ng-surface or steam jets are required. The
small open vessels or saucers on which some people rely, even when
located in the air-passages of a hot-air furnace, have only an
infinitesimal influence. Vertical wicks of felt with their lower ends in
water kept hot by the heating apparatus yield a rapid supply of
moisture. Evaporation is greatly facilitated if the water or wicks are
placed in the current of heated air entering the room. By a suitable
construction, the water may be replenished automatically. In very cold
dry weather, the air-supply of an ordinary medium-sized house requires
the addition of not less than 10 gallons of moisture every 24 hours, and
sometimes much more.

Some authorities doubt any ill effects from extreme dryness. This is a
subject yet to be cleared by experimental research.

[Sidenote: Freshness]

It is obvious that fresh pure air is preferable to impure air. Air may
be vitiated by poisonous gases, by dust and smoke, or by germs. Dust and
smoke often go together.

Lighting by electricity is preferable to lighting by gas, as some of the
gas is liable to escape and vitiate the air.

[Sidenote: Tobacco Smoke]

A very common and at the same time injurious form of air-vitiation is
that from tobacco smoke. Smoking, especially in a closed space such as a
smoking-room or smoking-car, vitiates the air very seriously, for smoker
and non-smoker alike.

[Sidenote: Dust]

As to dust, the morbidity and mortality rates in certain occupations,
particularly those known as the dusty trades, are appreciably and even
materially greater than in dustless trades.

An accumulation of house-dust should be avoided. The dust should be
removed--not by the old-fashioned feather duster which scatters the dust
into the air--but by a damp or oiled cloth. Dust-catching furniture and
hangings of plush, lace, etc., are not hygienic. A carpet-sweeper is
more hygienic than a broom, and a vacuum cleaner is better than a
carpet-sweeper. The removable rug is an improvement hygienically over
the fixed carpet.

[Sidenote: Bacteria]

The bacteria in air ride on the dust-particles. In a clean hospital
ward, when air was agitated by dry sweeping, the number of colonies of
bacteria collected on a given exposure rose twenty-fold, showing the
effect of ordinary broom-sweeping.

[Sidenote: Sunlight]

The air we breathe should be sunlit when possible. Many of our germ
enemies do not long survive in sunlight.

Section II--C