28 FRAGMENTS OF SCIENCE. 



tralize the former. When the neutralization is perfect, 

 the magnetic needle connected with the pile is no longer 

 deflected, but points to the zero of the graduated circle 

 over which it hangs. 



And now let us suppose the glass tube, through which 

 the waves from the he;ited plate of copper are passing, to 

 be exhausted by an air pump, the two sources of heat act- 

 ing at the same time on the two opposite faces of the pile. 

 When by means of an adjusting screen, perfectly equal 

 quantities of heat are imparted to the two faces, the needle 

 points to zero. Let any gas be now permitted to enter the 

 exhausted tube; if its molecules possess any power of in- 

 tercepting the calorific waves, the equilibrium previously 

 existing will be destroyed, the compensating source will 

 triumph, 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 following 

 table were examined, a small portion only of each being 

 admitted into the glass tube. The quantity admitted in 

 each case was just sufficient to depress a column of mer- 

 cury 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 



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 acidr ,.,.,,,,.,,,.., 6,480 



