THE SANITARY INFLUENCE OF FORESTS 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 
afew 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 
erowth 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. Huffel (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 
