SANITARY EFFECT. 173 



Although their exterior existence is unknown they probably also 

 come from the soil, and probably, also, warmth or wet soil, periodically 

 changing to dry in tlie ui)per strata, are the best conditions for their 

 development. As long as the water covers the soil there is, therefore, 

 no danger, which only begins with the recession of the same and the 

 admission of air necessary for the development of the plasmodia, which 

 accumulate then near the soil. Since they can not rise very high 

 houses placed on hills within malarial regions or high above the grouDd 

 are free from the disease. 



The same reasons which make the forest conditions unfavorable to 

 bacteria growth, namely reduced temperature and moister soil, with 

 more ready drainage usually, as well as the absence of dust and winds 

 raising the same and the filtration process shown by Serafiua by which 

 the bacteria coming from the outside are reduced in number, are also 

 reasons for the observed beneflcient efiect of forest plantations in ma- 

 larial districts, such as the Oampagna liomana. 



Finally, a word on the value of larger parks in cities. While they do 

 not, according to these expositions, purify the air directly by the func- 

 tion of the leaves, as has been claimed, theycertainly establish air cur- 

 rents that may bring fresher air to the ground; but their hygienic 

 function consists mainly in reducing the temperature by their shade 

 and in furnishing better drainage conditions of the soil and i)urifying 

 the same by absorbing the results of decomposition from animal mat- 

 ter, and lastly by preventing' or reducing at least dust and with it baC' 

 teria in the air, keeping it ^aiev than it would otherwise be, 



