56 
MISCELLANEA. 
bonic acid gas, the product of respiration, and sulphuretted 
hydrogen, the product of the sewers. In the day gases and 
various substances of all kinds rise in the air by the rare- 
faction of the heat. At night, when this rarefaction ceases, 
they fall by an increase of gravity, if imperfectly mixed with 
the atmosphere, while the gases evolved during the night, 
instead of ascending, remain at nearly the same level. It 
is known that carbonic acid gas, at a low temperature, par- 
takes so nearly of the nature of a fluid, that it may be 
poured out of one vessel into another. It rises at the 
temperature at which it is exhaled from the lungs, but 
its tendency is towards the floor, or the bed of the sleeper, 
in cold and unventilated rooms. At Hamburg, the alarm 
of cholera at night in some parts of the city was so great 
that many refused to go to bed, lest they should be at- 
tacked unawares in their sleep. Sitting up they probably 
kept their stoves or open fires burning for the sake of warmth, 
and that warmth giving the expansion to any deleterious 
gases present, which would best promote their escape, and 
promote their dilution in the atmosphere, the means of 
safety were then unconsciously assured. At Sierra Leone, 
the natives have a practice in the sickly season of keeping 
fires constantly burning in the huts at night, assigning that 
the fires keep away the evil spirits, to which in their ignor- 
ance they attributed fever and ague. Latterly, Europeans 
have begun to adopt the same practice, and those who have 
tried it assert that they have now entire immunity from the 
tropical fevers to which they were formerly subjected. In 
the epidemics of the middle ages fires used to be lighted in 
the streets for the purification of the air ; and in the plague 
of London, in 1685, fires in the streets were at one time kept 
burning incessantly, till extinguished by a violent storm of 
rain. Latterly trains of gunpowder have been fired, and 
cannon discharged for the same object ; but it is obvious 
that these measures, although sound in principle, must 
necessarily, though out of doors, be on too small a scale, as 
measured against an ocean of atmospheric air, to produce 
any sensible effect. Within doors, however, the case is 
different. It is quite possible to heat a room sufficiently to 
produce a rarefaction and consequent dilution of any malig- 
nant gases it may contain, and it is of course the air of the 
room, and that alone, at night, which comes in contact with 
the lungs of the person sleeping. — Westminster Review . 
