CHAP. II. ] its causes and characteristics. 29
A mixture of four parts of sulphuric acid and one part of nitric
acid was found to be as efficient as sulphuric acid in destroying the
plague microbe.
With organic acids.
The microbe was destroyed in five minutes by-
Formic acid of a strength of 1 in 100.
Acetic acid of a strength of 1 in 142.
Lactic acid in a strength of 1 in 333.
"E. Reducing agents.-The microbe was found to be extremely
resistant to the action of ferrous sulphate, a substance that has been
frequently recommended for the treatment of sewage and filth. It
appears generally to die out when evaporated to dryness in a thin film
on glass in the presence of the air, but was found still alive after five
days when evaporated to dryness in glass bulbs in a current of hydro-
gen gas.
"F. Oxidizing agents.-The microbe was found to be destroyed by
a solution of one in ten thousand of chloride of lime, but the lower
limit of the action of this substance was not detected. Permanganate
of potash was found to be capable of destroying it, under the artificial
conditions of my experiments, in a dilution of 1 in 50,000, that is to
say, in a solution in which the pink colour is but faintly marked. In
certain cases it appeared that the organic matter present was sufficient
to destroy all the permanganate used, after the lapse of some hours.
In these cases the microbes were first destroyed and afterwards the
permanganate was reduced. It was shown however by experiment
that the quantity of readily reducible organic matter present on a
cowdung floor is so great that probably a four per cent. solution of
permanganate would be necessary to produce a safe disinfection."
Laboratory
conditions
favourable to the
action of the
disinfectant.
Mr. Hankin recognised that the conditions under which the labor-
atory experiments were carried out were probably more favourable to
the disinfectant than would be the case in practice. "This fact while
tending to justify the condemnation of a disinfectant from the results
of such experiments, necessitates caution in using such experiments to
recommend a disinfectant. The fact that a disinfectant can destroy a
microbe suspended in water, as was the case in my experiments, does
not prove that it would be capable of destroying the microbe when
contained in human dejecta soaked into a cowdung floor. Hence it
appeared to me to be advisable to carry out some experiments in
which practical conditions would be more closely imitated."