RADAITION. 30 



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. 



Relative 

 Name of gas absorption 



Air 1 



Oxygen. . . . 1 



Nitrogen . . . 1 



Hydrogen . . . 1 



Carbonic oxide .... 760 



Carbonic acid .... 972 



Hydrochloric acid .... 1,005 



Nitric oxide 1,590 



Nitrous oxide .... 1,860 



Sulphide of hydrogen . . . 2,100 



Ammonia 5,460 



Olefiantgas 6,030 



Sulphurous acid .... 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 

 4 



