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POPULAR SCIENCE REVIEW. 
with the quantity of air to he supplied per hour. It is a matter 
for arithmetical calculation, but it seems to us by no means 
difficult to show that, whether the room he large or small (assu- 
ming it to be constantly in use), the quantity of air introduced 
must be the same in order to reduce its atmosphere to the 
standard demanded by hygiene. The quantity of carbonic 
acid developed per hour is a constant quantity ; and as it diffuses 
itself thoroughly through the room, it is evident that the quan- 
tity of air required to dilute it to innocuity must also be definite. 
Of course, in applying this in practice, it is necessary to assume 
that the room will ultimately arrive at a certain degree of pol- 
lution by carbonic acid. And this is just the particular which 
shows that if the room he not constantly occupied, Dr. Parkes’ 
view is correct. It takes a much shorter time to effect the 
pollution of the air to the required standard in a small room 
than in a large one. Hence in a room of great capacity the air 
might not become impure in the few hours during which it was 
in occupation. In hospitals and suchlike institutions it would, 
of course, be different. We cannot afford space to go into the 
calculation by which Professor Donkin arrives at his conclusion, 
hut we may state that he believes 3,000 cubic feet of air to be 
the minimum which should be supplied per hour to each indi- 
vidual; this being, of course, independent of the size of the 
room. The question of size, however, cannot he passed over as 
unimportant, for the simple reason that in rooms of small size 
the necessary supply could not be introduced without the 
employment of strong currents of air, which would he not only 
troublesome, but might be even dangerous to health. But it 
must never he forgotten, that though a smaller supply of air 
may suffice to ventilate a larger than a smaller room for a 
short space of time, at a certain period, sooner or later according 
to the capacity of the apartment, the same quantity of air per 
hour must be supplied to both, and this must invariably be 
3,000 cubic feet per hour for each person present. 
Having arrived at the determination of the principle which 
should guide us in ventilation, and having established a rule 
for the quantity of air which must be introduced per head per 
hour into a room constantly occupied, we can now proceed 
with the second part of our enquiry — the methods by which the 
introduction of fresh air is effected. This part of our subject 
is of the highest importance, and, we might add, also of the 
greatest difficulty. It is obvious that in all arrangements for 
ventilating an apartment, it is the same thing whether we pro- 
vide for the removal of a definite amount of foul air, or for 
the introduction of the same quantity of fresh. “ Nature 
abhors a vacuum,” and the elimination of the consumed or 
partially consumed air involves the introduction of an equivalent 
