It is pretty obvious that experiments upon the human subject cannot be pushed far enough to ascertain the highest amount of temperature that can he acquired under the influence of exposure to air of excessively high tempera ture. To judge of this analogically, recourse must be had to warm-bloode4 animals of the two classes, Mammalia and "Birds. Messrs. Delaroche and Berger consequently exposed different species of Mammalia and Birds to dry hot air of different temperatures, from to 93°, 75 c. to F.), leaving them im mersed till they died. The whole of the ani mals that were made subjects of experiment, in spite of diversity of class and species, and of the varieties of temperature to which they were exposed, had gained an increase of tem perature nearly equal at the moment of their death. The limits of the variations being be tween the terms 6°, 25 and 18 c., the amount of difference did not exceed 93 c. which is a very triflingquantity. It may therefore be inferred that man and the warm-blooded animals cannot, under the influence of exposure to dry air of excessively high temperature, have the heat of their body raised during life to a greater extent than from 7° to c. The temperature of the body being increased to this extent becomes fatal. It is in fact only attained at the moment of dissolution ; perhaps death has virtually taken place before it is attained.
We have seen that Franklin observed the temperature of his body to be lower than that of the air on a very hot day. Such a circum stance is rare in what may be called natural con ditions as regards man and the warm-blooded animals ; inasmuch as it rarely happens that the temperature of the air surpasses that of their bodies generally. The case is different, however, as regards the cold-blooded tribes. It is not at all necessary that the temperature of the air be very high to afford opportunities of observing the phenomenon in question among cold-blooded animals. This was ob served for the first time by Sir Charles Illagden in a frog, which on a summer's day, when the heat was by no means excessive, he observed to be lower in temperature than the surrounding air. A fact of this kind could not remain isolated and unconnected with others. Accord ingly we observe among the experiments of Dr. Davy such facts as the following :—The temperature of the atmosphere being c. (90° F.), that of a tortoise was only 4 (85° F.). The air marking 7 (80° F.), a frog indicated 25° (77° F.). The air being at 28°, 3 (83° F.), the blatta orientalis was at 9 F.). The air at 26°, 19 c., (79°,5 F.), a scorpion was at 3 F.). It is therefore apparent that the phenomenon is general among animals with cold blood ; that during the highest heats of summer, the tem perature still falling short of excessive, the heat of their bodies is below that of the air. There is thus a limit of summer temperature which separates two orders of phenomena relative to the temperature of cold-blooded animals.
Starting from a mean temperature of the air, that of cold-blooded animals, the vertebrate as well as the invertebrate tribes, is superior to this mean, only varying in this respect within the narrow limits of from a few fractional parts of a degree to about four degrees centigrade, until the air attains the summer heat. Towards this limit the differences decrease, and the term or c. to F.) attained, they become nil. The inverse phenomenon is also observed : the temperature of the greater num ber is inferior to that of the air, and the dif ferences go on increasing with the rise in temperature of the external air.
These phenomena are of great interest in themselves, but of still greater from the light they cast on questions of a similar kind relative to man and the warm-blooded tribes of crea tion. The slight evolution of heat by the cold blooded animals rendering their condition more simple, allows us to appreciate distinctly the influence of external causes.
We now proceed to Ueat of a third condition influencing temperature, namely, Evaporation.—The fluids so far surpass the solids in the bodies of animals that they cer tainly constitute the larger portion of their masses ; and, further, the exterior surface of animal bodies generally is extremely porous. Animals are consequently subjected to the ordinary physical laws of evaporation. It is very long since, in addition to the sweat or visible perspiration, the existence of an invisible perspiration has been recognized. The latter is owing in great part to the effects of evaporation. Now evaporation cannot take place without the occurrence of cooling or loss of temperature in the ratio of the quantity of vapour formed. Without keeping this cause of refrigeration in view, we should fall into serious mistakes in estimating the heat of animals. If, for ex ample, we would compare the heat of two animals, which, unwittingly to the observer, should be under different conditions of eva poration, we should deceive ourselves greatly in regard to their respective temperatures.
It is even so with reference to another fact bearing upon temperature, which is oRen forced on the attention, and which has almost always led inquirers into error. There are many ani mals among the inferior classes of the Invetie brata, which tried by the thermometer exhibit no difference in temperature from that of the surrounding air. These creatures do not, con sequently, appear to have any Eiculty of pro ducingheat. But in the mere fact of their main raining the temperature of the air about them, an inherent capacity to produce heat is apparent. Did they evolve no caloric, they would fall below the temperature of the air, in conse quence of the evaporation which goes on from the surface of their bodies. They must of necessity produce as much as is necessary to repair the loss which takes place from this cause.