AND THE ACTINIC CONSTITUTION OF THE ATMOSPHERE. 41 



I have ascertained since that this destruction and this attenuation go on in 

 the superficial layers of the soil and even down to some dejjth. If, in the many 

 attempts to count the microbes of the soil, it has so often been found that the 

 number is less near the surface than at the depth of a few centimetres, we must 

 attribute this result much rather to solar combustion than to desiccation. The 

 sanitary action of oxygen, which is pursued and completed in the atmosphere, 

 begins therefore, thanks to light, at the surface of the soil, and the healthiest coun- 

 tries are those in which the actinic power of the sun is greatest. 



By a curious mechanism, which I have tried to make generally known, the 

 solar action which neutralizes the microbes which it encounters, can act like them, 

 and take their place. I have in fact shown in an extensive work ' that the changes 

 which carbohydrates undergo upon exposure to sunlight are exactly like those 

 which they undergo under the action of ferments. Starting from the same point, 

 these two modes of transformation, apparently so different, resemble each other 

 not only in their variety and marvellous flexibility of conduct, but still more in 

 their intermediary and final products. 



Thus, invert sugar in alkaline solution, oxidized in sunlight, gives intermediary 

 products which are colloidal and identical mth humic acids, except that they are 

 not nitrogenous. These black acids are afterwards consumed by light, exactly 

 as we see in the bleaching of the black soil which the spade or the plow has 

 turned up. 



The extreme terms of the transformation of this sugar or of its humic deriva- 

 tives are as numerous and as varied in solar combustion as when produced by the 

 action of ferments. Thus, by contact with potash, or with soda, we obtain alcohol 

 through an interior combustion which is identical with that produced during alco- 

 holic fermentation. On the other hand, in the presence of baryta, no alcohol, but 

 lactic acid is produced. In this there is analogy, not with alcoholic fermentation, 

 but with lactic fermentation, and this analogy is all the closer since — as is recog- 

 nized — ^there may be several lactic acids of different rotary power, which may be 

 produced as well by the action of light as by that of fermentation. 



This solar, lactic fei-mentation is accompanied by the production of acetic acid, 

 as in the case of microbian fermentation. In other cases, butyiic acid is formed, 

 formic acid, oxalic acid— in short, all the ordinary residues of the ferment action. 

 Finally, carbonic acid represents in all cases the extreme term of the change of 

 organic matter into gas. 



The luminous action, varying in quantity according to place and season, as the 



' AniHiles de I'lintitiit Agroiiomii/i/r. \o\. x., t886. 



