182 



CONDUCTION OF HEAT. 



one ; and a constant series of currents upward and downward is thus estab- 

 lished. The portion of the liquid which receives heat below, is thus continu- 

 ally mixed through the other parts, and the heat is diffused by the motion of 

 ihe particles among each other ; the same effect takes place in gases. If a 

 lower stratum be heated, it acquires a tendency to ascend to the higher, and the 

 colder strata descend. 



If, however, heat be applied to the highest stratum of the liquid, this effect 

 cannot ensue ; and it is found that, in this case, the particles maintaining their 

 mutual arrangement, the transmission of heat takes place in the same manner 

 as if the liquid were solid. In fact, the heat is, in this case, conducted through 

 the liquid. Liquids, in this manner, are observed to have extremely low con- 

 ducting powers ; so low that, for a long period, they were supposed to be alto- 

 gether incapable of conducting heat. They have been ascertained by experi- 

 ment, however, not to be altogether destitute of the power of conduction. 



Let a small quantity of spirits of wine be poured on the surface of water, 

 at the temperature of 32°, and let a thermometer be immersed in the water at 

 a small depth below the common surface of the water and spirits ; let the spir- 

 its be now inflamed and caused to burn on the surface of the water. After the 

 lapse of a considerable time the thermometer will show a very slight indication 

 of increased temperature, by the downward transmission of heat from the burn- 

 ing spirits. 



This, and other experiments of a like nature, are extremely difficult of man- 

 agement, and very uncertain in their results. It often happens that the eleva- 

 tion of the thermometer is caused by currents of the liquid produced by heat 

 conducted downward by the sides of the vessels containing the liquid. Al- 

 though the liquid itself may fail to conduct the heat downward, yet the vessel 

 containing it, having a better conducting power, will transmit the heat to in- 

 ferior strata of the liquid, and currents may thus, to a certain extent, be estab- 

 lished. An ingenious method of evading this difficulty was suggested by Mr. 

 Murray, who conducted the experiment in vessels composed of ice. The heat 

 received by the sides of the vessel was, in this case, expended in the liquefac- 

 tion of the ice, and had no tendency, therefore, to disturb the result of the in- 

 vestigation. 



The process of cooling, which a hot body undergoes when suspended in air, 

 is chiefly owing to the radiation of heat from its surface : but another cause 

 of the diminution of heat conspires with this. The particles of air in contact 

 with the surface of the body, receive heat from it, and thus becoming specifi- 

 cally lighter by their dilatation, ascend, and give place to others, on which a 

 like effect is produced. Thus heat is imparted, constantly, to fresh portions 

 of the air, and carried off by them. If a hot body be suspended in a liquid, 

 the process, as to its cooling, is altogether produced by this means, for in that 

 case no radiation takes place. 



The covering of wool and feathers, which nature has provided for the infe- 

 rior classes of animals, has a property of conducting heat very imperfectly, 

 and hence, it has the effect of keeping the body cool in hot weather, and warm 

 in cold weather. The heat which is produced by powers provided in the ani- 

 mal economy, within the body, has a tendency, when in a cold atmosphere, to 

 escape faster than it is generated, the covering being a non-conductor, intercepts 

 it, and keeps it confined. 



Man is endowed with faculties which enable him to fabricate, for himself, 

 covering similar to that with which nature has provided other animals. Clothes 

 are generally composed of some light, non-conducting substances, which pro- 

 tect the body from the inclement heat or cold of the external air. In summer, 

 clothing keeps the body cool, and in winter, warm. Woollen substances are 



