Emission and Transmission of Heat 57 



current of air. This arrangement may be useful, especially 

 when it is important to effectuate the transfer of heat in a small 

 space; but it often has drawbacks, either on account of difficulty 

 in construction, or in cleaning the surfaces of absorption and 

 transmission. 



840. Most important of all, whenever it is a question of 

 transferring the heat of one body in motion to another, is it that 

 the latter body moves in a direction opposite to that of the 

 first, for as the hot body progresses it encounters the body, to 

 which it is to transfer its heat, at a lower temperature, and the 

 transmission continues, which would not be the case if the two 

 bodies were moving in the same direction. Consider, for ex- 

 ample, two concentric tubes, the inner tube conveying hot air or 

 water, and the interval between the two tubes, which we will 

 assume equal in sectional area to the inner tube, conveying cold 

 air or water in the opposite direction. If the tubes are suffi- 

 ciently long and the motion of the fluid slow enough, it is evi- 

 dent that there will be a complete exchange of temperature be- 

 fore the exit of the air, or water, whilst if the two fluids trav- 

 elled in the same direction, as one cooled and the other heated, 

 their temperatures would constantly approach and would finally 

 become the same, and from this point on all transmission of 

 heat would cease. 



CONDUCTIVITY OF POOR CONDUCTORS OF HEAT. 



841. The case of materials which are poor conductors of 

 heat is different from that of the metals, in nearly every case they 

 propagate all the heat which they are really able to transmit and it 

 is very important to know their conductive power. 



M. Despretz is the single physicist who has occupied himself 

 with the conductivity of a few of these bodies. Experiments on 

 the propagation of heat through bars of marble, porcelain and 

 brick gave 23.6, 12.2, and 11.4 as the relative values of the con- 

 ductivities of the three materials. But the manner of making 

 these experiments presents not only the causes of error already 

 explained (829) in speaking of the conductivity of metals, but also 

 that resulting from the hypothesis of uniformity of temperature 

 at all points of the same section, an hypothesis the more untrue 

 the less the conductivity of the material under experiment. Even 



