32 Prof. Tyndall on the Relation of 



intended to keep the circumferential portions of the plate of salt 

 perfectly dry. The flux of heat coming from the source C being 

 converged upon the central portion of the salt, completely chases 

 every trace of humidity from the surface on which it falls. 



With this arrangement I repeated all my former experiments 

 on humid and dry air. The result was the same as before. On 

 a day of average humidity the quantity of vapour diffused in London 

 air produced upwards of 60 times the absorption of the air itself 



It has been suggested to me that the air of our laboratory 

 might be impure ; the suspended carbon particles in a London 

 atmosphere have also been mentioned to me as a possible cause 

 of the absorption which I had ascribed to aqueous vapour. With 

 regard to the first objection, I may say that the same results were 

 obtained when the apparatus was removed to a large room at a 

 distance from the laboratory; and with regard to the second 

 cause of doubt, I met it by procuring air from the following 

 places : — 



1. Hyde Park. 



2. Primrose Hill. 



3. Hampstead Heath. 



4. Epsom race-course. 



5. A field near Newport, Isle of Wight. 



6. St. Catharine's Down, Isle of Wight. 



7. The sea-beach near Black Gang Chine. 



The aqueous vapour of the air from these localities exerted absorp- 

 tions from 60 to 70 times that of the air in which the vapour was 

 diffused. 



I then purposely experimented with smoke, by carrying air 

 through a receiver in which ignited brown paper had been 

 permitted to smoulder for a time, and drying it afterwards. 

 It was easy, of course, in this way to intercept the calorific rays ; 

 but, confining myself to the lengths of air actually experimented 

 on, I convinced myself that, even when the east wind blows, and 

 pours the carbon of the city upon the west end of London, the 

 heat intercepted by the suspended carbon particles is but a minute 

 fraction of that absorbed by the aqueous vapour. 



Further, I purified the air of the laboratory so well that its 

 absorption was less than unity ; the purified air was then con- 

 ducted through two U-tubes filled with fragments of clean glass 

 moistened with distilled water* Its neutrality when dry proved 

 that all prejudicial substances had been removed from the air; 

 and in passing through the U-tubes it could have contracted 

 nothing save the pure vapour of water. The vapour thus carried 

 into the experimental tube exerted an absorption 90 times as great 

 as that of the air which carried it. 



I have had the pleasure of showing the experiments on atmo- 



