16 FORESTS, WOODS, AND TREES 
matter in the floor of the forest (8). The occurrence of 
ozone in the air of forests, of mountains, and of seaside 
places, and its absence from the air of streets in towns and 
of inhabited houses, has given rise to the idea that this gas 
has some health-giving properties. It may purify the air 
by oxidising animal or vegetable matter in the course of 
decay and by uniting with the gases produced by their 
decomposition ; but the minute quantity of ozone found in 
the air of forests can have little effect of this kind, and is 
evidently of no hygienic importance. Recent experiments 
have shown that ozone is useless as a disinfectant, since 
the quantity necessary to kill pathogenic bacteria is very 
uritating to the lungs and proves fatal to animals 
experimented on. Hydrogen peroxide also exists in minute 
quantities in the air of forests; but there is considerable 
difficulty in distinguishing by its effects this substance from 
ozone. 
Forests depress the level of the underground water; and 
drainage can often be done effectually by planting trees. 
Diseases like phthisis, bronchitis, rheumatism, neuralgia, 
might then be diminished in forest areas. In damp marshy 
soils, pools are common, and serve as breeding grounds for 
mosquitoes. The planting of Eucalyptus trees in the Cam- 
pagna Romana diminished malaria undoubtedly, by the 
permanent lowering of the subsoil water, which dried up 
the pools that bred the mosquitoes. ‘The réle of the forest 
in draining marshy places, where water stagnates for some 
months of the year, is not doubted by the French, who have 
seen this effect in the pine forests of the Landes and Sologne. 
In the forest of Mondon near Nancy the level of the under- 
ground water is throughout the year at least 12 inches 
lower than in the cultivated land adjoining, to cite only one 
of the extensive series of observations that were carried out 
by Prof. E. Henry of the Nancy School of Forestry. Similar 
results have been established by Ototzky in the forests of 
the steppes of Vorone} province in Southern Russia. 
The most important hygienic asset of the forest is the 
purity of the air therein. Smoke, particles of dust, injurious 
