40 
Mr Tredgold cm the Principles and Practice 
preserved the mass of air in a room in a state of purity, the 
prime object of ventilation would have been accomplished ; but 
it is an obvious truth, that, unless we extract all the air which is 
injured, it must accumulate ; for, in consequence of the tendency 
of gaseous bodies to mix, when suffered to remain, long in con- 
tact, the air given off* from the lungs must mix with, and so far 
deteriorate all, the air in the room. Now, the mere change of a 
portion of this mixture for an equal portion of fresh air, will 
only improve the air of the room, by the removal of as much of 
the whole quantity of injured air, as is 'expressed by the fraction 
of which the numerator is the air extracted, and the denomina- 
tor the bulk of air in the room. Therefore, either a very great 
proportion of air must be removed by ventilation, or, we must 
endeavour to find the means of removing that which is unfit for 
supporting life, as soon as it is generated. 
In practice it is always inconvenient to introduce fresh air in 
large quantities ; it is expensive in winter, and fills every thing 
with dust in summer ; and, in this variable climate, the process 
becomes quite unmanageable in spring and autumn, Hence we 
are compelled to seek for the means of removing the noxious air 
before it has had time to mix with the air of the room ; and we 
are not a little encouraged in the research, by observing that 
our Creator has provided for the removal of the air we eject 
from the lungs in such a manner that we cannot inspire it again 
in a free atmosphere. The air in respiration loses its oxygen, 
and this loss is replaced by about an equal bulk of carbonic acid 
gas, which is heavier than oxygen in the ratio of 1 : .725j but 
the air expelled from the lungs is given out at a temperature of 
about 90°, and is nearly, if not quite, saturated with the quan- 
tity of vapour due to that temperature, which vapour and azote 
are both lighter than common air. Consequently the mixture 
of azote, carbonic acid gas, and vapour ejected from the lungs, 
is specifically lighter than common air, and ascends with con- 
siderable velocity ; the remarkable pause, which occurs imme- 
diately after an expiration, gives time for its ascent, and for a 
fresh supply of air to approach for the succeeding inspiration. 
It may be remarked, that the ejected air gradually diffuses 
itself among the air it rises through, which renders it necessary 
to provide for the removal of a much greater quantity than that 
