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The reaction of the sewage in the two experiments with manganate of soda and sulphuric acid
was distinctly acid, and to this acidity I ascribe in great measure the diminution in the number of microorganisms,
for I have shown in another place (Report of the Medical Officer to the Local Government
Board) that sulphuric acid has a powerful disinfectant action.
With reference to the deodorising effect, manganate of soda and sulphuric acid completely
removed the indescribable yet intensely disagreeable smell of sewer air, whereas manganate of soda
alone is not nearly so efficient.
Sulphurous acid, when present in sufficient quantity to remove all odour, becomes irritating to
those engaged in the sewer, and I think its use, at any rate in the manner hitherto employed, is not to
be recommended for another reason, viz., that unless there is a fairly constant current of air the
deodorising effect is entirely localised. Carbolic acid used in the strength above mentioned produces an
excellent result so far as odour is concerned. This is completely masked and the smell of the acid is
not disagreeable, but there is some doubt whether it would be perfectly harmless to those exposed to it,
if breathed for any considerable time.
The positive results of these experiments are the following—
I.—The micro-organisms in the sewer air are related to the micro-organisms in the air
outside, and not to the micro-organisms of the sewage.
II.-—In the air both within and without the sewer the forms of micro-organisms present
are almost exclusively moulds and micrococci; on the contrary, the micro-organisms of sewage
are for the most part bacilli. Of the latter sometimes as many as 25 per cent. very rapidly
liquefy the gelatine on which they grow, whereas in the whole course of my experiments
with fresh air and sewer air I only met with one colony, and that a micrococcus rapidly
liquefying gelatine.
III.—That for purposes of deodorising, manganate of soda and sulphuric acid and carbolic
acid are the most efficient of the several chemicals tried, and setting aside the question of
relative cost, the former is decidedly preferable for the reason stated above.
The whole of my results point unmistakably to the conclusion that the principal, if not the only,
source of micro-organisms in sewer air is the air without the sewer and not the sewage, and they also
tend to prove that there is very little ground for supposing that the micro-organisms of sewage, in the
absence of violent splashing, become disseminated in the sewer air.
In carrying out this work I have made a very large number of cultivations of the various
organisms met with, in order to study them with the microscope both in stained and unstained
specimens. It is in this particular direction that my experiments are so much more complete than
those of Drs. Carnelly and Haldane.
In conclusion let me emphasize the fact that these observations only hold good for sewers in
which the conditions are the same as those under which I experimented. What the condition of the
air would be in sewers in which there is only an intermittent flow of sewage, or in which the sewage
becomes stagnant and highly putrefactive, further experiment alone can show.
If the Committee at any time see fit to further extend their investigations, it would be most
desirable to determine exhaustively the effect of violent splashing on the micro-organisms of sewer air,
and also to make a very minute study of the various organisms met with in sewer air and sewage.
I am, Gentlemen,
Yours very faithfully,
May, 1892. J. Parry Laws.