594 POPULAR SCIENCE MONTHLY. 



was received as proved, because the essential facts at least of the 

 new departments were deducible from its premises. 



The life of the undulatory theory as a mechanical hypothesis 

 has, however, been a limited one, for it has been borne to the grave 

 in our own time without display, and been replaced by the electro- 

 magnetic theory. The cause of its death is shown very plainly 

 when we dissect the corpse. It was carried down by its mechan- 

 ical constituents. The hypothetical ether, on which the task of 

 undulating was imposed, had to do this under peculiarly hard 

 conditions, for the phenomena of polarization demanded per- 

 emptorily that the undulations should be transversal ; but such 

 undulations presuppose a rigid body, and Lord Kelvin's calcula- 

 tions have shown, as a final result, that a medium with such prop- 

 erties as this ether must have is not stable ; whence the con- 

 clusion is inevitable that it can have no physical existence. In 

 order to spare the now accepted electro-magnetic theory of light 

 from such a fate, the lamented Hertz, to whom this theory owes 

 so much, expressly denied that he saw anything else in it than 

 a system of six differential equations. This termination of the 

 evolution speaks more impressively than I ever can against 

 the permanent usefulness of the formerly current mechanical 

 theorizing. 



But I hear it said, Those theories have been so fertile. Yes, 

 they have been so through the sum of correct constituents in 

 them, as they have been damaging through their false ones. t 

 What were the correct and what the false had to be determined 

 by long and costly experiments. The result of our discussion so 

 far is first a pure negative. We have learned how not to do it, 

 and it seems to be of little use to follow out such negative results. 

 Yet we can point out a gain here, which will not appear worthless 

 to many of you. We find it possible, as we go, to refute critically 

 a view which had no small credit in its time, and caused great 

 concern to many of those interested in the discussion. I refer to 

 the widely known propositions which the eminent physiologist of 

 the University of Berlin, Emil Du Bois-Reymond, made, first 

 twenty-three years ago, on the occasion of the meeting of natu- 

 ralists at Leipsic, and afterward in some more widely read writ- 

 ings, relative to the prospects of our future knowledge of nature, 

 and which culminated in the famous expression ignorabimus. In 

 the long controversy which followed this address, Du Bois-Key- 

 mond, so far as I can see, remained essentially the victor against 

 all attacks, for all his antagonists proceeded from the same prin- 

 ciple from which he inferred his ignorabimus, and his conclusions 

 stood as firmly as that principle. This principle, which in the 

 meanwhile had never been brought in question by any one, was 

 the mechanical theory of the world the supposition that the solu- 



