through the Earth's Atmosphere. 203 



80 times more powerful than the 200 ; and hence, comparing a 

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

 vapour, 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 oppositftm, based on the philosophic 

 reluctance to accept a result 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 the human character gathers force from the healthy antago- 

 nisms of active life. It was urged that the result was on the 

 face of it improbable; that there were, moreover, many ways of 

 accounting for it, without ascribing so enormous a comparative 

 action to aqueous vapour. 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 colours of thin plates (soap-bubble colours) 

 flashed forth immediately 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 absorption of a layer of brine 

 which was measured, instead of the absorption of aqueous 

 vapour. 



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 altogether, 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 bringing air from Hyde Park, Hampstead Heath, 

 Primrose Hill, Epsom Downs, a field near Newport in the Isle 

 of Wight, St. Catharine's Down, and the sea-beach near Black 

 Gang Chine. The aqueous vapour of the air from these localities 

 intercepted at least seventy times the amount of radiant heat 

 absorbed by the air in which the vapour was diffused. Experi- 

 ments made with smoky air proved that the suspended smoke of 

 the atmosphere of West London, even when an east wind pours 

 over it the smoke of the city, exerts only a fraction of the destruc- 



