LECTURE x.x.xI.

_Of Animal Heat._

Since all animals, and especially those that have red blood, are much hotter than the medium in which they live, the source of this heat has become the subject of much investigation; and as the most probable theory is that of Dr. Crawford, I shall give a short detail of the reasons on which it is founded.

Having, with the most scrupulous attention, ascertained the _latent_, or, as he calls it, the _absolute_ heat of blood, and also that of the aliments of which it is composed, he finds that it contains more than could have been derived from _them_. Also finding that the absolute heat of arterial blood exceeds that of venous blood, in the proportion of 11 to 10, he concludes that it derives its heat from the air respired in the lungs, and that it parts with this _latent_ heat, so that it becomes sensible, in the course of its circulation, in which it becomes loaded with phlogiston, which it communicates to the air in the lungs.

That this heat is furnished by the _air_, he proves, by finding, that that which we inspire contains more heat than that which we expire, or than the aqueous humor which we expire along with it, in a very considerable proportion; so that if the heat contained in the pure air did not become latent in the blood, it would raise its temperature higher than that of red-hot iron. And again, if the venous blood, in being converted into arterial blood, did not receive a supply of latent heat from the air, its temperature would fall from 96 to 104 below 0 in Fahrenheit's thermometer.

That the heat procured by combustion has the same source, viz. the dephlogisticated air that is decomposed in the process, is generally allowed; and Dr. Crawford finds, that when equal portions of air are altered by the respiration of a Guinea pig, or by the burning of charcoal, the quant.i.ty of heat communicated by the two processes is nearly equal.

The following facts are also alleged in favour of his theory. Whereas animals which have much red blood, and respire much, have the power of keeping themselves in a temperature considerably higher than that of the surrounding atmosphere, other animals, as _frogs_ and _serpents_, are nearly of the same temperature with it; and those animals which have the largest respiratory organs, as birds, are the warmest; also the degree of heat is in some measure proportionable to the quant.i.ty of air that is respired in a given time, as in violent exercise.

It has been observed, that animals in a medium hotter than the blood have a power of preserving themselves in the same temperature. In this case the heat is probably carried off by perspiration, while the blood ceases to receive, or give out, any heat; and Dr. Crawford finds, that when an animal is placed in a warm medium the colour of the venous blood approaches nearer to that of the arterial than when it is placed in a colder medium; and also, that it phlogisticates the air less than in the former case; so that in these circ.u.mstances respiration has not the same effect that it has in a colder temperature, in giving the body an additional quant.i.ty of heat; which is an excellent provision in nature, as the heat is not wanted, but, on the contrary, would prove inconvenient.

LECTURE x.x.xII.

_Of Light._

Another most important agent in nature, and one that has a near connexion with heat, is _light_, being emitted by all bodies in a state of ignition, and especially by the sun, the great source of light and of heat to this habitable world.

Whether light consists of particles of matter (which is most probable) or be the undulation of a peculiar fluid, filling all s.p.a.ce, it is emitted from all luminous bodies in right lines.

Falling upon other bodies, part of the light is _reflected_ at an angle equal to that of its incidence, though not by impinging on the reflecting surface, but by a power acting at a small distance from it.

But another part of the light enters the body, and is _refracted_ or bent _towards_, or _from_, the perpendicular to the surface of the new medium, if the incidence be oblique to it. In general, rays of light falling obliquely on any medium are bent as if they were attracted by it, when it has a greater density, or contains more of the inflammable principle, than the medium through which it was transmitted to it. More of the rays are reflected when they fall upon a body with a small degree of obliquity to its surface, and more of them are transmitted, or enter the body, when their incidence is nearer to a perpendicular.

The velocity with which light is emitted or reflected is the same, and so great that it pa.s.ses from the sun to the earth in about eight minutes and twelve seconds.

Rays of light emitted or reflected from a body entering the pupil of the eye, are so refracted by the humours of it, as to be united at the surface of the retina, and so make images of the objects, by means of which they are visible to us; and the magnifying power of telescopes or microscopes depends upon contriving, by means of reflections or refractions, that pencils of rays issuing from every point of any object shall first diverge, and then converge, as they would have done from a much larger object, or from one placed much nearer to the eye.

