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pervading simple one of enormous extent. And further, the 
more a subject consists of compound forces of strong inten- 
sity the more it is capable of making itself manifest to the 
senses. And the less the subject of a conatus is com- 
pounded, whether small or large, the less it can make itself 
sensibly manifest. 
The general uses of the air we breathe are well known. 
O 
It is equ ally a well known that the air is the vehicle of what 
we call sound. Conatus acting in and through the air pro- 
duces vibrations. But these vibrations are not sound until 
this conatus by the vibrations of the air has affected 
another subject and produced the reaction of the ear. 
Then as the conatus and the subject of the ear harmonize 
sound is manifest. It is plain that if the vehicle of the vi- 
brations is removed sound cannot exist, or if we can conceive 
of the conatus of the vibrations existing without the air, it 
cannot make itself known. Further, if vibrations are made 
in the air for the production of sound, and another order of 
vibrations, as it were, crosses them so that a crest of one 
wave coalesces with the depression of another, no sound can 
be produced. But just as sound could not exist without air 
so also without it this silence could not be produced. And 
further, in the production of sound the law of the squares 
of distance is observed, and that for a reason which I think 
will shortly appear. Atmospheric air, however, is not a 
simple subject, for many conatus act in it. Hence it 
is a substance to that degree manifest to the senses, and 
we are so far physically conscious of its existence. But if 
atmospheric air be not a simple subject of enormous size, 
then it may be controlled and permeated by something more 
simple and more enormous. 
Because the undulatory theory of light explains more of 
its phenomena than the emission theory, the former has 
been adopted. This undulatory theory assumes the existence 
of another medium than atmospheric air called luminiferous 
