Page:Popular Science Monthly Volume 10.djvu/677

Rh within a cooler atmosphere. Such bodies lose their heat in three different ways: 1. Radiation. 2. Evaporation. 3. Conduction. This triple arrangement is of great advantage for the heat-department of our organism, inasmuch as the existence of these different routes allows of a delicate regulation—that, for instance, which we lose in a given case by radiation can be made up by diminution of loss through the other routes, and vice versa. The losses by radiation and by conduction are the most constant under equal conditions, and evaporation of water is the principal means for equalizing differences resulting from varying production of heat or from difficulties of the two other routes. Allow me to illustrate this by drawing your attention to some every-day phenomena.

You arrive, for instance, in an hotel after a journey during a cold winter's day, and have at once a fire lit in your room. Let the fire be ever so bright, the thermometer even rise to a reassuring degree—you must stick to the fireplace; the room does not get warm. If you continue to live in the same room and have the fire kept in, it will by-and-by get comfortable even if the thermometer in the room should stand lower than on the first day, and you will think quite correctly that the room wanted time to get warmed through and through. Before that had taken place, the loss of heat by increased radiation into the incompletely warmed space made itself sensibly felt in the heating department of your body. Radiation is the stronger the greater the difference of temperature between the two bodies. Surrounded as you are in a room not only by air, say of 68° Fahr., but also by walls, furniture, etc., which stand, perhaps, at 38° to 40°, your body radiates its heat particularly toward these colder objects, till they also get warmer. For a room to be warm, it must get warmed with all which it contains.

Let us now look at the contrary case, when our loss by radiation is uncommonly limited; for instance, in a thronged room on a warm and moist day. You feel an oppressive heat, and scarcely trust the thermometer, which marks only 68°, perhaps your favorite temperature. Quite correctly, you accuse the throng of people, and retire into an adjoining room, where you find the air delicious, and seem to receive new life; there, again, the thermometer is suspected by you, as it is scarcely different from its colleague inside; and if the air in the two rooms were to be examined eudiometrically, the difference would be so small as to leave unexplained the difference in your sensations. What, then, causes this difference? It is the suppression of your lateral radiation of heat, when you are in the midst of other equally warm bodies; your receipts and expenditure by radiation cover each other, and the cooling of the individual limits itself to the two other routes, conduction by the air moving round him, and evaporation of water from his surface. On such occasions the pores of your skin pour forth a quantity of water, and, at the same time, you