When a beam of light is bent out of its course by refraction, all the rays of which it consists are not equally refracted, but some of them more and others less; and the colour which they are disposed to exhibit is connected invariably with the degree of their refrangibility; the red-coloured rays being the least, and the violet the most refrangible, and the rest being more or less so in proportion to their nearness to these, which are the extremes, in the following order, violet, indigo, blue, green, yellow, orange, red.

These colours, when separated as much possible, are still contiguous; and all the shades of each colour have likewise their separate and invariable degrees of refrangibility. When separated as distinctly as possible, they divide the whole s.p.a.ce between them exactly as a musical chord is divided in order to found the several notes and half notes of an octave.

These differently-coloured rays of light are also separated in pa.s.sing through the transparent medium of air and water, in consequence of which the sky appears blue and the sea green, these rays being returned, while the red ones proceed to a greater distance. By this means also objects at the bottom of the sea appear to divers red, and so do all objects enlightened by an evening sun.

The mixture of all the differently-coloured rays, in the proportions in which they cover the coloured image above mentioned, makes a _white_, and the absence of all light is _blackness_.

By means of the different refrangibility of light, the colours of the rainbow may be explained.

The distance to which the differently-coloured rays are separated from each other is not in proportion to the mean refractive power of the medium, but depends upon the peculiar const.i.tution of the substance by which they are refracted. The _dispersing power_ of gla.s.s, into the composition of which _lead_ enters, is great in proportion to the mean refraction; and it is proportionally little in that gla.s.s in which there is much alkaline salt. The construction of _achromatic telescopes_ depends upon this principle.

Not only have different rays of light these different properties with respect to bodies, so as to be more or less refracted, or dispersed, by them, but different sides of the same rays seem to have different properties, for they are differently affected on entering a piece of _island crystal_. With the same degree of incidence; part of the pencil of rays, consisting of all the colours, proceeds in one direction, and the rest in a different one; so that objects seen through a piece of this substance appear double.

At the surface of all bodies rays of light are promiscuously reflected, or transmitted.

But if the next surface be very near to it, the rays of one colour chiefly are reflected, and the rest transmitted, and these places occur alternately for rays of each of the colours in pa.s.sing from the thinnest to the thickest parts of the medium; so that several series, or orders, of colours will be visible on the surface of the same thin transparent body. On this principle coloured rings appear between a plane and a convex lens, in a little oil on the surface of water, and in bubbles made with soap and water.

When rays of light pa.s.s near to any body, so as to come within the sphere of its attraction and repulsion, an _inflection_ takes place; all the kinds of rays being bent _towards_, or _from_, the body, and these powers affecting some rays more than others, they are by this means also separated from each other, so that coloured streaks appear both within the shadow, and the outside of it, the red rays being inflected at the greatest distance from the body.

Part of the light which enters bodies is retained within them, and proceeds no farther; but so loosely in some kinds of bodies, that a small degree of heat is sufficient to expel it again, so as to make the body visible in the dark: but the more heat is applied, the sooner is all the light expelled. This is a strong argument for the materiality of light. _Bolognian phosphorus_ is a substance which has this property; but a composition made by Mr. Canton, of calcined oyster-sh.e.l.ls and sulphur, in a much greater degree. However, white paper, and most substances, except the metals, are possessed of this property in a small degree.

Some bodies, especially phosphorus, and animal substances tending to putrefaction, emit light without being sensibly hot.

The _colours_ of vegetables, and likewise their _taste_ and _smell_, depend upon light. It is also by means of light falling on the leaves and other green parts of plants, that they emit dephlogisticated air, which preserves the atmosphere fit for respiration.

It is light that imparts colour to the skins of men, by means of the fluid immediately under them. This is the cause of _tanning_, of the _copper colour_ of the North Americans, and the _black_ of the Negroes.

Light also gives colour to several other substances, especially the solutions of mercury in acids.

LECTURE x.x.xIII.

