162 RADIATION. 



of cold air will continually come in contact with, tie plants, they inust 

 gradually get colder and colder, even though no evaporation takes 

 place, till they are as cold as the air itself. 



Radiation, the .third mode in which plants are influenced by cold, 

 depends upon the curious fact, that when two substances are placed 

 opposite to each other in the free and open air, if the one is warmer 

 than the other, it will immediately begin to give out its heat, which 

 will be received by the colder substance. The diflPerenee between this 

 mode of cooling and mere conduction is, that in the latter heat travels 

 from the hot to the colder surface by contact, and therefore only when 

 they absolutely touch each other, whilst in radiation, the two surfaces 

 are at a distance, and the heat passes at once through the air, and 

 without in any way warming it. The heat of the sun is radiant heat — 

 it passes through the clear air without communicating any warnith to 

 it, though it warms the earth below ; and then when the sun's rays 

 have warmed the earth, the latter in turn begins to warm the surround- 

 ing air — ^but this effect is no longer one of radiation, it is simply an 

 effect of conduction. On a clear night the surface of the ground may 

 be exposed to all three of these cooling influences at once ; it may be 

 cooled by evaporation, by contact with cold air, and by radiation. In 

 reality, however, it is very seldom that all these cooling influences are 

 in operation at the same time, because there are several counteracting 

 powers at work tending to keep the surface of the soil at a tolerably 

 uniform temperature ; and foremost of these is the formation of dew. 

 As the evaporation of water is a cooling process, heat being absorbed, 

 so the condensation of moisture is a warming process, an equal amount 

 of heat being given out ; consequently, just in proportion as the surface 

 of the earth after sunset is cooled by radiation, it will acquire the 

 power of condensing the moisture in the air, and by that very act will 

 gain. heat. It must also be remembered that radiation only takes place 

 in a still and clear night ; when there are clouds or mist, radiation does 

 not occur. • 



"Water may be cooled either by evaporation, or by the contact of cold 

 air, but it differs from the soil in the fiacility with which it is moved, 

 and the readiness with which currents are formed m it. When the 

 . earth is exposed to cooling inftuences, the surface soon becomes cold, 

 .but as heat travels very slowly through the porous soU, it takes a very 

 long time before,the cold penetrates, or rather before the heat escapes 

 .from any depth below the surface; in the case of water it is quite 

 different, because when the surface is in any way cooled, the water 

 becomes heavier or denser, and a kind of circulation is immediately 

 established, the cold water descending, and the warmer water rising to 

 its surface. In this manner, then, so long as the cooling influences 

 1 continue, the water goes on sinking in temperature, the whole body of 

 it getting colder ; this, however, does not continue after it has arrived 



