in physiology, one of the secondary qualities of bodies, produced by fire, and opposed to cold.

Under the article fire, we considered the sun as the principal source of heat upon the earth's surface, and the confines of the earth and atmosphere; without this, all the bodies upon our globe would doubtless grow rigid, lifeless, and fixed. It is this that stirs within them, as the main spring of their actions. Hence vegetation and animalization are evidently promoted; and hence the ocean and the atmosphere continue in a fluid state.

Heat in us is properly a sensation, excited by the action of fire; or it is the effect of fire on our organs of feeling. Hence it follows, that what we call heat is a particular idea or modification of our own mind, and not anything existing in that form in the body that occasions it. Heat, says Mr Locke, is no more in the fire that burns the finger, than pain is in the needle that pricks it. In effect, heat in the body that gives it, is only motion; and in the mind, only a particular idea.

Heat in the hot body, according to 'S Graveshane, is an agitation of the parts of the body, made by means of the fire contained in it; by such an agitation a motion is produced in our bodies, which excites the idea of heat in our mind; so that heat in respect of us is nothing but that idea, and in the hot body nothing but motion. If such motion expel the fire in right lines, it gives us the idea of light; if in a various and irregular motion, only heat.

Heat, with respect to our sensations, or the effect produced on us by a hot body, is estimated by its relation to the organ of feeling; no object appearing to be hot, unless its heat exceed that of our body. Whence the same thing to different persons, or at different times to the same person, shall appear both hot and cold. The degree of heat is measured by the expansion of the air, or spirit in the thermometer.

| The several animals | Weight of each | Height of the blood in the tube from the jugular vein | Height of the blood in tubes fixed to arteries | Capacity of the left ventricle of the heart | Area of the orifice of the aorta | Velocity of the blood in the aorta | Quantities of blood equal to the weight of the animal, in what time | Weight of the blood frustrated by the left ventricle contracting | N° of pulses in a minute | Area of tranverse section of ascending aorta | Square inches | |---------------------|---------------|-------------------------------------------------|------------------------------------------|----------------------------------------|-------------------------------|-------------------------|---------------------------------|-----------------------------|-----------------------------|-----------------------------| | Man | 160 Pounds | On straining. | 7 6 | 1.659 | 0.4187 | 56.55 | 34.18 | 4.38 | 51.5 | 75 | | Horse 1st. | | | 3 | 3.318 | 113.3 | 17.5 | 9.36 | | | | | 2d. | | | 9 | 8 | | | | | | | | 3d. | | | 12 | 52 | 10 | 1.036 | 86.85 | 60 | 13.75 | 113.22 | 86 | 0.677 | 0.369 | | Ox | | | 1600 | | 12.5 | 1.539 | 76.95 | 88 | 18.14 | 38 | 0.912 | 0.84 | | Sheep | | | 91 | 5½ | 9 | 1.85 | 0.172 | 174.5 | 20 | 4.593 | 36.56 | 65 | 0.094 | 0.07 | 0.012 | 0.246 | 0.383 | | Doe | | | 4 | 2 | 9 | 0.476 | | | | | | Dogs 1st. | | | 52 | 0 | 6 8 | 1.172 | 0.196 | 144.77 | 11.9 | 4.34 | 33.61 | 97 | 0.106 | 0.041 | 0.034 | | 2d. | | | 24 | 5 | 7 2 8 | 1 | 0.185 | 130.9 | 6.48 | 3.7 | | | | | | 3d. | | | 18 | 5 | 4 8 | 0.633 | 0.118 | 130 | 7.8 | 2.3 | 19.8 | | | | | 4th. | | | 12 | 8 | 4 3 3 | 0.101 | 120 | 6.7 | 1.85 | 11.1 | | | | |

It It has been justly observed, by some of our modern philosophers, that actual or absolute heat, is to sensible or relative heat, the same as motion is to velocity; for absolute heat is nothing but the whole motion of all the parts of the ignited body; and sensible or relative heat, respects only the comparative velocity of the parts. Thus, equal bulks of mercury and water set in a sand heat, where the heat of the fire may be uniformly communicated to both, will acquire in equal times equal degrees of absolute heat; but the relative heat of the water, or that which is sensible to the finger, will be near 14 times as great as that of the mercury, because the water, having 14 times a less quantity of matter, will admit of velocity so much in proportion greater.

Again, if mercury and water have the same relative or sensible heat, that is, if both are heated in such a manner as to cause an equal ascent in the thermometer, then a quantity of mercury will heat 14 times as much water as the same quantity of water will do; or it will make the same quantity of cold water 14 times hotter than the same quantity of hot water can. All which is easy to be shewn by experiment, and abundantly proves, that heat and fire are wholly owing to the velocity of the parts of the heated or ardent body: on which theory the various phenomena of heat, cold, fire, burning, &c., are rationally accounted for. For, first, we are to consider, that cold and heat are only comparative terms, or that the same thing may either be too hot, or too cold, according to the relative idea or standard-degree. Thus, ice or snow is said to be cold with respect to the finger, but ice or snow is warm if compared to a freezing mixture; so that if (as we commonly do) we make the hand or any part of the body the standard of heat or cold, or the term of comparison; then it is evident, 1. If the parts of any body, applied to the hand, have the same velocity as the parts of the hand, such a body we naturally pronounce is neither hot nor cold. 2. If the particles of the body have a greater velocity than those of the hand, we pronounce it warm, if the excess be small; but hot, if it be great. 3. If the velocity of the parts of the body applied be less than that in the hand, the sensation then is what we call cold, which also may be in various degrees. 4. Hence it is plain, there can be no such thing as absolute cold, but where the particles of matter are absolutely quiescent or at rest. 5. Hence also, there can be no such thing as absolute heat, because no degree of velocity can be assigned but a greater is still assignable, till we come to infinity, where we are quite lost, as having no idea of infinite velocity or heat.

