41 
of Warming and Ventilating Buildings. 
which is expired ; but it will be evident, that, if the whole mass 
of air be ascending with a slow current, and there be apertures 
for its escape at the top of the room, the diffusion will be less 
than in still air, and much less than it would be, if the ascent 
were interrupted by descending streams of cold air. While the 
vitiated air retains its heat, it may easily be shown that it will 
be lighter than common air, and consequently will ascend with 
greater velocity, and go off by the apertures ; but if it be re- 
tarded, so as to become of equal temperature with the common 
air, it will descend, become diffused, and deteriorate the rest of 
the air in the room. 
It will be evident, then, that ventilation should be continual, 
during the time a room is in use ; that the heated air should be 
given out at the highest parts of the room, and the cooler fresh 
air should enter at the lower parts. That, previous to a room 
being used, it should be ventilated freely ; and also, immediate- 
ly after it has been used, in order that any effluvia, which has 
collected through imperfect action of the ventilating process, 
may be removed. In warm weather, the latter change would 
be assisted, by washing, or sprinkling with water. 
But it is too common to let a room acquire an oppressive tem- 
perature before ventilation is given ; to provide no places for 
supplying cold air, except what chance furnishes ; or if it be 
supplied at all, it is at the upper part of the room, so as to in- 
terrupt the ventilation, instead of amending it. 
It will sometimes happen, that, through want of attention to 
ventilation, the air will arrive at that state of density, which ren- 
ders it in equilibrium with the external air, though of a higher 
temperature. In such a case, opening windows, or ventilators, 
produces no effect in still weather; and it becomes necessary to 
resort, either to mechanical power, or heat, to change the air. 
In hospitals, and buildings of a like description, it is therefore 
desirable to provide such means of ensuring a regular change 
of air. 
Whatever abstracts from the air the carbonic acid gas, which 
has accumulated by respiration, or otherwise, renders it more 
fit to sustain life, but .1 do not imagine the simple increase of 
carbonic acid gas to be the.cause of air being unhealthy ; it seems 
more likely to be owing to the avidity with which carbon unites 
