72

Sir ISAAC N E W T O N’s Book I.

The resistance which arises from the tenacity or adhesionof the parts of fluids may be diminished ; but still the re-sistance which arises from the inertia of the matter remains :if this could be taken away, as the matter would have no re-sistance, so it is not easy to conceive how it could have anyactivity or mechanical force to impell bodies, or to produce anyof the effects which are attributed to the subtile matter of theCartejiam. For action and reaction are always equal, andwe know of no force in bodies but what arises from their re-sistance to change their state, or their inertia. Without thisthere could be no centrifugal force, the favourite power bywhich those philosophers endeavour to explain the phænomenaof nature.

They suppose the particles of those subtile fluids to moveconstantly and equally in all directions ; and, by the favour ofthis hypothesis, they imagine that they may suppose them toact but not resist. But they have neither made this strangesupposition probable, nor even credible, nor can they shewthat it would answer their purpose. A motion of a fluidsavours the motion of a body in it, only as far as it is in thesame direction ; and an intestine motion of the parts of thefluid, equal in all directions, cannot make the resistance lessthan if there was no motion of the parts. It is supposed bymany that the particles of common fluids, water or air forexample, are in a constant intestine motion ; but this does nothinder those fluids from resisting in proportion to theirdensity.

We are told by some, that it is impossible to conceive avacuum. But this surely must proceed from their having im-bibed Des Cartes s doctrine, that the essence of body is con-stituted