THE FOREST AS A CONDITION. 77 



The experiences of France in this particular 

 are incontrovertible arguments, and furnish, in 

 later years, experimental evidence of the effec- 

 tiveness of a forest cover in arresting the progress 

 of erosion. France, too, furnishes perhaps the 

 most striking and most extensive example of how 

 the loose, shifting sands, the dunes and sand hills 

 in the plain, may be changed by a forest cover 

 from a useless, nay dangerous, condition into one 

 of profitable occupation. 



Regarding the sanitary influence of forests, there 

 have also been many claims made which cannot be 

 substantiated. The original principal claim was 

 that the physiological action of the foliage, in ab- 

 sorbing carbonic acid from the air and exhaling 

 oxygen, made forest air healthier, but it has been 

 calculated that the amount of oxygen so exhaled is 

 insignificant in proportion to the needs of human 

 respiration, and is probably offset by the increase 

 of carbonic acid resulting from the decomposition 

 of organic matter in the forest. 



Then it was claimed that by the transpiration of 

 the foliage wet ground may be drained, and thus 

 made healthier, and in this connection the Eucalyp- 

 tus plantations at the monastery of Tre Fontane in 

 the Campagna Romana are frequently cited as hav- 

 ing removed the malarial conditions of that region. 

 As a matter of fact, the fevers still occur, even 

 under the Eucalyptus plantation, although more 

 rarely. This comparative improvement seems 



