Air usually contains a proportion of solid matter suspended in small particles. The nature and the
quantity of such impurities depends, of course, upon circumstances : thus, the air of towns will contain more
solid impurities—of dust, carbon (in smoke), etc., than the air over country places. The air of crowded rooms
wiU contain particles of organic matter (usually poisonous), given off by the lungs. Gaseous impurities emanate
from sewers, marshes, graveyards, chemical works, etc., and give rise to various ailments, including vomiting,
diarrhoea, etc. Sulphurous acid, which is prevalent in towns where coal and gas are largely burnt, is one of the
chief causes of the difficulty experienced in cultivating trees or shrubs in such localities.