406 Prof. Tyndallon Absorption and Radiation of Heat ^ ^c, [Jan. 17, 



dynamically, dynamic absorption. Placing a thermo-electric pile at 

 the end of his experimental tube, the latter being exhausted, the gas 

 to be examined is permitted to enter the tube ; the gas is heated, and 

 if it possess any sensible radiative power, the pile will receive its 

 radiation, and the galvanometer connected with the pile will de- 

 clare it. 



Proceeding in this way with gases, Professor Tyndall found that 

 the radiation thus manifested, and which was sometimes so intense as 

 to urge the needle of the galvanometer through an arc of more than 

 sixty degrees, followed the exact order of the absorptions which he had 

 already determined. After the heat of the radiating column of gas 

 had wasted itself, the air-pump was worked at a certain rate, the 

 rarefied gas within the tube became chilled, and the face of the pile 

 turned towards the chilled gas became correspondingly lowered in 

 temperature. The dynamic absorptions of various gases were thus 

 determined, and they were found to go strictly hand in hand with the 

 dynamic radiation. 



In the case of vapours the following method was pursued. A 

 quantity of the vapour sufficient to depress the mercury column 0*5 of 

 an inch was admitted into the tube, and this was heated dynamically 

 by allowing dry air to enter till the tube was filled. The radiation of 

 the vapours thus determined followed exactly the same order as the 

 absorption which had already been measured. The dynamic absorp- 

 tion of the vapour was obtained by pumping out in the manner just 

 described, and it was found to follow the same order as the dynamic 

 radiation. In these experiments the air bore the same relationship to 

 the vapour that a polished silver surface does to a coat of varnish laid 

 over it. Neither the silver nor the air, both of which are elements or 

 mixtures of elements, possesses the power of agitating in any marked 

 degree the luminiferous ether. But the motion of the silver being 

 communicated to the varnish, and the motion of the air being commu- 

 nicated to the vapour, molecules are agitated which have the power 

 of disturbing, in a very considerable degree, the ether in which they 

 swing. 



The speaker finds by strict experiments that the dynamic radia- 

 tion of an amount of boracic ether vapour, possessing a tension of only 

 10 1 as'ooooo ^^ ^^ ^" atmosphere is easily measurable. He also shows 

 and explains the fact that with a tube 33 inches long, the dynamic 

 radiation of acetic ether considerably exceeds that of olefiant gas ; 

 while in a tube 3 inches long, the dynamic radiation of olefiant gas 

 considerably exceeds that of the ether. Aqueous vapour has been 

 subjected to a special examination, and Professor Tyndall finds it a 

 common fact for the aqueous vapour contained in the atmosphere to 

 exercise 60 times the absorption of the air itself. The further he has 

 pursued his attempts to obtain perfectly pure and dry air, the more 

 has the air approached the character of a vacuum. He further points 

 to the possibility of determining the temperature of space by direct 

 experiment. 



