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penetrated by these vapours. Between the actually mechanical 
method of taking solid matter into the air, such as when 
waves are agitated by the wind, and the purely chemical 
method, such as when a liquid is transformed into a vapour 
by heat, there must be many intermediate stages. These 
stages are required, apparently, as we can scarcely imagine 
pure gases undergoing transformation similar to bodies in a 
putrefactive state ; and we are not prepared with a theory by 
which diseases will be communicated without the agency of 
bodies in such a state of change — one of the oldest of theories 
and one promising to live long. Besides, the fact of a 
substance being found capable of being absorbed by metallic 
salts, and containing carbon with nitrogen in such a large 
amount, leads us to believe that bodies not very far removed 
from the substances decomposed are found in the vapours, 
and, if not far removed, capable of undergoing transformations 
so as to become farther removed, and by such transformations 
exercising their special influence. 
