STATE AGRICULTURAL SOCIETY. 115 



Conduction is destroyed by interruption of contact. Metals con- 

 duct heat most rapidly, while earthy matters have but a small 

 conducting power. Liquids and gases conduct heat least of all. 

 Porous bodies, like feathers, wool, cotton, &c., which enclose 

 much air in their interstices, are therefore among the poorest 

 conductors. Soils generally, must therefore rank among poor 

 conductors, although it is probable that there are considerable 

 differences among them. Humus, and soils rich in decaying 

 organic matters, are doubtless slower conductors of heat than 

 dense clays, but to my knowledge we have no precise experiments 

 on this subject. Mr. Hutchinson in an investigation of building 

 materials, found that if we assume the conducting power of slate 

 to be 100, that of soft chalk is 56, of gypsum 20, of sand 19. 



6. Convection. — Though liquids and gases are almost perfect 

 non-conductors of heat, yet it can diffuse through them rapidly, 

 if advantage be taken of the fact that by heating they expand and 

 therefore become specifically lighter. If heat be applied to the 

 upper surface of liquids or gases they remain for a long time 

 nearly unaffected, if it be applied beneath them, the lower layers 

 of particles become heated and rise, their place is supplied by 

 others, and so currents upward and downward are established, 

 whereby the heat is rapidly and uniformly distributed. This 

 process of convection can rarely have any influence in the soil. 

 What we have stated concerning it, shows, however, in what way 

 the atmosphere may constantly act in removing heat from the 

 surface of the soil. 



VII. The relations of water to heat. — The soil consists not 

 merely of mineral and vegetable matter — not merely of clay, sand 

 and liuuius — but it is always more or less penetrated with water. 

 The relations of this universally diffused liquid to heat, ai-e there- 

 fore of the utmost importance in understanding the conditions of 

 fertility. 



Three states of water. — Water may exist in three states — solid, 

 liipiid and gaseous. In each of these forms it has a separate sig- 

 nificance in connection with our subject, and in its passage from 

 one of these states to another, phenomena are occasioned which 

 have great influence on vegetable production. 



It is a matter of connnon observation that water exposed to the 

 air in a shallow vessel, ra]>idly decreases in bulk, and finally dis- 



