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classes, and to give them very expressive names, if the minerals which they contain were well known. The unicoloured crystalline traps occasionally contain labradorite, and the presence of this mineral may account for the soda that has generally been found in them, as well as in basalt, to which they are closely allied; but it does not, in some cases, account for the whole of the soda, so that they probably contain other minerals, of which this alkali forms a constituent part. I have analyzed two of these rocks, that contained labradorite, and the analysis differed but little from those of some basalts. The neighbourhood of Glasgow affords some very fine specimens of columnar trap, which all contain labradorite. The range of large and regular columns that stands on the side of a wooded hill, a little to the south of Strathblane, is well known. It is composed of a dark green trap or greenstone, containing crystals of that mineral. The same may be said of the black basalt, that forms the remarkable horizontal columns, near Altmarry turnpike, on the road from Glasgow to Drymen; and also of the columns that are found in more than one place on the Kilpatrick and Cathkin Hills. So that labradorite is probably one of the minerals of which basalt is often composed.

I have made these remarks upon the situations in which I have found labradorite, because I believe that we cannot attempt to arrange the trap-rocks, or give them expressive names, until we are better acquainted with their composition. The present nomenclature of these rocks is very defective, and, not being formed upon any fixed principle, it cannot be improved. The names must either be multiplied to such an extent as to render them very burdensome to the memory, or be applied so loosely, as always to require a definition. The short and simple terms now in use have not the flexibility which names, applied to ohjects that are constantly varying in their characters, should possess. It would be better to employ in geology, as often as possible, compound descriptive names, that would indicate the structure and composition of rocks. Such names would, in many cases, be shorter than those now employed, as they never would require a definition ; and, when formed upon a fixed principle, they might be multiplied without loading the memory. As far as the trap-rocks are concerned, they can be accurately named in this manner only; for we frequently find among them rocks of the same composition, but differing in structure; and, on the contrary, their structure is often the same, when their composition is different. They are easily arranged in a few classes, by their external characters; and the class to which the rock belongs may be expressed in a single word, while its composition may be denoted by two or three. There are some cases in which we should still be obliged to use arbitrary terms; but this should not induce us to reject the only principle upon which an expressive and convenient nomenclature can be formed for the greater number of these rocks. I have long been in the habit of using those compound terms in a district which presents trap under a great variety of forms; and I should mention the mode that I have adopted, to render them as short and expressive as possible, but that my observations and experiments on the subject are far from being completed.

Physiological Investigations arising from the Mechanical

Effects of Atmospherical Pressure on the Animal Frame. By

John Dalton, F. R. S. A

PERIOD of a century and a half has elapsed since the inventions of the barometer and the air-pump. In this time the weight of the atmosphere, its elasticity, its specific gravity, and many other properties, have been ascertained experimentally with almost mathematical precision. The weight of the atmosphere, that quality we have more particularly to consider in the present essay, is not constantly the same, as is proved by the rising and falling of the barometer. It varies in this part of the earth from işth to i' th of the whole weight at certain times; but those variations are gradual, so that it requires some days or weeks before the weight passes from one extreme to the other. On an average the weight or pressure of the atmosphere amounts to 141 lb. on each square inch of surface of the earth ; and, as fluids press equally in all directions, every square inch of surface, whatever may be its position, must be subject to the same pressure. The surface of the human body, as well as that of animals in general, 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 importapce, 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 6 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 now examine how far such a notion is countenanced by the preceding statement of facts.

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.)

bones, may

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,

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