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. 



- &amp;lt;& 



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 &quot;. 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 



