816 SPECIFIC HEAT. 



them remain at rest and become heated only very 

 slowly. When the motion of gaseous molecules is 

 restrained, they do not convey much heat, even if they 

 are heated from the side or from below. The free 

 motion of the air in a given space may be prevented by 

 dividing the whole space, by numerous partitions, into 

 smaller spaces. A free current is no longer possible 

 under these circumstances, and such a space could no 

 longer be effectually heated even by convection. It is 

 in this way that substances like fur, feathers, ashes, 

 fabrics, straw, &c., become bad conductors; the air 

 remains stationary between their particles, and offers 

 thus great resistance to the propagation of heat by pre- 

 venting a free circulation of heated molecules. 



58. Specific and Latent Heat. Two bullets of equal 

 size, one of zinc and the other of lead, each attached to 

 a piece of iron wire which serves as a handle, are placed 

 in boiling water and left in it until their temperature 

 may be reckoned with certainty to be at 100. They 

 are then quickly taken out and placed on a cake of 

 beeswax or tallow. The zinc will be seen to melt its 

 way pretty deep into the cake while the lead penetrates 

 but a small distance. Both metal balls are at the same 

 temperature, and have the same size, and yet, although 

 the leaden ball is more than half as heavy again as 

 the zinc ball, the lead cannot melt as much wax, that 

 is, does not part with as much heat as the zinc ball, 

 whilst both are cooling from 100 to the temperature of 

 the air. 



Different bodies at the same temperature contain 

 different quantities of heat; they part with different 

 quantities when cooled through the same number of 



