42 Mr Tredgold on the Principles cmd Practice 
with nearly every species of effluvia, and bears it to that part of 
the human frame which'is most susceptible of injury. Not only 
the gas from the lungs, but also the whole portion of carbonic 
acid which the air contains, is soon charged with deleterious 
matter, when ventilation is not properly attended to ; and the 
effluvia must either be neutralised by the agency of a volatile 
acid, which possesses a greater affinity for it, or the carbonic 
acid must be removed. 
In considering the principles of ventilation, it must be ob- 
vious, that it is much more necessary in some places than in 
others ; in isolated houses it may be neglected with impunity, 
but, in the confined streets of extensive towns, it must not be 
left to chance. Even in planning towns, the importance of 
thorough scope for the winds to follow the valleys, should be 
regarded, that the heavy impure air may be driven away. 
When a narrow street crosses a valley, without being crossed 
by another street, at the lowest part, it becomes very difficult 
to keep it in a proper state. But, in many cases, we meet with 
streets on level ground, planned as if it were to render it im- 
possible for a current of air to follow them ; and from the very 
circumstance of their not being pervious to the fresh air, they 
become the resort of the wretched, with a tenfold increase of 
filth and misery. By forming the New Street in London, much 
good has been done ; and one cannot well let this opportunity 
pass, without expressing a wish, that other openings may be ef- 
fected, planned with a more direct view to the health and con- 
venience of the metropolis, unencumbered by massive colonades. 
The giant members of the Doric column were never designed 
for a screen to a toy shop. 
The usual construction of prisons renders them similar to 
the interrupted and confined streets of towns, but the improve- 
ment of raising the cells above the ground storey must be very 
beneficial ; and the extensive area, inclosed by the walls, and the 
isolated buildings of some of the best prisons, must render them 
very healthy, when a proper attention to cleanliness is observed. 
An elevated site is clearly the best for any building which is to 
be inclosed by high walls ; there should be as few internal divi- 
sions of the area as possible ; and long rectangular yards, with 
open railing at the ends, seem better adapted for ventilation and 
