RADIATION. 



39 



and a deflection of the magnetic needle will be the 

 immediate consequence. From the deflections thus 

 produced by different gases, we can readily deduce the 

 relative amounts of wave-motion which their molecules 

 intercept. 



In this way the substances mentioned in the follow- 

 ing table were examined, a small portion only of each 

 being admitted into the glass tube. The quantity ad- 

 mitted in each case was just sufficient to depress a column 

 of mercury associated with the tube one inch : in other 

 words, the gases were examined at a pressure of one- 

 thirtieth of an atmosphere. The numbers in the table 

 express the relative amounts of wave-motion absorbed 

 by the respective gases, the quantity intercepted by 

 atmospheric air being taken as unity. 



Radiation through Gases. 



Name of gas 

 Air ... 



Oxygen . 

 Nitrogen 

 Hydrogen 

 Carbonic oxide 

 Carbonic acid 

 Hydrochloric acid . 

 Nitric oxide . 

 Nitrous oxide 

 Sulphide of hydrogen 

 Ammonia 

 Olefiant gas . . 

 Sulphurous acid 



Relative 

 absorption 

 1 

 1 

 1 

 1 



. 750 

 . 972 

 . 1,005 

 . 1,590 

 . 1,860 

 . 2,100 

 . 5,460 

 . 6,030 

 . 6,480 



Every gas in this table is perfectly transparent to 

 light, that is to say, all waves within the limits of the 

 visible spectrum pass through it without obstruction ; 

 but for the waves of slower period, emanating from our 

 heated plate of copper, enormous differences of absorp- 

 tive power are manifested. These differences illustrate 



