180 



CONDUCTION OF HEAT. 



B C, in small cavities provided for the purpose and filled with mercury. This 

 mercury will take the temperature of the bar, and will communicate it to each 

 thermometer successively. Before the bar is placed in contact with the red- 

 hot block A, the thermometers will all indicate the same temperature. At the 

 first moment, when the bar is placed in contact with A, none of the thermome- 

 ters will be affected by it ; but, after the lapse of a short time, the first ther- 

 mometer, t, will be observed to rise slowly ; after another interval, the ther- 

 mometer t' will begin to be affected ; and the other thermometers, after like 

 intervals, will be successively affected in the same way ; but the thermometer 

 t, by continuing to rise, will indicate a higher temperature than t', and t' a 

 higher temperature than *", and so on. After the lapse of a considerable time, 

 the temperatures of all the thermometers will be the same ; and if the block A 

 be observed, it will be found to have the common temperature indicated by all 

 the thermometers. 



It appears, from this experiment, that the propagation of heat in this manner 

 through the dimensions of the bar is very slow, and it would seem to take place 

 from particle to particle of the matter composing the bar. The first particle in 

 contact with the source of heat acquires a certain temperature ; this being 

 greater than the contiguous particles, an interchange takes place between the 

 two, on a principle exactly similar to the interchange of heat by radiation. In 

 fact, two contiguous particles in this case may be regarded, under the same 

 circumstances, as two bodies having different temperatures placed in the foci 

 of the two reflectors,//, fig. 2. In that case, the hotter body radiates heat 



Fig. 2. 



on the colder, and the colder on the hotter, in unequal quantities, until their 

 temperatures are equalized. Every two successive particles in the bar B C, 

 fig. 1, beginning from the source of heat, appear to act on each other in the 

 same way. 



Let a number of bars of different substances of equal dimensions, be suc- 

 cessively exposed in this manner, to the same source of heat, and let thermom- 

 eters be applied to similar points in them, it will be found that thermometers 

 in the same situation on different bars, will, after the lapse of the same time 

 from the commencement of the contact, be differently affected. In those bars 

 which are good conductors the thermometer will be more elevated than in those 

 which are bad conductors ; and, in general, the conducting power of the diffe- 

 rent bars may be estimated by the effect produced on thermometers at a given 

 distance from the source of heat, after the lapse of a given time. In experi- 

 ments of this nature it is, however, necessary to guard against the effects of 

 radiation ; because, if two different bars radiate differently, it is possible that 

 the indications of the thermometer may be so interfered with, by their different 

 powers of radiation, that their conducting power cannot, with certainty, be in- 

 ferred. In a course of experiments instituted on this subject by Despretz, he 



