144 THE PRINCIPLES OF SCIENCE. 



rays. Such an hypothesis then is inconsistent with the 

 main body of our knowledge concerning gases. 



,Provided that there be no clear and absolute conflict 

 with known laws of nature, there is nothing so im- 

 probable or apparently inconceivable that it may not be 

 rendered highly probable, or even approximately certain, 

 by a sufficient number of concordances. In fact the two 

 best founded and most conspicuously successful theories 

 in the whole range of physical science involve the most 

 absurd suppositions. Gravity is a force which appears to 

 act between bodies through vacuous space ; it is in 

 positive contradiction to the old dictum that nothing 

 could act but through some intervening medium or sub- 

 stance. It is even more puzzling that the force acts in 

 perfect indifference to all intervening obstacles. Light in 

 spite of its extreme velocity, shows much respect to 

 matter, for it is almost instantaneously stopped by opaque 

 substances, and to a considerable extent absorbed and de- 

 flected by transparent ones. But to gravity all media are, as 

 it were, absolutely transparent, nay non-existent ; and two 

 particles at opposite points of the earth affect each other 

 exactly as if the globe were not between. To complete the 

 apparent impossibility, the action is, so far as we can ob- 

 serve, absolutely instantaneous, so that every particle of the 

 universe is at every moment in separate cognizance, as it 

 were, of the relative position of every other particle 

 throughout the universe at that same moment of absolute 

 time. Compared with such incomprehensible conditions, 

 the theory of vortices deals with common-place realities. 

 Newton's celebrated saying, hypotheses non Jingo, bears 

 the appearance of pure irony ; and it was not without 

 apparent grounds that Leibnitz and the greatest con- 

 tinental philosophers charged Newton with re-introducing 

 occult powers and qualities. 



The undulatory theory of light presents almost equal 



