286 IMPORTANOH OF FREE VENTILATION. — 
oxygen, produced by the burning of gas-lights, lamps, or 
candles, 
337. Hence we see the great importance of providing for 
free ventilation, wherever large assemblages of persons ate 
collected together, even in buildings that seem quite adequate 
in point of size to receive them ; and much of the weariness 
which is experienced after attendanee on crowded assemblies 
of any kind, may be traced to this cause. In schools, facto- 
ries, and other places where a large number of persons remain 
curing a considerable proportion of the twenty-four houts, it 
-is impossible to give too much attention to the subject of 
ventilation ; and as, the smaller the room, the larger will be 
the proportion of carbonic acid its atmosphere will contain, 
after a certain number of persons have been breathing in it 
for a given time, it is necessary to take the greatest precaution 
when several persons are collected in those narrow dwellings, 
in which, unfortunately, the poorer classes are compelled to 
reside. Even the want of food, of clothing, and of fuel, are 
less fertile sources of disease than insufficient ventilation ; 
which particularly favours the spread of contagious diseases, 
on the one hand by keeping-in the poison, and thus concen- 
trating it upon those who expose themselves to its influence ; 
and, on the other, by obstructing the elimination of the waste 
matter from the system, the presence of which im the blood 
renders it peculiarly liable to be acted-on by all poisons 
having the nature of “ferments.” 
338. When the quantity of carbonic acid in the air accu- 
mulates beyond a certain point, it speedily produces suffocation 
and death. ‘This is occasioned by the obstruction to the flow 
of blood through the capillaries of the lungs, which takes 
place when it is no longer able to get rid of the carbonic acid 
with which it is charged, and to absorb oxygen in its stead. 
The general principle to which this stagnation may be referred 
has already been noticed (§ 280). Now, as all the blood of 
the system, in warm-blooded animals, is sent through the 
lungs before it is again transmitted to the body, it follows 
that any such obstruction in the lungs must bring the whole 
citeulation to a stand. The functions of the nervous system 
aré directly dependent upon a constané supply of arterial — 
blood (Chap. x.) ; and, accordingly, as this supply becomes 
progressively diminished in quantity and deteriorated in qua- 
