1872.] Notices of Books. 
499 
to be sure that air free from floating particles would be whole- 
some ; we have only the proof that, if there is an excess of some 
kind, it is unwholesome. Apparently everyone can breathe air 
tainted with any disease without being hurt, if the taint is small 
enough. Inconceivably small particles injure; but we must 
learn to divide even the inconceivably small. We can bear a 
larger amount of taint if it is diluted enough. Dilute sufficiently 
the air of a hospital, and infection ceases. One short time of 
the infected air produces disease; a long period of the diluted 
air produces none, although the number of particles that must 
pass over a certain spot must be much greater in the long time 
than when the stronger mixture passes in the short time. We 
learn from this that the amount that does injury is not infinitesi- 
mal; there must be a certain quantity. I do not doubt that we 
shall measure that readily: we can readily measure the amount 
of ammonia and organic matter in the infectious and non-infec- 
tious atmospheres. The really practical work must begin after 
this. It was my desire to have done so in one of the hospitals, 
but having begun I was interrupted, and it must not be done in 
a hurry. Ido not feel hopeless of being able to say that in a 
scarlet-fever atmosphere there must not only be so much nitro- 
genous organic matter and so many germs, otherwise infection 
will be certain,” 
For the actual chemical instruction relative to the matter 
before us we must refer the reader to the work itself; but to the 
general subject of ventilation we may here give some attention. 
‘‘The demands of ventilation would be best explained,” says 
Dr. Smith, “if we could reply to these questions :—What is the 
smallest amount of carbonic acid which may be called injurious ? 
and what is the smallest amount of organic matter?” The 
answer to the former question Dr. Smith has determined very 
accurately by means of a hermetically-sealed leaden chamber, 
and the effects on his own person of remaining in different states 
of the atmosphere. The amount for a healthy place is below 
0*4 per cent, while about five times that amount affects a candle 
sensibly. But the practical question—How often must we renew 
the air of a room in order to maintain a given degree of purity ? 
—is that most nearly affecting the general reader, and which we 
shall endeavour to answer from the work before us. Suppose 
that a man brings 100 cubic feet of air to contain 0°4 per cent of 
carbonic acid in an hour from zero, he will bring 1000 cubic feet 
to contain 0-04 in the same time, being at the end of the hour 
ready for another 1000 cubic feet, in order not to exceed this limit. 
But in actuality the air supplied also contains 0-04 per cent of 
carbonic acid, and the space in which the man is situate cannot 
be maintained at the degree of purity. The limit usually 
assigned is 0°06: this being o:o2 higher than that which he re- 
ceives as fresh, a man would require a constant supply of 2000 
feet per hour. But then the air-supply of the majority is not 
