21 
tions of those organs which are frequently found in persons 
thus engaged. To remove this dust by ventilation, is certain- 
ly impracticable, on account of the vast volume of air re- 
quired to pass through such rooms. It has been tried, how- 
ever, but, as might have been expected, with only insignifi- 
cant effect. 
Respirators, constructed of wire gauze, filled in with cotton, 
and which must cover both the mouth and nose, are the only 
means of effectually protecting the workman. These have 
lately been again recommended and introduced in England, 
and are said to accomplish all that could be expected. 
Against the other impurities found in the air of inhabited 
places, as mentioned above, we have but one remedy, and 
that is ventilation, or speedy renewal of the air. 
A certain amount of ventilation is always going on through 
the walls, if they be dry and not constructed of impenetrable 
material, and also through cracks in doors and windows. 
For ordinary purposes, this ventilation might suffice to 
keep the air in a room about at the proper standard ; but 
this is not the case in rcoms which are uncommonly crowded. 
Here recourse must be had to artificial means for promoting 
the renewal of the air. I cannot go into the details of the vari- 
ous modes of ventilation in use ; but can only mention, that the 
main object is, not only a desirable activity in the flues used 
for supplying good air and removing that which is waste, 
but also a system insuring a good distribution of the supply 
of air. The latter can be obtained, either by causing an ade- 
quate circulation of the air ina room, or by supplying the 
air through numerous openings in the floor. The first 
method was found to be in use insome of our public schools, 
the latter in two of our New York theatres. 
The bad influence of damp walls is found not only in the 
fact that they do not admit any outside air, but that they 
also condense the dangerous portions of exhaled air. The 
air in under-ground tenements is for this reason always 
richer in organic substances than might be anticipated from 
the amount of carbonic acid found therein. 
As I have used Pettenkofer’s method of the examination of 
air, which consists merely in a determination of the carbonic 
acid present, it was necessary to calculate on certain occasions 
the amount of carbonic acid formed by the combustion of 
gas. It was thus ascertained, that in theatres the amount of 
carbonic acid thus produced exceeds that due to respiration 
from 8 to 7 times, according to the more or less crowded state 
of the place. 
