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air to be about five of the former to one of the latter, a ratio which would in all probability be reversed
in the case of sewage. There are two points with reference to the organisms in sewer air to which I
wish to call special attention—firstly, the absence of bacillus coli communis and micrococcus ureae,
two organisms which must be present in sewage in immense numbers; secondly, the almost entire
absence of any organisms capable of very rapidly liquefying gelatine, the only exceptions being the
common hay bacillus (bacillus subtilis) and a micrococcus mentioned in ray first report. In sewage on
the other hand a large number of organisms, for the most part bacilli, possess this property of very
rapidly liquefying gelatine. All the micro-organisms tabulated above belong to that class known as
non-pathogenic or harmless bacteria, and with one exception are common in air and water.
Two organisms, a bacillus and a micrococcus, concerning which there was some slight suspicion as
to their non-pathogenic properties, I submitted to Dr. F. Andrewes, as instructed by your chairman.
From his report it is seen that the bacillus was one of the many lactic acid ferments, and
that the micrococcus which occurred only once during the course of these investigations was the
staphylococcus cereus flavus of Passet, a rather rare organism occurring in pus, but quite devoid of
any pathogenic property.
The results of these further investigations strengthen very considerably the conclusion arrived
at from my previous experiments, viz., that 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.
As our knowledge of the various organisms occurring in sewage under varying conditions is so
limited, it is in my opinion most desirable they should be submitted to careful examination. A
knowledge of the various species would in all probability considerably strengthen the conclusion
that the micro-organisms in sewer air are derived solely from the fresh air gaining access to the sewer by
ventilation.
Although one is led almost irresistibly to the conclusion that the organisms found in sewer air
probably do not constitute any source of danger, it is impossible to ignore the evidence, though it be only
circumstantial, that sewer air in some instances has had some causal relation to zymotic disease. It is
quite conceivable, though at present no evidence is forthcoming, that the danger of sewer air causing
disease is an indirect one; it may contain some highly poisonous chemical substance, possibly of an
alkaloidal nature, which though present in but minute quantities may, nevertheless, produce in
conjunction with the large excess of carbonic acid a profound effect upon the general vitality.
I am, Gentlemen,
Yours very faithfully,
7th December, 1893. J. Parry Laws.