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substances? But the atmosphere as a body is self-purifying, and 
itis therefore the object of ventilation to cause the air of inhabited 
rooms to mingle with the outside air, and to return purified. 
The atmosphere contains a condensed form of oxygen called 
ozone, and the part which this gas plays in the purification of the 
atmosphere is no doubt very important, owing to its great oxidizing 
properties: thus acting on putrid substances, and destroying their 
putridity. The air consists chiefly of two gases, oxygen and 
nitrogen—about twenty-one per cent of the former to seventy-nine of 
the latter; and as many of you may be aware of the properties of 
these gases, I will at once proceed to mention the general constitu- 
ents present along with them in minute quantities. They are— 
Water; Carbonic Acid, about 004 per cent.; Ammonia; Ozone; 
Nitrous and Nitric Acids ; Sulphurous and Sulphuric Acids. We 
have also in the air, in addition to these gaseous substances, various 
solid bodies, which are indeed familiar to us as seen floating in a 
sunbeam. They consist of every conceivable form of rubbish, 
including (probably the more important) substances of a fungoid 
nature, or the actual germs of those fungi commonly called mould 
and mildew. 
Again, there are sometimes present smaller particles, which, 
by the aid of a beam of light, or even with the assistance of 
powerful lenses, we are unable to see, or, at all events, to distinguish ; 
these may include disease spores or germs, which, passing into 
the blood, arrive at their destination where suitable materials exist 
for their development. 
Ozone, already spoken of as a natural element of atmospheric 
purification, and a condensed form of oxygen (oxygen being repre- 
sented by the figure O, while ozone by the figure O¢), is present 
in the atmosphere, however, in very small quantity. The best 
country air seldom contains more than one volumein 700,000. It 
may be prepared artificially by passing electricity through air 
gradually and silently, and it is therefore highly probable that the 
ozone of the atmosphere is produced by continuous silent passages 
of electricity between the clouds and the earth. Owing to the 
value of the presence of ozone in the atmosphere for the purpose 
