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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

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

to

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 {th, 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 of giaoth 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

of a cubic inch, for the weight of an additional atmosphere.
Now as the body consists of solids and liquids of almost in-
compressible matter, and there is only a small part of the vo-
lume consisting of elastic fluid that is compressible, no mate-
rial change of volume can take place, but on the sudden tran-
sition from one atmospheric pressure to another; and 'unless
a change of volume take place, we cannot feel any pressure,
either inward or outward. The phenomena of the water ham-
mer shew, that the particles of water are hard, as they strike
each other like flint and steel ; and it is exceedingly probable
that other bodies, solids as well as liquids, are constituted in
like manner.

A general pressure on the system, then, only in-
creases in a small degree the attraction of the ultimate particles,
and it is met by a corresponding increase of repulsion from the
atmosphere of heat; so that the system remains as nearly as
possible the same, and unaffected by such pressure.

I can scarcely forbear observing on the present occasion the absurdity of those who remark, that all people might swim, and that it is only from fear or ignorance of the art that some fail in the attempt. When we see that some persons are heavier than water, and others only .8 of that weight, it would be just as plausible for a piece of deal to upbraid a piece of lignum vitæ with the inability to swim from fear, or from want of skill in the art, which the deal considered of easy acquisition.-Manchester Memoirs, vol. v. New Series.

Chemical Analyses of Spinel, Gahnite, and Chrome Ore.

By Mr HERMANN ABICH.

Constituent Parts.

Silica,
Alumina,
Oxidulated Chrome,
Magnesia,
Oxidulated Iron,
Zinc-Oxide,
Oxidul. Manganese,

Massive Crystalliz. Blue Spinel, Red Spinel, Pleonaste, Pleonaste, Gahnite, Gahnite, Chrome Chrome Sweden. Ceylon. Ural. Vesuvius. Sweden. America. Ore. Ore.

2.25 2.02 2.5 2.38 3.84 1.22 0.83 00.00 68.94 69.01 65.27 67.46 55.14 57.09 13.85

11.85 00.00 1.10 00.00 00.00 00.00 00.00 54.91 60.04 25.72 26.21 17.58 25.94 5.25

2.22

9.69 7.45 0.71 13.97 5.06 5.85 4.55 18.97 20.13

30.02 34.80 Trace 00

3.40

...

Sum,

Trace

100.40

99.05

99.32 100.84 | 100.10

99.38 | 98.25

99.55

On the Uniform Permeability of all known Substances to the

Magnetic Influence, and the Application of the fact in Engineering and Mining, for the Determination of the Thickness of Solid Substances not otherwise Measurable. By the Rev. William SCORESBY, F.R. S. Lond. & Edin., Correspondent of the Institute of France, &c. &c. Communicated by the Author. Concluded from p. 334 of preceding Vol.

INVESTIGATION OF THE LAW OF THE DIRECTIVE POWER OF BAR

MAGNETS, AND THE EXTENT TO WHICH THE MAGNETIC IN-
FLUENCE MAY BE CARRIED FOR MEASURING DISTANCES.

2. The law of the directive power of bar-magnets, at different distances, was the next subject of investigation.

Coulomb, I believe, was the first to establish, by the test of satisfactory and consistent experiments, the previously assumed law, that the force of magnetic attraction and repulsion is in the inverse ratio of the squares of the distance. The application of this law to the investigation in hand served at once to verify the law, and to render the results of my experiments of general application. In regard to the comparison of distances, it appeared to me to be of considerable advantage to estimate all distances in lengths of the bar made use of, by which the results for any one bar became applicable to all other bars of a proportional form and quality, and state of magnetic energy. And such, therefore, with a certain modification, afterwards found to be necessary, was the measure constantly adopted.

Placing now the magnet in the direction of the east or west point of the compass, or at right angles to the magnetic meridian, I proceeded to ascertain experimentally the deviations produced, either by the same pole constantly, or by the mean of the action of each pole alternately, first at the distance of one length, and then successively at other distances to the extent of ten lengths of the bar.

Preparatory, however, to a general application of the results thus obtained, it will be useful to ascertain by calculation the actual force exerted by any magnet on a compass at different distances, according to the above law of attractions.

VOL. XIII. NO. XXV.- JULY 1832.

G

When the bar is placed in the prescribed position, with the north pole at the distance of one length from the compass, then the action of the south pole, tending to counteract that of the north, will be in the inverse relation with the nearest pole of 22 to 12. That is, if the force of the nearest pole be called 1, then the force of the remote pole will be inversely as 4 or th, which being in the contrary direction to that of the nearest pole, redụces its action to ths. In like manner, at two lengths of the bar, the force of the nearest pole being now inversely as the square of 2 or th is reduced by the remote pole at three lengths distance, the action of which is inversely as the square of 3 or th; hence -= , representing the actual influence or general resultant of the whole of the magnetic forces in the magnet acting upon the compass.

But we may obtain a general expression for all distances, either in lengths or fractions of lengths of the bar. Let F be the influence of the nearer pole at the distance a.

Fa? Then will

pod represent the influence at the distance x, and

Fa?

the counteracting influence of the remote pole at the dis

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X + 12

2

2

2 X

• X + 1

tance x +1.
Hence the resultant influence is,

2 x + 1
F a(
(-3) F a

F a

Q2.X + 1 As the force F, however, being the separate action of a pole not practicably separable, is not a quantity that can be immediately ascertained by experiment, this expression requires to be extended so as to connect it with the value of the assumed

force :

Let R be the resultant influence of a magnet on the compass at distance , and, in the first instance, let a be equal to x.

2ut1

2 a ti Then R = F a?

F al! at 1

2

-2

a + 1

- 2

a + 1

Therefore F = R : Hence,

2 a + 1 2 x + ]

a? a' + 12 2 x + 1

=R
x2
.X + 12 2a + 1

3.2.X + 1

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