r 



EVAPORATION. 173 



not understood by us, by which heat is generated in the system, and is con- 

 tinually given out by the body. If any cause withdraws heat faster from the 

 body than it is thus produced, a sensation of cold is felt ; and if, on the con- 

 trary, the heat be not withdrawn as fast as it is generated, the body becomes 

 unduly warm. A balance should, therefore, as much as possible, be maintain- 

 ed between the natural power of the body in the production of heat, and the ] 

 faculty of receiving that heat in surrounding objects. In cold weaiher all sur- 

 rounding objects being at a much lower temperature than the body, have a 

 tendency to receive heat faster than the body can supply it, and in this case, 

 artificial sources of external heat are sought, by which the temperature of sur- 

 rounding objects may be raised, so as to accommodate themselves to the ani- 

 mal system. In very hot weather, on the contrary, the temperature of surround- 

 ing objects is so near the temperature of the body, that the heat produced in the 

 system is not received with sufficient facility to keep the body sufficiently cool. 

 In this case, artificial means of keeping down the temperature of the body are 

 necessarily resorted to. 



If the clothes which cover the body are damp, the moisture which they 

 contain has a tendency to evaporate by the heat communicated to it by the 

 body. 



In fact, the body, in this case, is circumstanced exactly in the same manner 

 as the bulb of a thermometer, already described, surrounded by a damp sponge, 

 in which case we saw that the mercury rapidly fell. The heat absorbed in i| 

 the evaporation of the moisture contained in the clothes must be, in part, sup- 

 plied by the body, and will have a tendency to reduce the temperature of the 

 body in an undue degree, and thereby to produce cold. The effect of violent 

 labor or exercise is to cause the body to generate heat much faster than it would 

 do in a state of rest. Hence we see why, when the clothes have been ren- 

 dered wet by rain, or by perspiration, the taking of cold may be avoided by 

 keeping the body in a state of exercise or labor until the clothes can be changed, 

 or till they dry on the person ; for in this case, the heat carried off by the 

 moisture in evaporating is amply supplied by the redundant heat generated by 

 labor or exercise. 



A damp bed, however, is an evil which cannot be remedied by this means, 

 the object of bed-clothes being to check the escape of heat from the body, so 

 as to supply at night that warmth which may be obtained by exercise or labor i 

 during the day. This end is not only defeated, but the contrary effect produ- ' 

 ced, when the clothes by which the body is surrounded, contain moisture in 

 them. The heat supplied by the body is immediately absorbed by this mois- 

 ture, and passes off in vapor ; and this effect would continue until the clothes 

 were actually dried by the heat of the body. 



A damp bed may be frequently detected by the use of a warming-pan. 

 The introduction of the hot metal causes the moisture of the bed-clothes to be 

 immediately converted into steam, which issues into the open space in which 

 the warming-pan is introduced. When the warming-pan is withdrawn, this 

 vapor is again partially condensed, and deposited on the surface of the sheets, 

 the dampness will be then distinctly felt, a film of water being, in fact, deposited 

 on their surface. 



The danger of leaving damp or wet clothes to dry in an inhabited apartment, < \ 

 and more especially in a sleeping-room, will be readily understood from what > 

 has been just explained. The evaporation which takes place in the process ' 

 of drying causes an absorption of heat, and produces a corresponding depression 

 i of temperature in the apartment. 



A striking example of the effects of cold produced by evaporation is exhibited in 

 ! an experiment contrived by Dr. Wollaston, and made with an instrument which 



