BY P. OLSSON-SEFFER, PH. D. 103 



to go through the atmosphere, which contains besides the prin- 

 cipal ingredients of air, aqueous vapour, carbonic acid, small 

 quantities of ozone, nitric oxides, ammonia and water in solid 

 and liquid form, and particles of dust of various kinds. 



The gaseous matter round the earth hinders the radiation 

 of heat, as it returns theheat to the earth more or less, and thus 

 compensates for the loss. 



The degree of the fall of temperature caused by radiation 

 depends on the following circumstances : — 



I. — The Aqueous Vapour of the Air. 



The heat which radiates into space ccmes in most cases 

 from plants on the surface of the earth. The plants receive 

 heat from below by radiation from the bare earth, and by 

 conduction through the plants themselves, and the heat escapes 

 first to the atmosphere, on the state of which depends the 

 degree of the fall of temperature, and secondly into space. 



From the latest researches on the powers of emission and 

 absorption of gases we learn that while the pure and clear air 

 is nearly diathermous for heat, even the small quantities of 

 carbonic acid which are present in the atmosphere exercise a 

 perceptible absorption, which yet is not determined with 

 sufficient exactitude. 



It is probable that other gaseous matters in the air have 

 very little influence, which also seems to be the case with the 

 nitric oxides and the ammonia though they are the most 

 absorbent gases. 



Aqueous Vapour and Water. 



Thinly scattered as the molecules of aqueous vapour are 

 in the atmosphere, we might be inclined to disregard them as 

 carriers to the waves of heat, and imagine that these undulations 

 must be intercepted by the gases which form the great bulk of 

 the atmosphere, and not by the aqueous vapour which is 

 sparingly diffused among them. 



According to Tyndall, the action of a single atom of aqueous 

 vapour is 10,000 times than that of a single atom of oxygen or 

 nitrogen. According to others, the absorbing power of vapour 

 of water on the dark rays of heat is hardly greater than that of 

 air. Concerning the power of liquid water, that is to say in 

 this case condensed vapour, all agree that it is great and 

 attains nearly 90 per cent, of the radiated heat. 



Although dust or the solid particles of different kinds in 

 the air exists only to a small extent, its influence is still very 



