AND PUEVENTION OF MALARIA. 227 



of forces in it has uot been studied. The surface is acid, 

 the lower part is alkaline ; it is a filter, with a power of 

 retention and selection. The flow of air and water is from 

 the surface downwards. The surface therefore becomes 

 first oxidised, and is made acid. This acidity is itself a 

 preventative of great putrefaction, and this oxidation is a 

 destruction of those substances which may have resulted 

 from the rotting masses. When land is well drained these 

 processes will go on with greater rapidity ; it is as if the 

 lungs were enlarged. But notwithstanding this oxidation 

 and prevention of decomposition we must not forget that it 

 is useful to counteract another agency of great importance, 

 that by which the plant rots. This rotting is accompanied 

 by grubs, insects, animalcules, and vapours and gases ; it 

 closely resembles, and no doubt in part is a suppressed 

 putrefaction. The soil has the power of retaining the 

 products to a great extent and until it has destroyed 

 them, when they are not excessive. 



If the passage of air through a soil is not sufficient the 

 products of decomposition are not destroyed. If the 

 passage of water through a soil be not sufficient the soluble 

 products are not removed into the soil, but left, and as- 

 sisted to putrefy. 



It is certainly an object of great interest to be able to 

 know by the examination of a soil if it could or would 

 produce malaria, or if it were really in the act of producing 

 it. By the view taken most soils, indeed all containing 

 putrescent organic matter, will be subject to malaria when 

 the oxidising influence of the air and the balance of vege- 

 table and chemical life are disturbed. The soils most 

 productive of malaria are low grounds, and moist ones with 

 abundant vegetation. At the same time there are others 

 not described as such, and not very thoroughly investi- 

 gated. If we examine the surface water flowing from such 

 grounds in this country we shall find it to contain animal 



