456 Prof. Tyndall on the Action of Free Molecules on 



temperature of 15° C. was sent through the tube, the deflection 

 rose to five divisions; when the water was warmed to 60° or 

 80° Fahr., the deflection was 20 divisions ; and when the 

 water boiled, the deflection was 100 divisions. In this last 

 experiment, however, a mist appeared; so that, as urged at 

 the time, the radiation could not be said to have been purely 

 from vapour. In the other cases no mist was visible ; but it 

 was nevertheless concluded that the 20-di vision deflection was 

 due to the formation of mist at the boundary of the ascending 

 current. 



I should be disposed to claim these experiments as telling 

 in my favour. The first of them, in my opinion, dealt with 

 the radiation, not from dry air, but from the adjacent aqueous 

 vapour which had been warmed by the dry air. That the 

 deflection in the second experiment was small is not sur- 

 prising. The radiation which could reach the pile from a jet 

 of air only 15 millimetres in diameter, and containing such 

 moisture as could be taken up at 15° C, must have been 

 extremely small under any circumstances. But in the present 

 case, even this small radiation was diminished by the passage 

 of the heat through 400 millimetres of undried air. I should 

 demur to the explanation of the third experiment, and question 

 the warrant to imagine a mist which could not be seen. Even 

 the fourth experiment, where mist was visible, 3'ielded, I doubt 

 not, a mixed result — part of the effect, and probably the smallest 

 part, being due to the mist, and part of it to the vapour. 



With regard to the radiation fz-om hot aqueous vapour, the 

 following experiment is typical of some hundreds which I 

 have had occasion to make. A burner, consisting of two 

 rings provided with numerous small apertures, was placed 

 within a square tin chimney. At some height above the 

 burner the chimney was perforated, so as to enable the radia- 

 tion from a heated gaseous column within the chimney to 

 reach a distant thermopile. The side of the chimney facing 

 the pile was so protected by screens that the radiation from 

 the chimney itself was nil. Connecting the burner with a 

 bottle of compressed hydrogen, the gas was ignited. A 

 column of hot vapour rose from the burner and passed the 

 aperture in the chimney through which it sent its rays to the 

 pile. Mere tips of flame were first employed, the column of 

 vapour rising from them sufficing nevertheless to produce a 

 permanent deflection of 



40°. 

 A slight augmentation of the flame sent the needle up to 

 60°. 



