6

being lighter than air, if there are two storeys begin at the top; and if there are
many rooms in a corridor begin at the farthest from the exit. An assistant may
go in front mixing the acid and water. Great quickness and care are essential
as the gas is so poisonous.

    Of course if a tenement is to be disinfected all inhabitants must be cleared
out first. It is essential to have a clear space round the building to be fumigated.
It is recorded in one of the American experiments that persons walking 100
feet from the building could detect the odour of the gas the whole time.

    The period of exposure should be two hours at least. It is better not to try
ventilation till 12—24 hours have elapsed. So far as we have seen in our
experiments (with the 1-2-4 formula) in 50 minutes the action of the sulphuric
acid on the potassium cyanide has been completed. So really the ventilation of
the building can be carried out in two hours if the doors and windows can be
opened from the outside. It is at the moment of entering the building after fumi-
gation that danger is most to be guarded against.

    After opening the doors and windows from the outside certainly half an
hour should elapse before any one enters the building, and it is safer to
wait three hours. It depends altogether on the character of the building and if
there are many doors and windows which can be opened from the outside. We
have entered a room five minutes after the delivery of the gas has stopped.
There is a very good test suggested to us by Captain Dickinson, the Chemical
Analyst of Bombay. It depends on the formation of prussian blue by the gas if
there is any free in the room. A paste is made up consisting of ferrous sulphate
and caustic potash. A little is put on a rod of glass and the rod introduced into
the room. It is exposed to the air of the room for five minutes. If hydrocyanic
acid gas is present sodium ferrocyanide is formed. A few drops of pure hydro-
chloric acid are then run on the paste and a little water containing a few drops.
of ferric chloride added in a beaker. If there is any hydrocyanic acid gas at all
in the room blue colour is formed—deep blue if much gas; faint green blue if a
little. If only a yellow colour is present it is quite safe to enter the room. This.
is a most delicate test. It will detect 1/780 grain of hydrocyanic acid in a very
dilute liquid (Watt's Dictionary of Chemistry). It is much safer to rely on this.
test than on the odour of almonds, as the sense of smell seems to be paralysed
by the diluted gas after the first few whiffs. Even a trace of hydrocyanic acid in
the air leads to headache and nausea. It is so deadly that a whiff of fairly strong
gas kills at once.

    It is obvious that the greatest care must be taken during the whole pro-
cedure. No naked lights should be left about a building as hydrocyanic acid gas
is somewhat inflammable.