r,7 



are the molion of the air (Ihe sirenglh of Ihe wind), hn/.c, and 

 pressure. The lime of year, loo, may liuve an iiillucnce, htil lliis is 

 in any case imicli less Ihan llial of llie daily vari;ilions. llic CO., 

 percenlage seems in general lo be soniewliat grealer in winU-r llian 

 in suninier. My figuies also give a sliglil incioase from June to 

 Seplember (see p. 52). 



To the non-periodical lluclualions jjeiong dilTerenl average ligures 

 for dillerent years. For Ihe CO.^ percenlage duiing Ihe years J«9H 

 — 1901 BiiouN and Escomhk oblained Ihe average values of 2,8s, 

 2.88, 2,8(;, and 3.u per len thousand. Rkinau (H)2(), p. (>) considers 

 Ihat the high value for 1901 was due lo Ihe facl lh;il il was a rainy 

 summer and that there was therefore less assimilaloiy aclivily in 

 the vegelation. Mahik-Davy (Comples rend., hSSO, p. ;J2) also found 

 an inverse proporlionaUly belween condilions favourahle for assi- 

 milalion and the average CO,^ percenlage for the years 1876 — 1879. 



Far grealer varialions Ihan those menlioned above are seen in 

 the case of tiie air near the ground, e>pecially where Ihick vegetalion 

 is present. It is a well-known fact that hirge (juanlilies of earbon 

 dioxide are found in the air enclosed in cavilies of Ihe firound 

 FonOR, Pette.nkofhh, and olhers have made numerous in\estigalions 

 inlo Ihe CO.2 percenlage of the groiind-air under varying condilions 

 (see WoLLNY, Forsch. d. AgriUullurphysik, Vol. ö). Il varies belween 

 about 0.3 and 3.5%. The inlluence of Ihe teiuperalure is of special 

 imporlance. The CO.^ percenlage rises rapidly wilh Ihe leniperalure 

 up to about 60", and this is no doubl largely due lo the facl Ihal 

 the CO2 produclion is a biological process, induced by bacleria and 

 fungi. The dampness of the ground also has a great inlluence. 

 The CO2 percenlage rises wilh walering. The ualer loo jjrobably 

 works here purely physically, by driving oul tlie carbon dioxide 

 present in the ground through absorplion. 



In regard to the quanlilies of COo carried över lo Ihe almosphere 

 from the ground hardly any stalemenls are to be found. A priori 

 it is of course probable Ihat the carbon dioxide of Ihe air pailly 

 comes from the ground, and Ihal all Ihe fac ors Ihal increase Ihe 

 GO2 produclion of the ground also increase Ihe CO.^ percenlage in 

 the lowesl regions of the air. Purely Iheorelical disrussions 011 ihe 

 basis of the CO.^ values of the ground-air or the giving olTofCO., 

 from isolaled clods of soil (see on Ihis poinl Pktkhskn, Versuchs- 

 slalionen 1871, p. 165) wiJl be found in Letts and Blake (p. 215) and 

 Reinau (1920, p. 158). Besides high temperalure and dampness, wind 



