RADIATION. 177 



faces, the needle points to zero. Let any gas be now per- 

 mitted to enter the exhausted tube; if the molecules pos- 

 sess any power of intercepting the calorific waves, the 

 equilibrium previously existing will be destroyed, the com- 

 pensating source will triumph, and a deflection of the mag- 

 netic 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 following 

 table were examined, a small portion only of each being 

 admitted into the glass tube. The quantity admitted 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. 



N— fgas. a*S^ 



Air 1 



Oxygen 1 



Nitrogen 1 



Hydrogen 1 



Carbonic oxide 750 



Carbonic acid 972 



Hydrochloric acid 1,005 



Nitric oxide 1,590 



Nitrous oxide 1,860 



Sulphide of hydrogen 2,100 



Ammonia 5,460 



Olefiant gas 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 



