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the doors and windows, and through the chimneys. This may 
be easily proved, experimentally, by holding a lighted taper near 
any of these points. But it is equally true a priori. It happens 
generally that the inlet is not sufficient to remove the carbonic 
acid rapidly enough to reduce its proportions to the standard 
above given. The question then arises, How are the con- 
ditions necessary for the production of this standard ascertained ? 
This is exactly what we propose to discuss, for it supplies the 
key to the whole principle of ventilation. There is the less 
difficulty in popularising this subject, that of late it has received 
the studious attention of some of our ablest chemists and 
sanitarians, who have laid their, opinions before the public.* 
Before, however, we plunge in medias res of ventilation, we 
must ask the reader to remember that the physical law known 
as that of “ mutual diffusion,” plays an important part in all 
questions relating to the mixture of different gases, such as 
of oxygen, nitrogen, and carbonic acid, which make up our 
atmosphere. By virtue of this law, it occurs that two gases 
when brought together, no matter what their relative weights, 
become thoroughly mixed together, in proportions which are 
stated as being inversely as the square roots of their densities. 
Carbonic acid is a gas so heavy that it may be decanted from 
one vessel into another ; and hydrogen is so light that a balloon 
filled with it ascends, as we all know, into the air. Yet if a 
vessel filled with the latter be inverted over one containing 
the former, and a piece of membrane be placed between the 
mouths of the two, it will be found that, after a while, some 
of the carbonic acid has ascended into the upper vessel, and 
the hydrogen has descended into the lower one, and mingled 
with the carbonic acid. A mixture will be thus formed in both 
vessels. It is the same in nature. Animals are perpetually 
exhaling carbonic acid into the atmosphere, and were it not 
for this wonderful property of . “ diffusion,” a stratum of foul 
air would lie over the earth, and would possibly extinguish 
animal existence. This, then, is one of the great facts of venti- 
lation, as it is in nature. There is, however, another point in 
which natural ventilation is superior to all other forms — viz., that 
plants use up the carbonic acid as food, setting free the oxygen 
which helped to form it, and thus, as it were, manufacturing air 
for the use of animals. The law of diffusion the reader must 
consider as the starting-point in all ventilation problems. With 
this law as his guide, he is prepared for the consideration 
* See Papers by Dr. A. Smith, F.R.S., Dr. E. Smith, F.R.S., Professor 
Donkin, F.R.S., and Dr. Parkes, F.R.S., in the “Report of the Committee 
appointed to consider the Cubic Space of Metropolitan Workhouses.” Pre- 
sented to both Houses of Parliament, 1867. 
