THE SANITAEY INFLUENCE OF FOKESTS 17 



gases, and bacteria, which are all found in the air of cities, 

 are rare or completely absent in that of forests. Micro- 

 organisms are carried into the air from ordinary soils, when 

 their upper layers are dried and dust is formed, which is 

 easily moved by the wind. In the forest there is less 

 movement of air, and fewer microbes in the atmosphere, as 

 was verified by the observers Serafini and Arata, who found 

 all kinds of bacteria less numerous in the forest air than on 

 its outskirts, generally 23 to 28 times less. The foliage 

 of the trees acts as a kind of filter, and retains the dust and 

 other particles which are contained in the air that passes 

 over a forest or belt of trees. 



Apparently no experiments have been carried out with 

 respect to ionisation, dust, and nuclei in the air over forests. 

 Prof. A. S. Eve in a letter says : " The factor which 

 differentiates pure bracing air from relaxing air remains 

 obscure. No doubt the absence of dust and nuclei is a 

 factor, but there are probably other causes. Air which is well 

 cleaned by rain or snow, with all the water present as vapour, 

 and not as fog or mist, is bracing. Scots pine woods in 

 Surrey are better than mixed woods in, say, Bedfordshire. 

 In many cases the air passes from one state to the other in 

 a few hours. What are the changes which take place ? " 



With regard to the soil of the forest, Ebermayer showed 

 that it contains less albuminoid matter and salts suitable 

 for bacterial growth, and that the humus produced by the 

 growth of trees is antagonistic to pathogenic bacteria, which 

 have not been found up to the present time in the soil of 

 forests. Both the soil and the air of forests may then be 

 looked upon as pure from the point of view of health. 



It is stated that in India villages surrounded by forests 

 are never visited by cholera ; and troops are removed to 

 forest stations to arrest the disease. Huff el (9) confirms 

 this by the statement that the town of Haguenau in Alsace, 

 which is encircled by a magnificent forest nearly 50,000 

 acres in extent, was always free from the epidemics of 

 cholera which in the last century attacked several times 

 the other towns in the same district. The classic example 



c 



