390 Professor Tyndall [June 7, 



The simple atoms are found to be bad radiators ; the compound 

 atoms good ones : and the higher the degree of complexity in the 

 atomic grouping, the more potent, as a general rule, is the radiation 

 and absorption. Let us get definite ideas here, however gross, and 

 purify them afterwards 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 edgeways through the water, and im- 

 parts 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 this, and 

 yield up more speedily their motion to the^ ether. Mix oxygen and 

 nitrogen mechanically, they absorb and radiate a certain amount. 

 Cause these gases to combine chemically and form nitrous oxide, both 

 the absorption and radiation are thereby augmented 250 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 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 the requirements of the present discourse. Into a 

 glass cylinder, with its ends stopped by discs of plate-glass, a small 

 quantity of nitrous acid gas was 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 was cast upon a 

 screen. Introducing the cylinder containing the nitrous acid into the 

 path of the beam as it issued from the lamp, the splendid and con- 

 tinuous spectrum became instantly furrowed by numerous dark bands, 

 the rays answering to which were struck down by the nitric gas, while 

 it permitted the light which fell upon the intervening spaces to pass 

 with comparative impunity. 



Here also the principle of reciprocity, as regards radiation and 

 absorption, holds good ; and could we, without otherwise altering its 

 physical character, render that nitrous gas luminous, we should find 

 that the very rays which it absorbs are precisely those which it would 

 emit. When atmospheric air and other gases are brought to a state 

 of intense incandescence by the passage of an electric spark, the 

 spectra which we obtain from them consist of a series of bright bands. 

 But such spectra are produced with the greatest brilliancy, when, 

 instead of ordinary gases, we make use of metals heated so highly as 

 to volatilize them. This is easily done by the voltaic current. A 



