more, than 100 cubic inches. Besides the lungs there are no other receptacles for air, I believe, in the body except the stomach and bowels, which are occasionally more or less inflated with portions of air either from the atmosphere or other sources. If we allow 150 cubic inches for the volume of air contained in the whole man when entirely immersed in water, it will be as fair an estimate, perhaps, as can be made. But it may be imagined by some that the whole substance of the body is pervious to air; that the skin, the flesh, the blood and even the bones, may be imbued with air, somewhat in the same manner that water is, and yet have no cavities or cells in which the air is collected into a visible volume. Whether such an idea has ever been entertained or discussed I am not aware; but I presume no one has succeeded in determining either the nature or the quantity of the air so enveloped in the system. We shall

* Memoirs, vol. ii. (New Series, p. 26.)

now examine how far such a notion is countenanced by the ceding statement of facts.

pre

According to the preceding table of Robertson, the average bulk of the ten men was 2.618 cubic feet, 4.500 cubic inches nearly; but of this volume 150 inches according to the above estimate were air, and the remainder 4350 inches were solid and liquid parts of the body. Now the average specific gravity of those parts of the body has been estimated above at 1.05 when examined as dead matter: this would make their weight equal to 4567 cubic inches of water; whereas it was found by actual weighing, to be 146 lbs. as per table, = 4044 cubic inches: hence the observed weight was less than the calculated weight, a portion equal to the weight of 523 cubic inches of water, or more than one-ninth of the whole weight of the body.

Here is a discrepance that demands an investigation. Can Robertson's table of the specific gravities of men give too low an estimate? This is not likely; every one knows that the human subject generally floats in water till the lungs become filled with that element,-a proof that the body is lighter than water; and many persons are observed to swim with the whole head constantly above the surface of the water.

Have we overrated the specific gravities of the component parts of the body? I think not: bones, and flesh, and blood are certainly all heavier than water, some more, some less.

Has the capacity of the lungs for air been underrated? I cannot imagine that any one will contend that the lungs of a middle-sized man will hold, at a medium state of inflation, six times the volume of air we have assigned. Upon the whole, I am inclined to believe the true explanation of the difficulty will be found in this, that the substance of the body is pervious to air, and that a considerable portion of it constantly exists in the body during life, subject to increase and diminution according to the pressure of the atmosphere; in the same manner as it exists in water: and, further, that when life is extinct, this air in some degree escapes and renders the parts specifically heavier than when the vital functions were in a state of activity.

The facts that water absorbs air of all kinds, that the quantity of the air absorbed is proportioned to the pressure and density of the gas, whether it be alone or mixed with other gases,

5

and that certain laws of equilibrium take place, by which water acquires that state in which it is disposed neither to give out nor to take in any more gas, have been abundantly proved by Dr Henry and myself. M. Saussure has shown the like for other liquids, and for a great number of solid bodies. It may be seen, too, in my Chemistry, vol. i. p. 236, that a bladder, which is generally considered as an animal membrane, least pervious to air, may be filled with one gas, and being some time exposed to the atmosphere, it will be found to continue full blown as at first, but the contents will be chiefly atmospheric air. Messrs Allen and Pepys, in their ingenious and excellent essays on respiration, have proved that when a Guinea pig or a pigeon is confined for an hour, more or less, in a mixture of hydrogen and oxygen gases, in proportion as 78 to 22, a large portion of azotic gas is found in the residue, and an equal portion of hydrogen disappears. They ascribe this change to effects of respiration, but it appears to me more probably due to the principle we are advocating; namely, to the egress of azotic from the whole body, and the ingress of hydrogen in lieu of it, in consequence of withdrawing the external pressure of the former and substituting that of the latter.

When the palm of the hand is placed over the top of the receiver of an air-pump, and the air is exhausted, the pressure of the air on the outside is scarcely felt, but the inside is swollen and feels as if it was drawn or sucked into the receiver. Thus the sensation is on the inside and not without; but there is within, and the consequence is a tendency of the air in the hand to escape into the receiver, which occasions the pain and swelling. It is thus also that the issuing of blood in the surgical operation of cupping is effected.

Though it does not seem of much consequence what the pressure of the air may be on the animal frame within certain limits, yet sudden changes must always be accompanied with uneasy sensation. Climbing mountains, or ascending in a balloon, removes a part of the atmospheric pressure from the body; this causes the air in the body to tend outwards, and sometimes occasions bleedings. To supply oxygen to the lungs, a greater volume of air must be breathed, and this seems to produce an acceleration of the pulse. On the other hand, by descending

30 or 40 feet deep into the water in a diving bell, the pressure of the air upon the body is increased inwards; pains in the ears are felt from the difficulty of suddenly restoring a disturbed equilibrium; but if the descent is slow and interrupted, time is given for the air to enter the pores, and the pain is less sensible. To what limit warm-blooded animals could bear rarefaction of air so as to subsist, has not, that I am aware of, been determined with much precision. Ascents in balloons have been made till the atmospheric pressure was reduced more than one-half. Formerly I found that a mouse could subsist in th of atmospheric density and seemed not to have suffered much; but upon reducing the density below 4th, the animal was convulsed and expired immediately, notwithstanding the air was instantly admitted.

If the view we have expounded in this essay, in regard to the action of aerial pressure on the animal frame, be correct, it may be inferred, that the pressure admits of great latitude; perhaps an animal could subsist under the pressure of half an atmosphere, or of three or four, or more atmospheres. The uneasiness and danger would be found in the quick transition; if time is allowed for the air to enter the body, and to escape from it, the transition is gradual, and the sensation arising from it imperceptible. The animal economy would be adapted to it, like as in the transition from a cold to a warm climate. It may hereafter be found, what length of time is sufficient to adjust the equilibrium, and whether this subject is any way connected with certain diseased states of the body. As far as regards the absolute pressure on the body, and our insensibility of it generally, this question will be met by the argument, that the air within the body, by its elasticity, sustains a corresponding pressure from without; but this only accounts for our alleviation from a small fractional part of the whole exterior pressure. The greater part must still be supported by the body; and we must have recourse to the great incompressibility of matter to account for our insensibility of pressure. Canton found that water, pressed by one atmosphere more than ordinary, only exhibited a reduction ofth part of the whole; if the same rate, applied to the compression of the human body, the reduction or compression of the size of a man, 4500 cubic inches, would only be th