_Of Magnetism._

Magnetism is a property peculiar to iron, or some ores of it. The earth itself, owing probably to the iron ores contained in it, has the same property. But though all iron is acted upon by magnetism, _steel_ only is capable of having the power communicated to it.

Every magnet has two poles, denominated _north_ and _south_, each of which attracts the other, and repels that of the same kind with itself.

If a magnet be cut into two parts, between the two poles, it will make two magnets, the parts that were contiguous becoming opposite poles.

Though the poles of a magnet are denominated _north_ and _south_, they do not constantly, and in all parts of the earth, point due north or south, but in most places to the east or west of them, and with a considerable variation in a course of time. Also a magnet exactly balanced at its center will have a declination from an horizontal position of about 70 degrees. The former is called the _variation_, and the latter the _dipping_ of the magnetic needle.

A straight bar of iron which has been long fixed in a vertical position, will become a magnet, the lower end becoming a north pole, and the upper end a south one; for if it be suspended horizontally, the lower end will point towards the north, and the upper end towards the south. Also any bar of iron, not magnetical, held in a vertical position, will become a temporary magnet, the lower end becoming a north pole, and the upper end a south one; and a few strokes of a hammer will fix the poles for a short time, though the position of the ends be changed. Magnetism may likewise be given to a bar of iron by placing it firmly in the position of the dipping-needle, and rubbing it hard one way with a polished steel instrument. Iron will also become magnetical by ignition and quenching it in water in the position of the dipping-needle.

Magnetism acts, without any diminution of its force, through any medium; and iron not magnetical will have that power while it is in connexion with a magnet, or rather the power of the magnet is extended through the iron.

Steel filings gently thrown upon a magnet, adhere to it in a curious manner; and the filings, acquiring magnetism by the contact, adhere together, and form a number of small magnets, which arrange themselves according to the attraction of the poles of the original magnet. This experiment is made to the most advantage upon a piece of pasteboard, or paper, placed over the magnet.

Magnetism is communicated by the friction, or the near position, of a magnet to a piece of steel of a size less than it. For this reason a combination of magnetical bars will have a greater effect than a single one; and in the following manner, beginning without any magnetism at all, the greatest quant.i.ty may be procured. Six bars of steel may be rendered slightly magnetical by fixing each of them successively to an upright poker, and stroking it several times from the bottom to the top with the lower end of an old pair of tongs. If then four of these bars be joined, the magnetism of the remaining two will be much increased by a proper method of rubbing with them; and by changing their places, joining the strongest, and acting upon the weakest, they may all be made as magnetical as they are capable of being.

The strength of a natural magnet may be increased by covering its polar extremities with steel. This is called the _arming_ of the loadstone.

To account for the variation of the needle, Dr. Halley supposed the earth to consist of two parts, an external _sh.e.l.l_ and an internal _nucleus_, detached, and having a revolution distinct from it; and that the action of the poles of the sh.e.l.l and of the nucleus would explain all the varieties in the position of the needle. But others think that the cause of the magnetism of the earth is not _within_, but _without_ itself. One reason for this opinion is, that a magnet is liable to be affected by a strong aurora borealis; and another is, that the variation of the needle proceeds in such manner as supposes that the motion of the nucleus must be quicker than that of the sh.e.l.l of the earth; whereas, since it is most natural to suppose that motion was communicated to the nucleus by the sh.e.l.l, it would be slower.

Some idea of the quant.i.ty and the progress of the variation of the needle may be formed from the following facts.--At the Cape of Good Hope, when it was discovered by the Portuguese, in 1486, there was no variation, the needle there pointing due north; in 1622 it was about 2 degrees westward, in 1675 it was 8 W. in 1700 about 11 W. in 1756 about 18 W. and in 1774 about 21 W. In London, in 1580, the variation was 11 degrees 15 seconds E.; in 1622 it was 6 E. in 1634 it was 4 deg.

5 min. E. in 1657 it was nothing at all; in 1672 it was 2 deg. 30 min.

W. in 1692 it was 6 deg. W. in 1753 it was about 16 W. and at present it is about 21 W.