14 
ANNIVERSARY ADDRESS. 
obtains everywhere and always and which has already been hinted 
at in connection with the centripetal and centrifugal forces. If 
the impurities of respiration, coal-burning, and other cases of 
combustion obeyed the law of gravitation alone, these impurities 
would accumulate on the surface of the earth and would render 
all animal life impossible. They obey the law of gravitation; but 
they obey the law or decree of diffusion also, under which a heavier 
gas will pass up into a lighter notwithstanding gravitation, and a 
lighter will pass down into a heavier notwithstanding gravitation. 
And here we must allude to another law relating to diffusion ; an 
arithmetical law expressive of the rate of diffusion—a law to 
which indeed the term ‘ law of diffusion ’ is sometimes restricted. 
Truth-seekers have experimentally measured the rates at which 
gases diffuse into one another, and at first sight the results 
seemed to show that gases diffused at rates as different as their 
specific weights. But soon it was evident that although the 
lighter gases diffused faster than the heavier, they did not do so 
exactly in proportion to their lightness. Further careful search 
detected the law. Gases do not diffuse at as many different rates 
as there are different gases, nor even at as many unrelated rates as 
there are gases of different relative weight, but they all diffuse at a 
rate which is simple and single in kind, namely, a rate inversely 
proportional to the square root of their relative weights. Thus 
a gas which is sixteen times as light as another will diffuse four 
times as fast as that other. These laws of diffusion and rate 
of diffusion come into play in respiration and in all varieties 
of ventilation. Manifestly it is in obedience to these laws that the 
impure gas expired from the lungs does not accumulate in the 
neighbourhood of one’s mouth. Clearly it is in obedience to these laws 
that this heavy and, when concentrated, poisonous gas, yielded not 
only from the lungs but from all fires and flames, does not accumulate 
on the surface of the earth and render life impossible, but diffuses 
away into the air, becoming so diluted as to be harmless to man and 
all animals, and yet not so dilute but that it still is the food 
by which plants live and grow. If we are to live healthy lives as 
regards the inhalation of pure air and as regards the admission 
of pure air to our dwellings, some of us must study the laws of 
diffusion, respiration, and combustion. Knowledge of these laws 
is, in fact, beginning to be taught in all our higher schools 
and colleges in place of some of the classical teaching which 
formerly occupied so large a proportion of school hours. 
From the superhuman laws which govern and protect us during 
sleep we naturally turn to those of the force which dispels darkness, 
