SANITARY ESSENTIALS. 318 



water in their natural state, in their intimate structure ; and all 

 soils hold — under varying conditions of density or porosity, 

 modified by vegetable growth and the deeper impenetrable 

 layers, or stratas — either water or air in the spaces between 

 the particles of which they are composed. 



This water, chiefly supplied by rainfall and storms, obedi- 

 ent to the law of its nature, seeks by gravitation its level, 

 displacing and forcing air from the earth to the amount of its 

 bulk, it being heavier than air ; and, whenever the point of 

 saturation is reached, all the air is expelled. 



Nature is said to abhor a vacuum : the ground-air, therefore, 

 often reaches to a great depth, and is in constant circulation. 



Its currents are modified by the ground-water, by atmos- 

 pheric and telluric phenomena, such as heat, cold, storm, 

 wind, barometrical and electrical changes. 



The earth is constantly absorbing or emitting this ground- 

 air: it is always loaded with moisture, and more or less 

 impurity, being mixed with gases derived from the fermenta- 

 tion and decomposition of organic material in the soil. It 

 may be laden with miasmatic, morbific agents, causing fevers 

 and debilitating diseases. Therefore the purity of the 

 ground-water and ground-air in the vicinity of a site in- 

 tended for a dwelling is of the highest importance in their 

 influence upon the health, comfort, and happiness of its occu- 

 pants. 



The site selected may be improved b}^ grading to direct 

 the flow of surfa(?e-water from the buildings, and b}" under- 

 draining by means of tiles. The cellar should be drained by 

 placing tile-drains outside, and lower than the foundation- 

 walls. It should also be coated with a carefully prepared 

 layer of cement-concrete, to keep air and dampness from 

 entering the house from the earth ; dryness of soil and air 

 being the chief sanitary essential to be secured in selecting 

 a site for a residence. Speaking upon this point. Dr. Henry 

 I. Bowditch of Boston, who made the subject of consumption 

 (in relation to its causes, and its prevalence in connection 

 with influences derived from the soil) a subject of special 

 investigation, says, — 



" In choosing a site for a dwelling-house, the great deside- 

 ratum is to obtain, not a perfectly arid place (for no such spot 

 could be inhabited by man), but a portion of a township 



