Soil
The soil bears a close relation to health. The porous soils such as sand and gravel are considered the most healthful because they are driest. But there are objections to these, since they are easily polluted by leaky drains and cesspools, and are capable of holding a notable volume of ground water and ground air. Such air is almost invariably impure. The rain percolates through the soil until it reaches an impervious stratum which prevents it from penetrating any further, and it is held in the interstices or interspaces of the soil and forms the ground water.