126 SCIENCE AND FRUIT GROWING 



from this space to the earth to counterbalance the heat which 

 the earth is radiating into space. Any cloud in the sky, though 

 its temperature may be low, is warm in comparison with outer 

 space, and it consequently sends back some heat to the earth, 

 the fall of temperature of the latter being thus checked : this will 

 occur even if the cloud is not directly overhead. It is only on 

 clear, bright nights that the earth loses much heat by radiation, 

 and that heavy dews or night frosts occur. 



Different substances differ greatly in their powers of radiating 

 heat and of absorbing heat radiated to them; but the power 

 of radiation and absorption go strictly together : the better a 

 substance is as a radiator, the better is it also as an absorber. 

 Dry air is a bad radiator and absorber, yet, owing to the great 

 bulk of the atmosphere, the actual amount of heat which it 

 absorbs is very large, and hence the atmosphere prevents both 

 an excessive heating of the surface of the earth during the day, 

 and an excessive cooling during the night, such as occurs on the 

 moon, rendering the existence of life there impossible. 



Water, even when it is in the form of invisible moisture in 

 the air, is a good radiator and absorber, and hence there is less 

 cooling of the earth's surface when the air is moist than when 

 it is dry, even if there are no actual clouds (condensed moisture) 

 in the sky. 1 



For the production of dew or hoar frost there must, also, be 

 no wind, for with wind there is a continual circulation of air 

 over the ground, preventing the latter from getting cooled down 

 by radiation. The air, being a bad radiator, loses but little heat 

 directly by radiation at night, but it does, however, get colder 

 when radiation is active : it is ' cooled by contact with the 

 ground, which is radiating away its heat and becoming colder, 

 and, when the air gets sufficiently cold, it is no longer able 

 to retain the moisture present in it, and this latter separates 

 out in minute droplets, producing a mist ; these droplets attach 

 themselves to any solid with which they come in contact, forming 

 dew, or, if the temperature is below the freezing point, hoar 

 frost. Thus, dew neither falls, nor does it rise out of the ground, 

 but originates in the air itself. In the same way the mist, 

 which produces the dew, does not rise, though the layer of air 



1 For a summary of the evidence on this point, see Wells, " On Dew," 

 Appendix, by R. Strachan; edition by L. P. Casella, 1866, p. 123. Carbon 

 dioxide is also a better absorber than air, and attempts have been made, 

 though without success, to protect orchards from frost by flooding them 

 with streams of that gas (West and Edlefsen, Utah Expt. Station, Bulletin 

 161, 1917). 



