VENTILATION AND YENTILATOBS. 
403 
of the other principles of this branch of hygiene. Concerning 
the extent of its operation, however, the student encounters 
his first difficulty, for it must be confessed that all writers 
are not agreed as to whether, in the case of the atmosphere 
of dwelling-rooms, the diffusion which takes place is com- 
plete or partial. Decision in regard to this is “ a consum- 
mation most devoutly to be desired,” for in its absence we 
are driven to accept one of two alternatives. For instance, it 
being known how much aii* one individual vitiates per minute 
in breathing, if it were admitted that complete diffusion occurs, 
we could calculate with tolerable precision what quantity of 
fresh air should be introduced per minute into a room of known 
capacity, in order to enable healthy respiration to take place. 
But if we admit that the diffusion is only partial, and that 
atmospheric currents may sweep away the foul air as it issues 
from the lungs, and before it has had time to mingle thoroughly 
with the air of the chamber, then our basis of calculation is 
so uncertain and variable that nothing short of practical demon- 
stration can give us a clue to the quantity of air which it is 
necessary to supply for the purposes of health. These may 
not at first sight appear serious objections, but it must be 
borne in mind that the end of all efficient ventilation is 
the removal of exactly so much vitiated air as will leave the 
remaining atmosphere fit for healthy respiration, and no more 
than this, and the achievement of this without inconvenience or 
discomfort. It follows, then, that if any system of ventilation be 
based on erroneous calculation, either too much or too little air 
will be introduced into the apartment ventilated, and thus the 
inmates will either be supplied with impure air or be exposed to 
unpleasant draughts. The necessity for the establishment of a 
rigid elementary principle, as the starting-point of all systems 
of ventilation, is therefore obvious. 
It may be as well, before going further into the subject, to 
lay before the reader the views of two able writers, whose 
opinions are somewhat materially in conflict. Dr. Angus Smith 
and Dr. Parkes, of Netley, have both devoted much time and 
skill to the investigation of the conditions of ventilation, but 
they come to very different conclusions as to the question, Gan 
a small room be more thoroughly ventilated with a given sup- 
ply of air than a large one ? Dr. Smith advocates the em- 
ployment of the small room under the circumstances referred 
to. He says : “ Let us imagine a man in a small box, or having 
his head in a small box, in which he would be supplied with 
air sufficient for an inhalation as often as he required it. The 
total amount would be about 12 to 20 cubic feet in an hour. 
The stream of air is so rapid that the impure is removed as 
rapidly as the pure is supplied. Put the same man in the 
VOL. VI. — NO. XXV. G G 
