6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 63 



exposed plates in the forest air and on its outskirts and tabulated 

 their countings of bacteria for forty successive days from May 6. 

 They made three classes — molds, liquefying and non-liquefying 

 bacteria. They found that, with one exception, one or two of these 

 classes were always less numerous in the forest than on its outskirts 

 and generally from twenty-three to twenty-eight times less. Serafini 

 makes the point that bacteria coming from the outside are reduced in 

 number by a sort of filtration process. Thus we see that the air of 

 forests is comparatively free from endogenous and exogenous bac- 

 teria — none of them in any case being pathogenic.^ 



CARBON DIOXIDE IN FORESTS 



Puchner shows that the air in the forest contains generally more 

 carbonic acid gas than in the open, due to the decomposition of 

 litter.^ But this difference must be almost inappreciable. As we 

 know, the law of diffusion of gases renders it impossible for varia- 

 tions in the relative proportion of the atmospheric constituents to be 

 more than transitory. Diffusion is greatly favored by the winds 

 which sweep through the tree tops, especially where they are not 

 too crowded. 



The fact that so many sanatoria for tuberculosis are located in or 

 near forests makes it very important to dwell a little longer on the 

 constituents of the air in these localities. We know that forests, 

 as well as all other forms of vegetal growth, take up large quantities 

 of carbonic acid, retaining the carbon and rejecting the oxygen, 

 and the question naturally arises, does it sensibly change the relative 

 quality of either constituent so that the composition of the air is 

 slightly different in the woods? Prof. Mark W. Harrington, lately 

 chief of the United States Weather Bureau, undertook to answer 

 that question, both with reference to carbonic acid, oxygen, and 

 ozone, with some interesting results." Repeated observations show 

 that each constituent is curiously uniform in quantity in the free 

 air. It has been thought that carbonic acid is quite variable but 

 the introduction of better methods of observation shows that, except 

 in confined places where the gas is produced, the variations are very 



' See B. E. Fernow : Forest Influences, U. S. Dep. Agriculture, Forestry 

 Division Bulletin No. 7, pp. 171-173. 



* H. Puchner: Investigations of the Carbonic Acid Contents of the Atmos- 

 phere. 



* ,M. W. Harrington : Review of Forest Meteorological Observations, U. 

 S. Dep. Agriculture, Forestry Division Bulletin No. 7, p. 105. 



