PHYSICAL BASIS OF SOLAR CHEMISTRY. 333 



group of them united to form a system can produce. 

 An oar runs freely edgeways 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 vertical, 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 ab- 

 sorbs well. Simple atoms glide through the ether with- 

 out much resistance; compound ones encounter resist- 

 ance, and hence yield up more speedily their motion 

 to the ether. Mix oxygen and nitrogen mechanically, 

 they absorb and radiate a certain amount of heat. 

 Cause these gases to combine chemically and form ni- 

 trous oxide, both the absorption and radiation are 

 thereby augmented hundreds of times! 



In this way we look with the telescope of the in- 

 tellect into atomic systems, and obtain a conception of 

 processes which the eye of sense can never reach. But 

 gases and vapours 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 discs of plate-glass, a small quantity 

 of nitrous acid gas is introduced; the presence of the 

 gas being indicated by its rich brown colour. The 

 beam from an electric lamp being sent through two 

 prisms of bisulphide of carbon, a spectrum seven feet 

 long and eighteen inches wide is cast upon the screen. 

 Introducing the cylinder containing the nitrous acid 

 into the path of the beam as it issues from the lamp, 



