NATURAL PHILOSOPHY. 139 



nitrogen there is about one of aqueous vapor. This one, then, ig 

 eighty times more powerful than the two hundred ; and hence, compar- 

 ing a single atom of oxygen or nitrogen with a single atom of aqueous 

 vapor, we may infer that the action of the latter is 16,000 times that 

 of the former. This was a very astonishing result, and it naturally 

 excited opposition, based on the philosophic reluctance to accept a re- 

 sult so grave in consequences, before testing it to the uttermost. From 

 such opposition, a discovery, if it be worth the name, emerges with its 

 fibre strengthened ; as l^ie human character gathers force from the 

 healthy antagonisms of active life. It was urged that the result was 

 on the face of it improbable ; that there were, rqpreover, many ways 

 of accounting for it, without ascribing so enormous a comparative ac- 

 tion to aqueous vapor. For example, the cylinder which contained 

 the air in which these experiments were made was stopped at its ends 

 by plates of rock-salt, on account of their transparency to radiant heat. 

 Rock-salt is hygroscopic ; it attracts the moisture of the atmosphere. 

 Thus, a layer of brine readily forms on the surface of a plate of rock- 

 salt ; and it is well known that brine is very impervious to the rays of 

 heat. Illuminating a polished plate of salt, by the electric lamp, and 

 casting, by means of a lens, a magnified image of the plate upon a 

 screen, the speaker breathed through a tube for a moment on the salt ; 

 brilliant colors of thin plates (soap-bubble colors) flashed forth imme- 

 diately upon the screen, these being caused by the film of moisture 

 which overspread the salt. Such a film, it was contended, is formed 

 when undried air is sent into the cylinder ; it was, therefore, the ab- 

 sorption of a layer of brine which was measured, instead of the ab- 

 sorption of aqueous vapor. 



This objection was met in two ways. First, by showing that the 

 plates of salt, when subjected to the strictest examination, show no trace 

 of a film of moisture. Secondly, by abolishing the plates of salt alto- 

 gether, and obtaining the same results in a cylinder open at both ends. 



It was next surmised that the effect was due to the impurity of the 

 London air ; and the suspended carbon particles were pointed to as the 

 cause of the opacity to radiant heat. This objection was met by bring- 

 ing air from Epsom Downs, a field near Newport, in the Isle of Wight, 

 and a sea-beach. The aqueous vapor of the air from these localities in- 

 tercepted at least seventy times the amount of radiant heat absorbed 

 by the air in which the vapor was diffused. Experiments made with 

 smoky air proved that the suspended smoke of the atmosphere of 

 London, even when an east wind pours over it the smoke of the city, 

 exerts only a fraction of the destructive powers exercised by the 

 transparent and impalpable aqueous vapor diffused in the air. 



The cvlinder which contained the air through which the calorific 



/ c? 



rays passed was polished within, and the rays which struck the interior 

 surface were reflected from it to the thermo-electric pile which meas- 

 ured the radiation. The following objection was raised : You permit 

 moist air to enter your cylinder; a portion of this moisture is con- 

 densed as a liquid film upon the interior surface of your tube ; its re- 

 flective power is thereby diminished ; less heat therefore reaches the pile ; 

 and you incorrectly ascribe to the absorption of aqueous vapor an 

 effect which is really due to diminished reflection of the interior surface 

 of your cylinder. But why should the aqueous vapor so condense ? 



