"How powerful, indeed, must the resistance appear which the vital force supplies to leaves charged with oil of turpentine or tannic acid, when we consider the affinity of oxygen for these compounds! "This intensity of action, or of resistance, the plant obtains by means of the sun's light; the effect of which in chemical actions may be, and is, compared to that of a very high temperature, (moderate red heat.) "During the night, an opposite process goes on in the plant; we see then that the constituents of the leaves and green parts combine with the oxygen of the air--a property which in daylight they did not possess. "From these facts we can draw no other conclusion but this: that the intensity of the vital force diminishes with the abstraction of light; that, with the approach of night, a state of equilibrium is established; and that, in complete darkness, all those constituents of plants which, during the day, possessed the power of separating oxygen from chemical combinations, and of resisting its action, lose their power completely. "A precisely similar phenomenon is observed in animals. "The living animal body exhibits its peculiar manifestations of vitality only at certain temperatures. When exposed to a certain degree of cold, these vital phenomena entirely cease. "The abstraction of heat must, therefore, be viewed as quite equivalent to a diminution of the vital energy; the resistance opposed by the vital force to external causes of disturbance must diminish, in certain temperatures, in the same ratio in which the tendency of the elements of the body to combine with the oxygen of the air increases."