may render the gas active. For the purpose
of destroying all insects in a room an exposure of about two hours to
the gas are necessary, while for the destruction of bacteria an
exposure of at least fifteen to sixteen hours is required.

In the application of disinfection with sulphur dioxide, as with any
other gas, it must not be forgotten that gases very readily escape
through the many apertures, cracks, and openings in the room and
through the slits near doors and windows; and in order to confine the
gas in the room it is absolutely necessary to hermetically close all
such apertures, cracks, etc., before generating the gaseous
disinfectant. The closing of the openings, etc., is done by the
pasting over these strips of gummed paper, an important procedure
which must not be overlooked, and which must be carried out in a
conscientious manner.

When sulphur is used in candle form the expense is considerably
increased without any additional efficiency. When a solution of
sulphurous acid is employed, exposure of the liquid to the air
suffices to disengage the sulphur dioxide necessary for disinfection.
The quantity of the solution needed is double that of the crude drug,
i. e., ten pounds for every 1,000 cubic feet of room space.

=Formaldehyde.=--At present the tendency is to employ formaldehyde gas
instead of the sulphur so popular some time ago. The advantages of
formaldehyde over sulphur are: (1) its nonpoisonous nature; (2) it is
a very good germicide; (3) it has no injurious effect upon fabrics and
objects; (4) it does not change colors; and (5) it can be used for the
disinfection of rooms with the richest hangings, bric-a-brac, etc.,
without danger to these. Formaldehyde is evolved either from paraform
or from the liquid formalin; formerly it was also obtained by the
action of wood-alcohol vapor upon red-hot platinum.

Formaldehyde gas has not very great penetrating power; it is not an
insecticide, but kills bacteria in a very short time, and spores in an
hour or two.

Paraform (polymerized formaldehyde; trioxymethylene) is sold in
pastilles or in powder form, and when heated reverts again to
formaldehyde; it must not burn, for no gas is evolved when the heating
reaches the stage of burning. The lamps used for disinfection with
paraform are very simple in construction, but as the evolution of the
gas is very uncertain, this method is used only for small places, and
it demands two ounces of paraform for every 1,