GENERAL ACCOUNT OF RADIATION. 233 



If we wish to keep ice from melting, it is advisable to enclose it in a 

 box of low conducting power, lined outside with bright metal. The outer 

 surface will ultimately take a temperature below that of the room, and such 

 that the temperature slope from the outside to the inside of the box will 

 just conduct to the ice all the heat supplied by the surroundings. The 

 lower the absorbing power of the metal surface, the less will be the slope 

 in the non-conducting case needed to convey the heat to the ice, and the 

 less the heat so conveyed. This may be seen more exactly as follows : 



If A is the absorption of heat per second by the metal surface for each 

 degree that it is below the temperature of the room, and C the quantity 

 of heat conducted in through the box per second per 1 difference of 

 temperature between inside and outside, and if t is the temperature of 

 the room, that of the outer surface of the box, that of the inside, 

 the steady state is given by 



A(t-8) = C6 



whence 0= 



-; - ~ 



A + C 



The quantity of heat received by the ice is, therefore, 



AC* 



+ 



which diminishes as A and C diminish. 



Deposition of Dew on Different Surfaces. A very interesting 



illustration of the effect of variation in radiating power is afforded by 

 the phenomena of dew formation. We owe the explanation of the 

 formation of dew, now generally accepted, chiefly to the experiments and 

 observations of Dr. Wells, made about 1812. He found that dew is 

 most freely deposited on calm, clear nights, on substances close to the 

 surface of the earth and not shielded from the sky. These observations 

 led him to suppose that the deposition is due to the cooling of the 

 surface by radiation, and the consequent cooling of the air in contact 

 with the surface below the saturation-point of the vapour contained 

 in it. He verified this cooling by radiation by showing that the 

 surface cooled down far below the air a little above it, and he showed 

 also that the cooling was greater for high radiators than for low: 

 He also found that any protection from the sky, by hindering the 

 radiation outwards, lessened the cooling. The lesser deposit on surfaces 

 raised above the surface of the earth he attributed to the falling down 

 from them of the cooled and therefore heavier air. There was, there- 

 fore, a constant renewal of the air in contact with the elevated surface, 

 and this air had not time to cool sufficiently to deposit much of its 

 vapour. Wells's theory has since been supplemented by Aitken, who 

 has shown that much of the dew found on the surface of vegetation 

 arises from the vegetation. When the air is not saturated, the water 

 evaporates from the surface ; but when the air is cooled to saturation- 

 point, it cannot take up this water, which therefore remains as dew. 



Formation Of Ice in India. Ice has been formed in India on 

 nights when the air does not fall to freezing-point by forming square 

 pits (according to Dr. Wells) about 2 feet deep and 30 inches wide, and 

 filling them about 8 inches or 1 foot deep with straw. On this rows of 



