49 
To make my meaning clearer, let us suppose microbia pre- 
sent in a confined sewer. Their action results in the pro- 
duction of certain gases, and the presence of those gases 
again intensifies the action of the microbia. Or, to put 
another supposition-certain microbia present, say, in the 
peripheral regions of the nervous system, produce a given 
effect through the nerves upon the nerve centres, and that 
effect re-develops from the organic molecules ” of the nerve 
centres, organisms or ferments capable of acting precisely as 
the original microbia acted. Such suppositions appear to 
offer explanations of the varying virulence of zymotic 
diseases and of the discoveries by Pasteur and his disciples 
relating to the attenuation or intensifying of microbia. They 
may also provide the key to the mystery of protective inocu- 
lations. For the mild vaccine calls into existence a certain 
resisting power which appears to be intensified by the con- 
sequences of its own action. 
Leaving such speculations on one side, however, for the 
present, what I wish now to point out is that in Pasteur’s 
latest experiments we appear to have another illustration of 
the hygienic value of fresh air, and a confirmation of the 
suggestion that carbonic acid is a gas capable of at least 
preserving zymotic disease. May we not, hypothetically, 
generalise the idea, and carry it a little further, by saying 
not only that fresh air favours saprophytic life, while foul 
gases favour parasitic life, but that foul gases evolve para- 
sitic from saprophytic life. It is a remarkable fact that no 
genuine pathogenic microbia have ever been obtained from 
collections made from the atmosphere. Dr. Miquel has 
made a vast number of experiments in the cultivation of 
atmospheric germs, only to arrive at the conclusion that 
pathogenic microbia “ appear to be banished from the air.” 
I venture to submit, however, that this does not imply that 
infection may not be communicated through the atmosphere. 
Attention has been called to the fact that epidemics often 
