RADIATION. 181 



radiations from these substances when a source of heat 

 which was not luminous was employed. 



These researches led Dr. Wells, in the year 1815, to 

 his theory of dew. Dr. "Wells observed that if radiations 

 are taking place from all surfaces of the earth from the 

 surface of grass, metals, and other objects then when the 

 sun is away so that the atmosphere is not heated, those 

 objects ought all to get cool by radiation, throwing heat oft' 

 into space. But following out the conclusions of Leslie, he 

 said there are some substances which ought to radiate 

 heat very much more than other substances. For instance, 

 grass and the leaves of trees, or the substance of which a 

 spider's web is composed, are all good radiators, and those 

 ought to get cool quicker than a metal or any substance 

 like that which does not radiate so much. He 'also saw 

 that if any object be covered at a little height, however 

 thin that covering be, it will prevent free radiation from 

 the surface of the ground to the sky, and consequently it 

 will not cool so rapidly. He noticed that those were all 

 the conditions which were favourable to the deposition of 

 dew, and he saw in this an explanation of the formation 

 of dew. The atmosphere always contains a certain amount 

 of moisture ; if it were to be suddenly cooled the amount 

 of moisture which it would be capable of -containing in the 

 form of vapour would be sensibly diminished. If now in 

 the atmosphere there be a sufficient amount of moisture so 

 as to saturate it, and suppose that to be cooled, it will no 

 longer be able to contain this moisture in the form of 

 vapour, and consequently it will be deposited in the form 

 of dew on any surface which has cooled down to below 

 the dew-point of the air at that time. I shall be able to 

 make this clearer perhaps with the aid of a very rough 

 experiment. 



You know that if one breathes upon a piece of glass it 

 will be covered with vapour deposited in the form of 

 minute drops of water, the reason being that the breath is 

 saturated with aqueous vapour. The glass when the breath 

 falls upon it is cold ; at that temperature the air can no 

 longer suspend so much moisture in the form of vapour, 

 and consequently it must be condensed. This, then, illus- 

 trates the manner in which dew is formed on surfaces 



