CONDUCTION OF HEAT. 



181 



employed bars of the same size, covered with a coating of varnish. Heat was 

 applied by a lamp at one end, and its progress along the bar indicated by a 

 thermometer at the other ; the lamp was applied until its utmost effect on the 

 thermometer was ascertained ; and the greatest heat to which the thermometer 

 could thus be raised by the effect of the lamp, was taken as the measure of the 

 conducting power of the bar. The following table exhibits the results of 

 Despretz's experiments on different substances : — 



Conducting power. 



Gold, 100- 



Platinum, 98-1 



Silver, 97-3 



Copper, 89-82 



Iron, 37 41 



Zinc, 36-37 



Tin 30-38 



Lead 17-96 



Marble 2-34 



Porcelain 1-22 



Brick earth 1-13 



From this table it is obvious that the metals are, by far, the best conductors 

 of heat, and that the conducting power of earthy substances is prodigiously 

 inferior. 



Similar experiments were made on different species of wood, by MM. A. 

 Delarive, and A. Decandolle. From these experiments it appears that, gene- 

 rally, the more dense woods are those which conduct, heat best. This rule, 

 however, is not invariable, for the conducting power of nut-wood was found to 

 be considerably greater than that of oak. It was, also, found, that heat was 

 better conducted in the direction of the fibres than across them. 



In bodies of the same kind, the rate at which heat is conducted, from the 

 hotter to the colder, depends on the extent of the surface of contact, and is 

 proportional to that surface. Thus if two spheres or balls of metal, at different 

 temperatures, be placed in contact, they will touch only in a single point, 

 and the transmission of heat will be extremely slow ; but if two cubes of the 

 same metal be placed face to face, their surface of contact will be considerable, 

 and the transition of heat will be proportionally rapid. 



Bodies of a porous, soft, or spongy texture, and especially those of a fibrous 

 nature, such as wool, feathers, fur, &c, are the worst conductors of heat. 

 Such a body may be placed in contact with another body of a much higher or 

 much lower temperature than itself, without exhibiting any change of temper- 

 ature, for a long period of time. 



From what has been above explained, it appears that, besides a tendency 

 to equilibrium of temperature, which arises from the interchange of heat by 

 radiation, bodies have a like tendency to calorific equilibrium by the transmis- 

 sion of heat by contact. After the lapse of a sufficient time, every two bodies 

 in contact distribute between them the heat they contain in such portions as to 

 render their temperature equal. The manner in which this effect is, generally, 

 produced in liquids and gases differs, however, materially from the nature of 

 the process in solids. The constituent particles of solid bodies being incapa- 

 ble of changing their material position and arrangement, the heat can only pass 

 through them, from particle to particle, by a slow process ; but when the parti- 

 , cles forming any stratum of a liquid are heated, their mass, expanding, becomes 

 | lighter, bulk for bulk, than the stratum immediately above it, and ascends, al- 

 , lowing the superior strata to descend. Thus a source of heat applied to the 

 [ bottom of a vessel containing a liquid, immediately causes the liquid near the 

 > bottom to form an upward current, while the superior liquid forms a downward 



