RADIATION. 39 



umph, 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 col- 

 umn 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 ab- 

 sorbed by the respective gases, the quantity intercepted 

 by atmospheric air being taken as unity. 



Radiation through Oases. 



Relative 

 Name of gas absorption 



Air 1 



Oxygen 1 



Nitrogen 1 



Hydrogen 1 



Carbonic oxido . '. . ." . v . . . ". 750 



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 periods, emanating from our 

 heated plate of copper, enormous differences of absorp- 

 tive power are manifested. These differences illustrate 

 4 



