June, 1889. 
president’s address. 
139 
not characteristic of all the Bacteria. Certain species and 
genera are extensively pleomorphic; to quote the words of 
an observer, about Bacterium luminance: “At a given moment, 
in a pure cultivation, elements of all sizes and forms are to 
be seen, some like Leptotlirix, Bacillus, and Bacterium, others 
like Vibrio, and others like Spirillum.” But, on the other 
hand, certain genera and species are, so far as the evidence 
goes at present, absolutely unchangeable and constant in form. 
This question has been frequently and warmly debated, 
but is now set at rest to an extent which would have been 
quite impossible but for Koch’s method of solid media. 
Whatever care was exercised with a liquid nutrient solution, 
the objection that foreign germs had been introduced during 
the process could never be entirely controverted. 
Distribution of Bacteria. 
But, perhaps, it will occur to some that there is one 
difficulty which nothing that has yet been said will enable 
us to surmount. In all our manipulations, the things that 
are used, after being sterilised, must be exposed to the air, 
and we have been taught that the atmosphere is full of the 
germs of Fungi. How then are we to prevent these germs 
from entering and contaminating everything ? We might, it 
is true, perform all our operations under a disinfecting spray; 
it would be difficult, but not absolutely impossible. Obviously, 
before we can tell what is our best course, we must carefully 
investigate the truth of the common belief in the abundance 
of germs in the atmosphere, and the method of plate cultivation 
offers exactly the means required for so doing. 
If a plate, prepared as before, but without any sowing of 
Bacteria, be exposed to the air in a certain place for a certain 
time, every germ which falls upon it will remain where it 
falls; and then, if the plate be kept at a suitable temperature, 
every one which is capable of growing under these conditions 
will develop and produce its colony. These colonies can then 
be counted, and by that means some idea may be formed of 
the number of Bacteria in the air of any given place. In 
this way and others the question has been investigated by 
several persons, notably by a Frenchman, Miquel, and 
interesting results have been obtained. 
Miquel directed a measured current of air to the surface 
of a gelatine plate, and counted the germs thus entrapped. 
He found that a cubic metre of air in the Park at Montsouris 
contained between ... ... ... ... 30 and 700 
In the same bulk from the Hue de Rivoli there 
were 
5,500 
