The necessity of keeping the atmosphere of workrooms pure and fresh makes it essential that there should be sufficient cubic space per person and that proper circulation of the air should be maintained. The minimum amount of cubic space legally fixed in many countries—10-15 cubic metres—is a minimum and should be greatly exceeded where possible. Natural ventilation which is dependent upon windows, porosity of building materials, cracks in the floors, &c., fails when, as is desirable for purposes of cleanliness, walls and floors are made of smooth impermeable material, and natural ventilation will rarely supply the requisite cubic feet of fresh air quickly enough. Ordinarily, under conditions of natural ventilation, the air in a workroom is renewed in from one to two hours. Artificial ventilation therefore becomes imperative. Natural ventilation by opening windows and doors can only be practised in intervals of work and as a rule only in small workrooms. During work time the draught and reduction of temperature so caused produce discomfort. _Artificial ventilation_ is effected by special openings and ducts placed at some suitable spot in the room to be ventilated and arranged so that either fresh air is introduced or air extracted from the room. The first method is called propulsion, the latter exhaust ventilation. Various agencies will produce a draught in the ventilating ducts, namely, difference of temperature between the outside and inside air, which can be artificially strengthened (_a_) by utilising the action of the wind, (_b_) by heating the air in the exhaust duct, (_c_) by heating apparatus, and (_d_) by mechanical power (use of fans). Where advantage is taken of the action of the wind the exit to the ventilating duct must be fitted with a cowl. The draught in pipes is materially increased if they are led into furnace flues or chimneys; in certain cases there is advantage in constructing perpendicular ventilating shafts in the building extending above the roof and fitted with cowls. Combination of heating and ventilation is very effective. [Illustration: FIG. 45.—Steam Injector (_after Körting_), showing steam injector and air entry]