498 Prof. Tyndall on the Action of Free Molecules on 



On Monday, accordingly, the experiments were resumed; 

 but they proved by no means so easy as I had hoped to find 

 them. For five clays I worked at the subject without coming 

 to any satisfactory conclusion. The source, the mirror, the 

 cell, the experimental tube, and the thermopile were all in 

 turn objects of scrutiny ; but there still remained a difference 

 between the action of the liquid and that of its vapour suffi- 

 cient to throw doubt on the assertion of their identity. 



At the conclusion of many trials and precautions I found 

 the absorption of the vapour still distinctly in excess of that 

 of the liquid. I had reduced the spherical aberration to a 

 minimum, by confining the reflection to a small central area 

 of the silvered mirror. Still the image of the incandescent 

 lime, formed at the end T' of the experimental tube, was large 

 enough to encroach a little on the annular space surrounding 

 the aperture closed by the plate of rock-salt. Diaphragms of 

 polished metal had also been used, to lessen the amount of heat 

 falling upon the pile. I figured to myself the heat impinging 

 on the annular space and diaphragm, reverberated back to 

 the end T of the experimental tube, reflected from the annular 

 space and diaphragm at that end, and thus in part sent back 

 to the pile. Such heat, instead of passing once through the 

 vapour, would pass through it three times; and if it formed a 

 sensible part of the total heat, might make vapour absorption 

 appear greater than liquid absorption. I had the tube dis- 

 mounted, and the annular spaces and diaphragms carefully 

 coated with lampblack. Eemounting the tube and measuring 

 once more the vapour absorption, it was found to be 



32'4 per cent. 

 This was the mean of five concurrent series of observations, 

 in which every care was taken to ensure exactitude. Lest 

 the total heat should vary during the execution of a series, 

 it was taken at the conclusion of every individual experiment. 

 The absorption of sulphuric ether, acting as a liquid, was 

 next determined. The mean of three series of experiments, 

 two of which yielded identical results, and the third of which 

 differed only 0*7 per cent, from the others, made the absorp- 

 tion of the liquid ether to be 



32-9 per cent., 



which, I need not say, is surprisingly close to the vapour 

 absorption, differing therefrom by only 05 per cent. 



Informed by experiment that the heat from the incandescent 

 platinum spiral was more powerfully absorbed than that of the 

 lime-light, I thought it worth while to inquire whether the 



