444 Prof. Tyndall on tlie Action of Free Molecules on 



heated dynamically. Incompetent to radiate itself, it im- 

 parted its warmth to the vapour, and this powerful radiator 

 poured the heat thus received against the pile. This heat 

 sufficed not only to neutralize the deflection of 50° due to 

 absorption, and indicating cold, but to carry the needle up to 

 50° on the side of heat. So likewise, when the experimental 

 tube was filled with mixed air and vapour, the needle being at 

 0°, a stroke of the pump, though opening a freer passage for 

 the rays from the source, caused a deflection indicative not of 

 heat, but of cold. Here, the vapour within the tube, being 

 chilled by the dilatation of the air, the pile radiated its un- 

 compensated warmth into the vapour and produced the 

 observed deflection. 



Such observationssuggested a new means of demonstrating 

 the absorption and radiation of heat by gases and vapours. 

 Abandoning all external sources of heat, and permitting the 

 various gases already examined to enter the experimental 

 tube at a common velocity, they became self-heated and 

 radiated against the pile. Their radiation, thus determined, 

 corresponded exactly with the results obtained when heated 

 columns of these gases were permitted to rise freely in the 

 atmosphere. 



Both the radiation and absorption of vapours were deter- 

 mined in the same manner. The external source of heat was 

 abandoned, and a measured quantity of every vapour was 

 introduced into the experimental tube. Through an orifice 

 of fixed dimensions dry air was then permitted to enter the 

 tube, where the destruction of its vis viva raised its tempe- 

 rature. The heated air warmed the vapour, which in its turn 

 radiated the heat imparted to it against the pile. The de- 

 flection of the galvanometer declared the strength of this 

 radiation. Absorption was determined by permitting the 

 mixed air and vapour to dilate by a measured quantity, the 

 pile being here the warm body, and the chilled vapour the 

 absorbent. The order in which the vapours stood as regards 

 absorption was here exactly the order of their radiation; 

 while both absorption and radiation, thus determined, agreed 

 with the results obtained by sending the rays from an external 

 source of heat through the pure vapours in the experimental 

 tube. 



What has been called " vapour-Lesion/' whereby liquid 

 films are produced, has been supposed to play a dominant 

 part in my experiments. But it can hardly be imagined that 

 an irregular action of this kind could produce result- of such 

 precision and consistency as those here recorded. Such 

 results are, in my opinion, only compatible with the con- 



