SECT. ii. OPALESCENCE OF THE ATMOSPHERE. 57 



sphere, the greater the height above the level of the 

 sea, the less the amount of diffused light and conse- 

 quently of actinic power. Hence photographers have 

 to expose their plates for a much longer time to the 

 light on the snowy peaks of the Alps and other great 

 heights than in England or at the level of the sea. 

 During Mr. Glaisher's tenth balloon ascent simultane- 

 ous observations were made at Greenwich Observatory 

 and in the balloon, when at more than three miles 

 above the surface of the earth, the standard paper 

 exposed to the full rays of the sun was not as much 

 coloured in half an hour as the corresponding paper at 

 Greenwich in one minute. 



By a series of observations at Heidelberg, Kew, and 

 Manchester, it has been proved that the very small 

 relative chemical action of the sun's direct light de- 

 creases rapidly with his altitude, and at these three 

 places of observation, it has frequently happened when 

 the sun's altitude was very low, as at 12, that his direct 

 light made no impression on a sensitive paper. ' The 

 sun's light had been robbed of its chemical power in 

 passing through the air.' This singular result is as- 

 cribed by Professor Eoscoe to what he calls the opales- 

 cence of the atmosphere. 



Opalescent glass, slightly milky liquids, pure water 

 with particles of sulphur floating in it, are impervious 

 to the chemical rays, whence Professor Eoscoe infers 

 that the atmosphere, more especially its lower regions, 

 possesses that property in consequence of multitudes of 

 solid particles floating in it. What they are is unknown, 

 but infinitesimal particles of soda seem to be every- 

 where, and no doubt particles of other substances mixed 

 with them may be often seen as motes dancing in the 

 sunbeams. Besides, it is clearly proved that myriads 

 of the eggs and germs of organized beings, though 

 invisible to the naked eye, are continually floating in 



