PHYSICAL BASTS OF SOLAR CHEMISTRY 351 



general rule, is the radiation and absorption. Let us get 

 definite ideas here, however gross, and purify them after- 

 ward by the process of abstraction. Imagine our simple 

 atoms swinging like single spheres in the ether; they 

 cannot create the swell which a group of them united to 

 form a system can produce. An oar runs freely edgewise 

 through the water, and imparts far less of its motion to 

 the water than when its broad, flat side is brought to bear 

 upon it. In our present language the oar, broad side ver- 

 tical, is a good radiator; broad side horizontal, it is a bad 

 radiator. Conversely the waves of water, impinging upon 

 the flat face of the oar- blade, will impart a greater amount 

 of motion to it than when impinging upon the edge. In 

 the position in which the oar radiates well, it also absorbs 

 well. Simple atoms glide through the ether without much 

 resistance ; compound ones encounter resistance, and hence 

 yield up more speedily their motion to the ether. Mix 

 oxygen and nitrogen mechanically, they absorb and radi- 

 ate a certain amount of heat. Cause these gases to com- 

 bine chemically and form nitrous oxide, both the absorp- 

 tion and radiation are thereby augmented hundreds of 

 times! 



In this way we look with the telescope of the intellect 

 into atomic systems, and obtain a conception of processes 

 which the eye of sense can never reach. But gases and 

 vapors possess a power of choice as to the rays which 

 they absorb. They single out certain groups of rays for 

 destruction, and allow other groups to pass unharmed. 

 This is best illustrated by a famous experiment of Sir 

 David Brewster's, modified to suit present requirements. 

 Into a glass cylinder, with its ends stopped by disks of 

 plate-glass, a small quantity of nitrous acid gas is intro- 



