Under circumstances of moderate nourishment and temperature, the tadpoles under water did not complete their changes but in a very partial and protracted manner, while the greater portion made no change. The great difference in the circumstances of the experiments seems to have been the access to the air of those which went through their transformations as usual. Exclusion from light made no difference in the results, and these were solely influenced by occasional renewal of air from pulmonary respiration.
These animals possessing a double respiration, cutaneous and pulmonic, that is, absorbing air from the water around them and inhaling it from the atmosphere on its surface, renders these facts highly curious. Fish possess only the means of aquatic respiration, and the influence of temperature was tried upon them submersed in water deprived of its air by previous boiling, the heat being varied from 0° to 40°. The fish died quicker under these circumstances than the frog species in the same situation; but their lives were prolonged more in the descent of the thermometer than during its elevation, as also occurred with the experiments on frogs and salamanders; and, in both cases, the younger the animal, the less it could resist the higher temperatures. At 40° the young animals only survived about two minutes, and the adults many more.
Fish were also submersed in closed vessels of aërated water, and, by varying the temperature and the quantities of water, the duration of their lives was augmented in proportion to the increased volume of the liquid, the temperature remaining the same; but when these experiments were conducted in open vessels, the contact with the atmosphere altered the phenomena. At 20°, a small fish expired in four hours; and when the temperature was lowered to 10° or 12°, the same sort of animal lived several days; and when the water was kept clean by being changed every twenty-four hours, the fish lived indefinitely.
It is known that fish rise periodically to the surface of the waters to respire; and Dr. Edwards discovered that they did so when they have reduced the properties of the air [p298] dissolved in the water to a lower standard than is requisite for the proper aërification of their blood; thus renewing their supply of oxygen.
The functions of this class of animals have always been obscure; and their phenomena are different from those of others. Different species of fish die at various periods when deprived of water, some in a few minutes, others in a few hours; and it appears that their dissolution arises not so much from incapability of atmospheric respiration (for the experiments of Sylvester prove that they can respire pure air), as from the different state of the air.
Some experiments on lizards, snakes, and turtles conclude the researches among cold-blooded animals. The skins of these, like those of the frogs and salamanders, received vivifying influence from the air, mainly acting, in conjunction with pulmonary respiration, to promote their existence. Snakes and turtle, their pulmonary respiration being insulated, from their skins being guarded from atmospheric influence, were found alive; but the lizards died in a few hours, when the vivifying contact of the air was removed from their bodies, and they breathed only by their mouths. Animals naturally defended by scales transpire much less than such as have their skins free. Thus frogs, toads, and salamanders yielded more by perspiration than lizards, snakes, and turtle, in a given time; and the porosity of the skin of course regulates the facility of transpiration in all cases.
With these experiments and remarks, Dr. Edwards concludes the second part of his researches. The third part includes animals of warm blood, in which will be found some curious and interesting remarks on the heat of young animals compared with that of adults.
Dr. Edwards refutes the common notion of young animals being necessarily hotter than adults. The heat of young puppies was very near that of the parent, or one or two degrees less, but this variation was not constant. Some new-born kittens and rabbits were also subjected to similar trials, and the results led to a conclusion that the temperature of young animals is less than that of adults.
According to these experiments, the power of resisting the cooling influence of the air acquires force as the animal grows up; and those examples related, in which artificial covering was adopted, show that nudity is not the only cause of the reduction of heat, which is, in fact, more referrible [p299] to their infantile constitution. At first the sucking animal shows little variation from the parent temperature; then this becomes more and more reduced, and about the fifteenth day it is a degree or two below the mother.
Birds, which are warmer than mammiferæ, were next made the objects of experimental inquiry, and the young recently hatched exhibited a lower temperature than the grown birds. After removal from the shelter of their nests into a mild atmosphere of 17°, in one hour they cooled down from 36° to 19°, thus losing 17° in an hour. At an elevation of 22° the same results were obtained, and they cooled down to within one degree of the surrounding air. The plumage of birds has little if any influence upon their temperature. The production of heat lies within, and not on the surface of the animal; and if it be strongly developed, the removal of natural coverings does not influence the heat produced; and if it be weak, their addition will not prevent cooling. Birds recently escaped from the shell cooled to within two degrees of the air, whereas the unplumed adult birds scarcely lost one degree.