2G CONDUCTION OF HEAT. 



In the case of solid substances, the phenomenon of the con- 

 duction of heat is so simple and familiar, that little need be said 

 on the subject. Different solid substances vary exceedingly 

 from each other in their power to conduct heat. Dense or 

 heavy substances are generally good conductors, while light and 

 porous bodies conduct heat imperfectly. Hence, the universal 

 use of substances of the latter class for the purposes of clothing. 

 Count Rumford observed that the finer the fabric of woollen 

 cloth is, the more imperfectly does it conduct. The down of 

 the eider-duck appears to be unrivalled in this respect. Bad 

 conductors are also the most suitable for keeping bodies cool, 

 protecting them from the access of heat. Hence, to preserve ice 

 in summer, we wrap it in flannel. Among good conductors of 

 heat, the metals are the best. The relative conducting power 

 of several bodies is expressed by the numbers in the following 

 table, from the experiments of Despretz. 



Gold 1000 Tin 303.9 



Silver 9J3 Lead J79.fi 



Copper 898 Marble 23.6* 



Platinum 381 Porcelain 14.2 



Iron 374.3 Clay 11.4 



Zinc 363 



Glass is an imperfect conductor, for we can fuse the point of 

 a glass rod in a lamp, holding it within an inch of the extremity. 

 On the contrary, we find it difficult to heat any part of a thick 

 metallic wire to redness in a lamp, owing to the rapidity with 

 which the heat is carried away by the contiguous parts. 



Certain vibrations were observed by Mr. Trevelyan to take 

 place between metallic masses having different temperatures, 

 occasioning particular sounds, which appear to be connected 

 with the conducting power of the metals.* Thus 

 if a heated curved bar of brass b, be laid upon 

 a cold support of lead /, of which the surface is 

 flat, as represented in the figure, the brass bar, 

 while communicating its heat to the lead, is 

 thrown into a state of vibration, accompanied 

 with a rocking motion and the production of a musical note, 

 like that of the glass harmonicon. The motion of the brass 

 bar appears to depend upon a certain repulsion existing be- 

 tween heated surfaces, enhanced in this case by the low 



* Phil. Mag. 3d series, vol. iii, 321, 



