000 cubic feet of space,
with an exposure of twelve hours. Formaldehyde is also used in the
form of the liquid formalin either by spraying and sprinkling the
objects to be disinfected with the liquid, and then placing them in a
tightly covered box, so that they are disinfected by the evolution of
the gas, or by wetting sheets with a formalin solution and letting
them hang in the room to be disinfected.

The method most frequently employed is to generate the formaldehyde in
generators, retorts, and in the so-called autoclaves, and then to
force it through apertures into the room.

Of the other gaseous disinfectants used, hydrocyanic acid and chlorine
may be mentioned, although they are very rarely used because of their
irritating and poisonous character.

=Hydrocyanic Acid= is frequently used as an insecticide in ships,
mills, and greenhouses, but its germicidal power is weak.

=Chlorine= is a good germicide, but is very irritating, poisonous, and
dangerous to handle; it is evolved by the decomposition of chlorinated
lime with sulphuric acid. Chlorine gas is very injurious to objects,
materials, and colors, and its use is therefore very limited.

_Chemicals Used as Disinfectants_

Solution of chemicals, in order to be effective, must be used
generously, in concentrated form, for a prolonged time, and, if
possible, warm or hot. The strength of the solution must depend upon
the work to be performed and the materials used. The method of
applying the solution differs. It may consist in immersing and soaking
the infected object in the solution; or the solution may be applied as
a wash to surfaces, or used in the form of sprays, atomizers, etc. The
most important solutions of chemicals and the ones most frequently
employed are those of carbolic acid and bichloride of mercury.

=Carbolic Acid.=--In the strength of 1:15,000 carbolic acid prevents
decomposition; a strength of 1:1,000 is needed for the destruction of
bacteria, and a three per cent to five per cent solution for the
destruction of spores. Carbolic acid is used, as a rule, in two per
cent to five per cent solutions, and is a very good disinfectant for
washing floors, walls, ceilings, woodwork, small objects, etc. The
cresols, creolin, lysol, and other solutions of the cresols are more
germicidal than carbolic acid, and are sometimes used for the same
purposes.

=Bichloride of Mercury= (corrosive sublimate) is a potent poison and a
powerful germicide; in s