44 FRAGMENTS OF SCIENCE 



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 respec- 

 tive gases, the quantity intercepted by atmospheric air 

 being taken as unity. 



Radiation through Gases 



Nitrous oxide 1,860 



Sulphide of hydrogen 2,100 



Ammonia * .... 5,460 



Olefiantgas . .... 6,030 



Sulphurous acid .*:.... 6,480 



