not the density of population which regulates the health. But if a dense
population is spread over the surface or close to the surface of the
ground, by which means all movement of air is prevented, and if there
are numerous corners in which refuse is accumulated, it will be
difficult to prevent disease. Dr. Angus Smith's experiments show that
while there is less oxygen and more carbonic acid in the eastern and
in the more crowded parts of London, yet in open spaces the amount of
oxygen rises and the carbonic acid diminishes very considerably; and
that we are exposed to distinct currents of good air in the worst, and
equally to currents of bad air in the best atmosphere, in towns like
Manchester.

Dr. Tyndall showed that where there is quiescence in the air the
tendency of his sterilized infusions to produce organisms was increased.
The conclusion from all these experiments is to show the importance of
laying out the general plan of dwellings in a town so that currents of
air shall be able to flow on all sides with as little impediment as
possible, by which means the air will be continually liable to renewal
by purer air. The dwellings which have been constructed in the place of
the very defective dwellings condemned by the medical officers of health
in various parts of London specially illustrate the importance of this
question of the circulation of air. These dwellings replace those in
which the normal mortality was as much as 33, 44, and 50 per 1,000. But
these improved dwellings provide ample space all round the blocks
of building, so that air can flow round and through them in every
direction, and so that there are no narrow courts and hidden corners for
the accumulation of refuse. The mortality in the new dwellings is as low
as 13 per 1,000 in some, and does not rise above 20 per 1,000 in any of
them, and upon an average of years it may be taken at from 14 to 16
per 1,000. It is to this point that I specially desire to draw