Atmospheric Actinometry and the Actinic Constitution 



OF the Atmosphere. 



By K. Duclaux, 



Professor of Physics in tlu- A^^rononucul lnslitiitt\ Paris. 



ATMOSPHERIC ACTINOMETRY. 



The progress made by science leads us more and more to attribute to chemical 

 rays a special action, which is dift'ei'ent from and, to a certain extent, independent 

 of that of the calorific and luminous rays. The chemical radiations of the sun, 

 reaching the limits of our atmosphere, become modified while passing through it, 

 according to a law which is peculiar to them ; and, so far as can be seen in so new 

 a subject, their absorption is not the same as that t)f the calorific or luminous parts 

 of the spectrum. 



Photographers, especially those who take landscapes, well know that days 

 which are equally warm or equally luminous do not always give the same results 

 for the same length of exposui'e, and that there are days when, for some unknown 

 reason, the chemical impression is much slower than on others. 



Another argument may be drawn from what often happens iu northern lands, 

 where vegetation, which is well known to be specially susceptible to the power of 

 chemical rays, makes much more rapid progi'ess than in tempeivate I'egions, notwith- 

 standing the fainter light and the lower temperature. 



To what are such diffei'ences due ? What law does the chemical absorption 

 of the atmosphere obey, and on what does it depend ? Ought we to attribute it to 

 its normal elements: oxygen, nitrogen, carbonic acid, and water vapor? Then it 

 should have some general uniformity. Or ought we to see in it, on the con- 

 trary, the action of solid or volatile elements, which incessantly reach it fi'ora the 

 bare or from the cultivated soil ? Then it should have a local character, leading 

 to a multiplicity of chemical climates. These are very impoi-tant questions, for 



