23 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 heated plate of copper are passing, to 
he 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 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, 
