PHYSICAL BASIS OF SOLAE CHEMISTRY. 333 



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 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 radiate a 

 certain amount of heat. Cause these gases to combine 

 chemically and form nitrous 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 experi- 

 ment 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, 

 the splendid and continuous spectrum becomes instantly 

 furrowed by numerous dark bands, the rays answering 



