That may be one reason: but I should think that the principal cause of the heat experienced in fevers, is, that there is no vent for the caloric which is generated in the body. One of the most considerable secretions is the insensible perspiration; this is constantly carrying off caloric in a latent state; but during the hot stage of a fever, the pores are so contracted, that all perspiration ceases, and the accumulation of caloric in the body occasions those burning sensations which are so painful. EMILY. This is, no doubt, the reason why the perspiration that often succeeds the hot stage of a fever affords so much relief. If I had known this theory of animal heat when I had a fever last summer, I think I should have found some amusement in watching the chemical processes that were going on within me. CAROLINE. But exercise likewise produces animal heat, and that must be quite in a different manner. MRS. B. Not so much so as you think; for the more exercise you take, the more the body is stimulated, and requires recruiting. For this purpose the circulation of the blood is quickened, the breath proportionably accelerated, and consequently a greater quantity of caloric evolved. CAROLINE. True; after running very fast, I gasp for breath, my respiration is quick and hard, and it is just then that I begin to feel hot. EMILY. It would seem, then, that violent exercise should produce fever. MRS. B. Not if the person is in a good state of health; for the additional caloric is then carried off by the perspiration which succeeds. EMILY. What admirable resources nature has provided for us! By the production of animal heat she has enabled us to keep up the temperature of our bodies above that of inanimate objects; and whenever this source becomes too abundant, the excess is carried off by perspiration. MRS. B. It is by the same law of nature that we are enabled, in all climates, and in all seasons, to preserve our bodies of an equal temperature, or at least very nearly so. CAROLINE. You cannot mean to say that our bodies are of the same temperature in summer, and in winter, in England, and in the West-Indies. MRS. B. Yes, I do; at least if you speak of the temperature of the blood, and the internal parts of the body; for those parts that are immediately in contact with the atmosphere, such as the hands and face, will occasionally get warmer, or colder, than the internal or more sheltered parts. But if you put the bulb of a thermometer in your mouth, which is the best way of ascertaining the real temperature of your body, you will scarcely perceive any difference in its indication, whatever may be the difference of temperature of the atmosphere. CAROLINE. And when I feel overcome by heat, I am really not hotter than when I am shivering with cold? MRS. B.