We must anticipate a little the doctrines of an approaching article (Caloric), by explaining some terms essentially necessary to the proper comprehension of Dr. Crawford's system. If we suppose at this moment the existence of heat as a separate principle admitted, absolute heat is the real quantity of this principle; rela-tive heat that quantity only which is obvious to the senses, or can be measured by a thermometer. Thus, according to the common instance, a pound of water and as much calx of antimony have the same temperature to the thermometer; but the water contains four times as much heat as the calx. The capacities of bodies for retaining this principle, are also different and measured by the degrees of sensible heat in each, after being exposed to the same temperature. Thus, if water and mercury are exposed to the same heat for a given time, while the temperature of the water is raised one degree, that of the mercury is raised 28°. The capacity of water then to that of mercury, is as 28 to 1. These two qualities, absolute heat, and capacity for heat, are often confounded, and particularly by Dr. Crawford, who was not aware, that in the diaphoretic antimony the heat was really a component part of the calx.

In pursuance of the general distinction between absolute and relative heat, Dr. Crawford examined the arterial blood compared with water, and found it to be as 1.03 to 1.00. He consequently suspected, that the blood absorbed heat from the air in the lungs; and this idea was confirmed by the intimate connection between the increase of temperature and the frequency of respiration, as well as the extent of the respiratory organs. On pursuing the enquiry, he found that the absolute heat of atmospheric air was changed by passing through the lungs; and, in general, air contained absolute heat in the same proportion that it was adapted for respiration. On examining the state of the blood in the pulmonary vein and artery, he found the heat greater in the former than in the latter. The colour of the venous blood resembles more nearly that of arteries in a warm than in a cold atmosphere, for less heat is of course absorbed; and, in general, the heat absorbed by air is nearly the quantity produced by burning a wax taper, for the air is vitiated in nearly the same proportion by both processes. The absolute heat of different animal substances he ascertained to be as follows. Supposing water 1.0000, inflammable air was 21.4000, oxygen gas 4.7490, atmospherical air i pour

1.5500, carbonic acid gas 1.0454, azote 0.7936,arterial blood 1.0300, venous blood 0.8928, fresh cows' milk milk

0.9999, hide of an ox with the hair 0.7870, lungs of a sheep 0.7690, lean beaf 0.7400.

To apply these facts to the subject before us, he found that the absolute heat of pure air, at the common temperature of the atmosphere, was equal to 1550. The The heat of fixed air and aqueous vapour being one third less, pure air changed to the two latter, would give out 3 X 1550° = 4650°. Many causes to reduce this quantity, but it will be evident that a large proportion of heat must be absorbed by the blood, a- sensible heat is produced.

The capacity of the heat in venous blood, appears to that of arterial as about 23: 20. If venous blood be therefore converted to arterial, there will be this proportional loss of heat; but venous blood contains 1 and consequently the loss from the change of venous into arterial blood, would be very nearly 200°, if the deficiency were not supplied from the air. We now know, also, more clearly than at the period Dr. C ford wrote, that oxygen contains a considerable proportion of caloric, and its abstraction is of course connected with a diminution of this principle. As oxygen, therefore, disappears in respiration, heat is lost to our senses but recovered again in the increased capacity of the blood, after it has circulated through the lungs. The blood in circulation becomes replete with azote, and, of course, its attraction for oxygen is diminished.

Heat, therefore, escapes in every part of the circulating system, and supports an equable warmth; till the blood, returning to the lungs, again absorbs a fresh proportion of oxygen from the air, to be again partly separated for the support of animal life: we say partly, for the capacity of the remaining fluids being increased, a portion is absorbed, and becomes of these a component part.

This doctrine is recommended by its simplicity,its conformity to other appearances, and the ready appli-cation it affords to different phenomena; particularly

S s 2 the connection of animal heat with the extent of the respiratory organs, and the frequency of respiration. It explains also some other facts which require a more ample consideration.

The heat of animals, at whatever degree it may be placed, is uniform. We see that the lower the surrounding temperature is, the separation of the oxygen from the air will be more complete, and, of course, the separation of heat in the circulation. The arterial and venous blood will, as we have said, differ nearly in the same proportion in their colour. Thus the changes balance each other; and in warm countries, where putrefaction powerfully vitiates the air, breathing has a proportionably less effect.

This balance of the effects of heat in the air, and of the production of animal heat, goes further; for, when heat is increased beyond its due bounds, the same principle produces cold. Mr. Tillet found that a girl could live for sonic time in an oven heated to 220°; and Dr. Fordyce observed that a dog could live, with little inconvenience, in a heat of 260°; and he himself endured the heat of 230° for fifteen minutes, while the thermometer under his tongue pointed only to 100°. Dr. Crawford proved, that when a living and a dead frog were exposed to a great degree of heat, in air or water, the former acquired the heat of the surrounding temperature more slowly than the latter. These facts are readily explained from our author's system. It appears, from what we have observed, that the blood brings with it to the lungs such an increased capacity for containing heat, that if its temperature were not supported by the oxygen of the air, it would sink 200°: but, in great beats, this capacity is supplied, and a small proportion only is absorbed; in very high degrees of temperature, probably none: and when, from the changes produced by the circulation, this extraordinary proportion of heat is separated, as it will be by the more rapid increase of the animal process, the superabundant heat is lost in the aqueous vapour, and in the evaporation, or rather the change of that vapour into air. Thus we see, also, why the heat in putrid fevers is so considerable, and why it may even increase after death; for the putrid fluids, having a less capacity of heat, lose whatever they contained in consequence of their former capacity, and putrefaction, hastening on rapidly after death, indeed more rapidly than the heat can be carried off, occasions its apparent increase.