CHAPTER VIII.. Earth, Air, and Water, in Relation to Man--How Modified by

Earth, air, and water, in relation to man--How modified by him--Results of that modification--Action and reaction--Antagonism of man to nature--Effects of human labour on the soil--How man protects his dwelling--Distinction between a drain and a sewer, a distinction first practically denied in England--Chemical elements of animal bodies--Nourishment of plants--Exhaustion of the soil in Virginia--Value of farm-yard manure--Agriculture in China--Effects of clearing the primæval forests of America--Causes of the hay-fever, typhus and typhoid fevers--Effects of bad ventilation--Importance of the infusoria in nature’s great scheme--Origin and action of humus--Functions of the humus and of the leaves--Means adopted in Holland for the conversion of a bog or morass into a polder--Antediluvian vegetation--Elements which require being restored to the soil--Belgian agriculturists--Statistics of Quetelet pp. 66–88