THE SOUTHERN PLANTER. 



681 



colored soils. This topic will be recurred 

 to. 



3. Reflection.— Bodies exposed to radiant 

 heat may reflect it to a great extent. This 

 is the case with polished metals, while 

 glass is a poor reflector. Reflection is op- 

 posed to absorption. 



4. Transmission. — Radiant heat may also 

 be transmitted through bodies precisely in 

 the way that light is. Rock salt transmits 

 92 per cent, of the heat that falls upon it: 

 alum allows onlv 12 per cent, to pass, 



process of convection can raiely have any 

 influence in the soil. What we have sta- 

 ted concerning it, shows, however, in what 

 way the atmosphere may constantly act in 

 removing heat from the surface of the soil. 



Vn. The relations of water to heat. — 

 The soil consists not merely of mineral 

 and vegetable matter — not merely of clay, 

 sand and humus — but it is always more or 

 less penetrated with water. The relations 

 of this universally difl?used liquid to heat, 

 are therefore of the utmost importance in 



while blue vitriol intercepts radiant heat junderstanding the conditions of fertility 

 totally; it is so to speak, opake to heat. — 

 On the other hand black glass, which is 

 opake to light, allows considerable heat to 

 pass through it. This kind of transmis- 

 sion is instantaneous and must be distin- 

 guished from 



5. Conduction. — This is a slower pro- 

 cess, and consists in the passage of heat 

 from particle to particle of a solid sub- 

 stance. Conduction is destroyed by inter- 

 ruption of contact. Metals conduct heat 

 most rapidly, while earthy matters have 

 but a small conducting power. Liquids 

 and gases conduct heat least of all. Po- 

 rous bodies, like feathers, wool, cotton, 

 &c., which enclose much air in their in- 

 terstices, are therefore among the poorest 

 conductors. Soils generally, must there- 

 fore 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 as- 

 sume 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 ga- 

 ses are almost perfect non-conductors of 

 heat, yet it can diffuse through them ra- 

 pidly, if advantage be taken of the fact 

 thE^t by heating they expand and therefore 

 become specifically lighter. If heat be 

 applied to the upper surface of liquids or 

 gases they remain lor a long time nearly 

 unaffected, if it be applied beneath them, 

 the lower layers of particles become heat- 

 ed and rise, their place is supplied by oth- 

 ers, and so currents upward and downward 

 are established, whereby the heat is ra 

 pidly and uniformly distributed. This 



Three states of water. — Water may exist 

 in three states- — solid, liquid and gaseous. 

 In each of these forms it has a separate 

 significance 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 common observation 

 that water exposed to the air in a shallow 

 vessel, rapidly decreases in bulk, and final- 

 ly disappears; it evaporates, it becomes 

 invisible vapor or steam, and passes into 

 the air. The higher the temperature to 

 which the water is exposed, the more ra- 

 pidly is this conversion accomplished. On 

 the other hand, when a glass of cold wa- 

 ter is brought into a warm, moist atmos- 

 phere, or held over the spout of a boiling 

 tea-kettle, a deposition of water takes 

 place on the cold surface; the vapour con- 

 denses, liquefies. Thus, by exposing wa- 

 ter to great cold it freezes, solidifies, be- 

 comes ice;\by elevating the temperature 

 of a piece of ice, it becomes first liquid 

 and then gaseous ; by cooling vapor, it 

 passes into the liquid and finally into the 

 solid form. Temperature and pressure are 

 the influences that affect the condition of 

 water. The first of these alone needs 

 lengthened consideration here 



LIQUE FACTION V APORIZ ATION- 



HEAT. 



•LATENT 



When a piece of ice 

 sel, whose temperature 

 means of a lamp, at the 



is placed in a ves- 

 is increasing:, by 



rate of one de- 



gree of the thermometer every minute, it 

 will be found that the temperature of the 

 ice rises until it attains 32°. When this 

 point is reached, it begins to melt, but does 

 not suddenly become fluid ; the melting 

 goes on very gradually. A thermometer 

 placed in the water, remains constantly at 



