OF TEMPERATURE. ->7 



availed themselves of it in protecting their tender plants from 

 frost, before the laws of the radiation of heat came to be 

 explained. 3. Dr. Wells proved by numerous experiments 

 that the quantity of dew which condenses on different objects 

 exposed in the same circumstances, is proportional to the 

 radiating power of those substances. Thus, when a polished 

 plate of metal and a quantity of wool are exposed together in 

 favourable circumstances, scarcely a trace of dew is to be 

 observed on the metal, while a large quantity condenses in 

 the wool, the latter substance being incomparably the best 

 radiator, and therefore falling to a much lower temperature 

 than the metal. 



The same theory has been applied to explain a process for 

 making ice followed by the native Indians near Calcutta. In that 

 climate the temperature of the air rarely falls below 40 in the 

 coldest nights ; but the sky is clear, and a powerful radiation 

 takes place from the surface of the ground. Hence, water con- 

 tained in shallow pans imbedded in straw, is often sheeted over 

 with ice by a night's exposure. The water is certainly cooled 

 by radiation from its surface, and not by evaporation ; for the 

 process succeeds best when the pans are placed in shallow 

 trenches dug in the ground, an arrangement which retards eva- 

 poration ; and no ice forms in windy weather, when evaporation 

 is greatest. 



The morning frosts of autumn are first felt in sequestered 

 situations, as in ravines closed on all sides, or along the low 

 courses of rivers, where the cooling of the earth's surface by 

 radiation is in the least degree checked by the movement of the 

 air over it. These are also the very situations upon which the 

 sun's rays produce the greatest effect in summer. 



Reverting again to the subject of the conduction of heat 

 through solid bodies, it may now be stated, that there is 

 every reason to believe that heat is propagated, even in that 

 case, in a manner not unlike radiation. Heat, in its passage 

 through a bar of iron, is probably radiated from particle to par- 

 ticle ; for the material atoms, of which the bar consists, are not 

 supposed to be in absolute contact, although held near each 

 other by a strong attraction. Radiation, as observed in air or a 

 vacuum, may thus pass into conduction in the case of solids, 

 without any breach of continuity in the natural law to which 

 heat in motion is subject. Baron Fourier proceeds upon such 

 an hypothesis in his mathematical investigation of the law of 