From this theory of heat and cold we may conclude, that there is no body in nature whose parts are not in motion, in some degree. Since we have yet been able to discover no ultimate degree or limit of cold; and if any such thing were to be found in nature, it is likely that it would be as impossible to bear or endure the test, as any extreme degree of heat; both heat and cold naturally tending to destroy the animated part, or test, in the extreme degrees: cold, by destroying the vital motion, and fixing the part rigid and inflexible; but heat, by putting the parts into too great an agitation, causing a greater velocity of the fluids, and dissipation and a force of tension in the solids beyond what the natural state of the body can bear; and therefore it will inevitably destroy it.

the animal economy, known by the several names of natural heat, vital heat, innate heat, and animal heat, is commonly supposed to be that generated by the attrition of the parts of the blood, occasioned by its circulatory motion, especially in the arteries.

To what organs, or operations, the heat of the human body, and other animal bodies, is owing, is hitherto extremely doubtful. The opinions that at present prevail are, 1. That the heat of animal-bodies is owing to the attrition betwixt the arteries and the blood. 2. That the lungs are the fountain of this heat. 3. That the attrition of the parts of the solids on one another produce it. 4. That it is owing to the mechanical attrition of the particles of our fluids. To which opinions Dr Stevenson of Edinburgh added a 5th, viz. That whole process by which our aliment and juices are constantly undergoing some alteration.

The reasonings in favour of these several opinions may be seen at large, as laid down by the above-mentioned author in an essay on the cause of animal-heat, in the Medical Essays, vol. vi. The chief arguments in favour of the first opinion, are, that if an artery is tied, or cut, the part to which it goes turns cold; and on the ceasing of the pulsation of the arteries, cold and death follow. An increase of heat attends a brisk circulation, and a languid circulation is accompanied with a small heat. One who burns in a fever, or is hot with exercise, has a full and frequent pulse. In cold faintings, chlorosis, &c., the pulse is small and slow. To these they add, that the thermometer shews the arterial blood to be a little hotter than that of the veins.

This is accounted for from the conical figure of the arteries, from their fluxes and branches into exquisitely small capillaries; whence the resistance, and consequently the attrition, must be great, from the number, strength, and elasticity of their coats, from the propelling power of the heart, and their strong resistance. From all these it is inferred, that the particles of blood perpetually getting new motions, directions, and rotations, are attenuated, condensed, have their angles grinded off, and are made homogeneous: hence, it is said, follows the fluidity, red colour, and heat of the mass, which is here perfected.

The second opinion is, that the lungs are the fountain of heat in the human body. All that has been said for the blood's being heated in the arteries is advanced to prove this hypothesis, with considerable additions, viz. that in the lungs the blood-vessels everywhere attend, divide, and subdivide, along with the ramifications of the wind-pipe; and as these are perpetually changing their situation and form, becoming longer or shorter, making more acute or more obtuse angles, angles, so must the concomitant blood-vessels every moment make new angles, and give the blood new directions; that at last it enters into an exquisitely fine network, spreads everywhere on the vastly thin air-vehicles, where these air-bladders are perpetually changing their angles, points of contact, their form, volume, interstices, and so forth. From these and the elasticity of the air, and weight of the atmosphere, the blood is said to be churned, pressed backward and forward, broken and kneaded together, dissolved and condensed, made red and hot in respiration.

The third opinion is, that the cause of the animal heat is owing to the action of the solid parts upon one another. The reason in support of this opinion, is, that the heart and arteries move most; thence that it is natural to think, that the heat should be owing to this motion.

The fourth opinion is, the mechanical attrition of the particles of the fluids upon one another. Dr Stevenson observes, that those who support this hypothesis, must not only suppose that mechanical attrition begets heat, but begets itself without diminution; that they must not only shew what sets this attrition going, but what maintains it, because all mechanical force perpetually decreases in a resisting medium; in short, that they must shew the possibility of a perpetuum mobile, the impossibility of which they themselves demonstrate.

The fifth opinion is, what Dr Stevenson calls the animal process, or that process by which our aliment and fluids are perpetually undergoing some alteration. This process, according to that writer, may be one sui generis, somewhat of a middle nature betwixt fermentation and putrefaction; and he thinks it comes so near to the latter, that he chooses to call it by that name. In putrefaction, which is a most powerful dissolvent of bodies, the intestine action of their minute particles creates, collects, or some way or other is the cause or means of heat. The doctor thinks it probable that this process is constantly carried on in all our juices, especially where there is blood; and this is chiefly in the veins, so that the blood is both the fountain of heat and the first spring and motion.

The late Dr Mortimer, in the Philos. Trans. no. 476. gives it as his opinion, that the heat of animals is explicable from the phosphorus and air they contain. Phosphorus exists, at least in a dormant state, in animal fluids; and it is also known, that they all contain air; it is therefore only necessary to bring the phosphoreal and aerial particles into contact, and heat must of consequence be generated.