18 ATMOSPHEKIC ACTLNOMETRY 



" Tbe light which has gone through a laytT of water is five times more active 

 than tliat which has traversed a solution of sulphate of quiniue of quite the same 

 depth. I sliall continue my obsei'vations at the time of the solstice." 



And on July ;»th : 



" I have again made an experiment with sulphate of ijuiMine. On June L'Ttli, 

 while there were consumed in the open air during the whole day 87 per cent of 

 the total amount of oxalic acid, and 78 per cent under a bell jar filled with water, 

 the decomiKKsition amounted only to 20 per cent under u precisely similar hell 

 filled with a solution of suli»hate of (piinine." 



Analogous results are obtained by comparing the effect of sifting solar rays 

 through a solution of potasssium bichromate, which by preference allows those 

 radiations to pass which are least refi'angible, with that of transmission through a 

 soluticm of sulphate of copper which allows the most refrangible radiations to pass. 

 All this proves that it is mainly the chemical radiations which are of importance, 

 and that when these radiations are employed in o.\idation, or more geneially in the 

 transformation of organic or even mineral substances in the aii*, they reach the sur- 

 face of the soil much weakened. 



Here is, therefoie, a local cause of variations in the actinometric degree; a 

 local and incidental cause, considering that it may be summed up thus : There may 

 exist actinic clouds, clouds scarcel}' visible to the naked eye and not accessible to 

 our senses, but the effect of which, at least as far as it can be measui'ed by solu- 

 tions of oxalic acid, exceeds by fai', in intensity, that of the variations in brilliancy 

 and obscurity produced by oidinary clouds. These clouds come and go, are no 

 longer today where they weie the day before; they dissolve, for they are, like 

 othei' clouds, no soouei' formed than they become subject to ceaseless causes of 

 destruction. Tliis explains very fully why the actinometric degree should vary 

 so greatly from day to day and from one year to another. It may also be that we 

 find here oui' e.\i)lanation of the greater actinic power which spring has, in other 

 words, that season during which the atmosphere is certainly poorest in organic 

 substances. 



Upon reaching this point we see new vist;is open l)efore us. It is well known 

 that the turning green of the foliage and the production of chlorophyll may take 

 place w hen the intensity of light is very feeble, as for instance in the back of 

 a room lighted by one window, but that, under such circumstances, the chlorophyll 

 does not begin to act and is not decomposed by carbonic acid. It requires a much 

 stronger luminous intensity for the process of assimilation to begin. This phe- 



