The mere temperature of the blood in each group is insufficient for our knowledge of their distinctive characters. We [p307] further find them characterised by a consumption of air in union with their heat, so as to unite these two functions, and thus render them dependent upon the same organs. Dr. Edwards has further shown, that from birth to maturity the production of heat goes on increasing with the consumption of air. And thus age (as well as the seasons) has been shown to be a modifier of animal heat; for, as the hot season advances, the consumption of air becomes diminished, and when the cold sets in, it increases; and this decrease and increase are accompanied by corresponding developements of heat. In cases of fainting, of hysteric and asthmatic fits, the principle here laid down, as to the balance between the air consumed and heat, is instinctively acted upon by the most ignorant persons, who open all the doors and windows to admit cold air, and dash cold water in the patient’s face. The addition or continuance of heat increases the affection. The application of cold produces instant relief. The state of asphyxia is relieved, the senses return, the pulse beats at the wrist, and the respiration goes on naturally. The _cooling_ renders the air, unfit before, fitted for the purposes of life. The effects of temperature upon the respiratory movements are indicated also in those constitutional changes which diminish the production of heat and the consumption of air. Organic affection of the heart or lungs may produce this change, which entails the necessity of a change of climate, or an alteration of temperature artificially, to restore the balance between the air and the animal heat. A very elaborate and complete argument, and series of experiments, are devoted to the subject of _transpiration_, and the effect upon it of the influence derived from repose of the body and sleep, by the air’s motion or stillness, and by the pressure of the atmosphere. We have, however, pursued the interesting points touched upon so far as to render it impossible to enter at present upon this portion of the work. The importance of the subject demands a fuller investigation and report than we have now room for; and we must, therefore, defer it to another opportunity. [p308] _Experiments on_ THOUGHT. By a Correspondent.