Part 13 (1/2)

LECTURE xxxI

_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 becoation; and as the ive 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 containsthat 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 fros, and that it parts with this _latent_ heat, so that it becomes sensible, in the course of its circulation, in which it becoiston, which it cos

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 hu with it, in a very considerable proportion; so that if the heat contained in the pure air did not becoher 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 coisticated air that is decoenerally 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 quantity of heat communicated by the two processes is nearly equal

The following facts are also alleged in favour of his theory Whereas animals which havetheher than that of the surrounding ats_ and _serpents_, are nearly of the saest respiratory organs, as birds, are the warree of heat is in some measure proportionable to the quantity of air that is respired in a given time, as in violent exercise

It has been observed, that animals in athemselves 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 isticates the air less than in the former case; so that in these circumstances respiration has not the sa the body an additional quantity of heat; which is an excellent provision in nature, as the heat is not wanted, but, on the contrary, would prove inconvenient

LECTURE xxxII

_Of Light_

Another ent in nature, and one that has a near connexion with heat, is _light_, being enition, 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 space, it is e upon other bodies, part of the light is _reflected_ at an angle equal to that of its incidence, though not by i at a sht enters the body, and is _refracted_ or bent _towards_, or _from_, the perpendicular to the surface of the new eneral, rays of light falling obliquely on any mediureater density, or contains h which it was transmitted to it More of the rays are reflected when they fall upon a body with a sree 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 hich light is ereat that it passes froht ht e 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 es of the objects, bypower of telescopes or , by means of reflections or refractions, that pencils of rays issuing froe, and then converge, as they would have done froer object, or froht 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 themore or less so in proportion to their nearness to these, which are the extrereen, yellow, orange, red

These colours, when separated as uous; 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 space 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 passing 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 bottohtened by an evening sun