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 trary, the heat be not withdrawn as fast as it is generated, the body becomes 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

 svstem is not received with sufficient facility to keep the body sufficiently coo" 

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

 necessarily resorted to. 



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

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

 body. 



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

 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 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 th 

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

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

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

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

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

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

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

 labor or exercise. 



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

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

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

 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 i 

 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 clothe 

 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 b 

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

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 When the warming-pan is withdrawn, thi 

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

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

 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 



of temperature in the apartment. 



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

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



