Radiation of Heat by Gaseous Matter. 285 



of a small glass funnel ; the stopcock (S) was then attached to 

 the flask and connected with a second air-pump, which was 

 always kept at hand. The air above the liquid was removed, 

 and the air dissolved in it was allowed to bubble away, until 

 nothing remained but the pure liquid below and the pure vapour 

 above it. The stopcock S was now shut off, and the flask united 

 to the experimental tube. The exhaustion of the tube and stop- 

 cocks being complete, and the needle of the galvanometer at 

 zero, the cock S was turned on and the mercury-gauge care- 

 fully observed at the same time. No bubbling of the liquid 

 was in any case permitted. The vapour entered silently and 

 without the slightest commotion; and when the mercurial 

 column was depressed to the extent required, the cock S was 

 promptly turned off. 



The energy with which the needle moves the moment a strong 

 vapour enters is so extraordinary, that I was compelled to remove 

 the stops which arrested the swing of the needle at 90°, lest the 

 shock against them should derange the equilibrated magnetism 

 of the astatic pair. The needle often swung far beyond a qua- 

 drant ; and after it had come finally and permanently to rest, its 

 position was observed in the following manner: — The dial of 

 the galvanometer being well illuminated, a looking-glass was 

 placed behind the instrument, at such an angle that when 

 looked at horizontally the image of the dial was clearly seen. 

 This image was observed by an excellent telescope fixed at a 

 distance of 11 feet from the galvanometer. Attached to the 

 needle, and in continuation of it, was a bit of glass fibre of 

 extreme fineness, which ranged over the graduated circle ; and 

 by means of it a very small fraction of a degree could be easily 

 read off. I resorted to the expedient of observing from a di- 

 stance, because I found that the approach of my person, perhaps 

 through the diamagnetic action of my own body, had a sensible 

 effect upon the needle of my instrument, which, I believe, sur- 

 passes in delicacy any hitherto constructed. 



The permanent deflection of the needle was noted in all these 

 experiments, and the value of the deflection, expressed in terms 

 of one of the lower degrees of the galvanometer, was obtained 

 from a table of calibration. To spare unnecessary labour I omit 

 the deflections in the following Table, and give the absorptions 

 only produced by the vapours mentioned, at 01, 0'5, and 1*0 

 inch of tension. 



