of Heat by Gaseous and Liquid Matter. 101 



A brass tube 3 feet long, and very slightly tarnished within, 

 was used for dynamic radiation. Dry air on entering the tube 

 produced a deflection of 12 degrees. The tube was then polished- 

 within, and the experiment repeated ; the action of dry air was 

 instantly reduced to 7*5 degrees. 



The rock-salt plate at the end of the tube was then removed, 

 and a lining of black paper 2 feet long was introduced within it. 

 The tube was again closed, and the experiment of allowing dry 

 air to enter it repeated. The deflections observed in three suc- 

 cessive experiments were 



80°, 81°, 80°. 



This result might be obtained as long as the lining continued 

 within the tube. 



The plate of rock-salt was again removed, and the length of 

 the lining was reduced to a foot ; the dynamic radiation on the 

 entrance of dry air in three successive experiments gave the 

 deflections 



76°, 74?, 75°. 



The plate was again removed and the lining reduced to 3 

 inches ; the deflections obtained in two successive experiments 

 were 



>°, 65°. 



Finally the lining was reduced to a ring only 1| inch in width ; 

 the dynamic radiation from this small surface gave in two suc- 

 cessive trials the deflections 



56°, 56°-5. 



The lining was then entirely removed, and the deflection 

 instantly fell to 



7°-5. 



A coating of lampblack within the tube produced the same 

 effect as the paper lining; common writing-paper was almost 

 equally effective; a coating of varnish also produced large 

 deflections, and the mere oxidation of the interior surface of the 

 tube is also very effective. 



In the above experiments the lining was first heated, and it 

 then radiated its heat through a thick plate of rock-salt against 

 the pile. The effect of the heat was enfeebled by distance, by 

 reflexion from the surfaces of the salt, and by partial absorption. 

 Still we see that the radiation thus weakened was competent to 

 drive the needle almost through the quadrant of a circle. If, 

 instead of being thus separated from the lining, the face of the 

 pile itself had formed part of the interior surface of the tube, 

 receiving there the direct impact of the particles of air, of course 

 the deflections would be far greater than the highest of those 



