THE USE OF HYPOTHESIS. 145 



difficulties of conception. We are asked by physical 

 philosophers to give up all our ordinary prepossessions, 

 and believe that the interstellar space which seemed so 

 empty is not empty at all, but filled with something 

 immensely more solid and elastic than steel. As Dr. 

 Young himself remarked f , * the luminiferous ether, per- 

 vading all space, and penetrating almost all substances, is 

 not only highly elastic, but absolutely solid ! ! ! ' Sir John 

 Herschel has calculated the amount of force which may be 

 supposed, according to the undulatory theory of light, to 

 be exerted at each point in space, and finds it to be 

 1,148,000,000,000 times the elastic force of ordinary air at 

 the earth's surface, so that the pressure of the ether upon 

 a square inch of surface must be about 17,000,000,000,000, 

 or seventeen billions of pounds s. Yet we live and move 

 without appreciable resistance through this medium, in- 

 definitely harder and more elastic than adamant. All our 

 ordinary notions must be laid aside in contemplating such 

 an hypothesis ; yet they are no more than the observed 

 phenomena of light and heat force us to accept. We 

 cannot deny even the strange suggestion of Dr. Young, 

 that there may be independent worlds, some possibly 

 existing in different parts of space, but others perhaps 

 pervading each other unseen and unknown in the same 

 space h . For if we are bound to admit the conception of 

 this adamantine firmament, it is equally easy to admit a 

 plurality of such. We see, then, that mere difficulties of 

 conception must not in the least discredit a theory which 

 otherwise agrees with facts, and we must only reject 

 hypotheses which are inconceivable in the sense of break- 

 ing distinctly the primary laws of thought and nature. 



f Young's 'Works/ vol. i. p. 415. 



g 'Familiar Lectures on Scientific Subjects/ p. 282. 



h Young's 'Works/ vol. i. p. 417. 



VOL. II. L 



