Page:Popular Science Monthly Volume 10.djvu/679

Rh of tissue, whether this increase was the consequence of increased nutrition, or of muscular exertion. We have experimented upon men at rest and at work, and we have found that on a day of rest they usually evaporated through lungs and skin about two pounds only during twenty-four hours, and on a day of hard work 4 pounds of water. In the first instance, about 2,016 caloric units, in the second, 4,480 had to leave the body in consequence of evaporation.

This explains to you how it can be that even with the hardest work our blood will not become warmer, but sometimes even cooler. The last observation has been made quite recently in mountaineering expeditions. Prof. Lortet, of Lyons, found, when he made an ascent of Mont Blanc, that the temperature in his mouth and armpit was less than normal, and became normal only when he was at rest. On such high mountains the lessened pressure of the atmosphere favors the peripheric circulation, there is a rush of water to the surface, and its evaporation takes place more readily, and increases with the altitude. At great heights persons in a balloon constantly complain of great dryness in the mouth.

Profs. Voit, Recknagel, and myself, are just now occupied in investigating the economy of animal heat, and we have found that after six hours' hard work the person leaves the apparatus in a cooler condition than when he went in, or after he had been at rest in the apparatus for the same space of time. Of course, the ventilation of the apparatus must work well, and send per hour about 11,100 gallons or 1,800 cubic feet of air through the chamber, else less water and less heat depart by evaporation.

You see what powerful means of cooling our body we have in the increase of our peripheric circulation, and consequent evaporation, at a time when the other routes are not open sufficiently but you see also how dangerous this means can become, if it is employed at a time when considerable quantities of heat depart on the other routes. If, heated and damp, you enter suddenly a cold space, where radiation increases at once, and a good deal of heat is also yielded by conduction to the cold air, you are in great danger of contracting an illness by the abnormal losses of heat, and the violent and sudden changes in the circulation. But if you undergo such changes slowly and gently, the three routes open themselves harmoniously. Our organism is a faithful and clever servant, who helps himself and his master, provided he is not hurried and ill-treated. When I come to speak of ventilation, I shall not forget to tell you of currents of air, called draughts.

The third route, that of conduction, by which we give up heat to the air, is also of great importance, and must in some circumstances replace the two others to a considerable degree. As long as our body is warmer than the surrounding air, this air gets warmer at every point of contact with our body, but at the same time lighter, and as