CONDUCTING POWERS OF BODIES. 89' 



However slight the additional heat may be to which a 

 body is subjected, it expands under its influence ; con- 

 sequently, every atom which goes to form the mass of 

 the earth moves under the excitation, and the first heat 

 ray of the morning which touches the earth's surface, 

 sets up a vibration which is continued as a tremor to its 

 very centre. The differences between the temperature 

 of day and night are considerable ; therefore all bodies 

 expand under the influence of the higher, and contract 

 under that of the lower temperature. During the day, 

 any cloud obscuring the sun produces, in every solid, 

 fluid, or aeriform body, within the range of solar in- 

 fluence, a check : the particles which had been expand- 

 ing under the force of heat suddenly contract. Thus 

 there must of necessity be, during the hours of sunshine, 

 a tendency in all bodies to dilate, and during the hours 

 of night they must be resuming their original conditions. 

 Not only do dissimilar bodies radiate heat in different 

 degrees, but they conduct it also with constantly varying 

 rates. Heat passes along silver or copper with readi- 

 ness, compared to its progress through platinum. It is 

 conducted by glass but slowly, and still more slowly by 

 wood and charcoal. We receive some important intima- 

 tions of the molecular structure of matter, from those 

 experiments which prove that heat is conducted more 

 readily along some lines than others. In some planes, 

 wood and other substances are better conductors than iu 

 others. The metallic oxides or earths are bad conduc- 

 tors of heat, by which provision the caloric absorbed by 

 the sun's rays is not carried away from the surface of this 

 planet so rapidly as it would have been had it been of 

 metal, but is retained in the superficial crust to produce 

 the due temperature for healthful germination and 

 vegetable growth. The wool and hair of animals are still 

 inferior conductors, and thus, under changes of climate 

 and of seasons, the beasts of the field are secured against 

 those violent transitions from heat to cold which would 



