59 6 POPULAR SCIENCE MONTHLY. 



ing a picture of the reality ? To such, a question I might reply, 

 " Thou shalt not make unto thee any image or any likeness ! " 

 Our object is not to see the world in a more or less darkened or 

 distorted glass, but as immediately as the constitution of our 

 mind will permit. To set realities, demonstrable and measurable 

 magnitudes, in definite relations with one another, so that when 

 one is given the others will follow that is the purpose of science, 

 and it can not be fulfilled by the substitution of any hypothetical 

 figure, but only by the demonstration of the mutual dependencies 

 of measurable magnitudes. 



This road is undoubtedly long arid toilsome, but it is the only 

 permissible one. We do not have to go upon it, however, de- 

 spairing of ourselves seeing the end of it, and merely hoping that 

 it may lead our children to the desired heights. No, we our- 

 selves are the happy ones, and the most hopeful scientific gift 

 which the departing century can offer the dawning one is the 

 replacement of the mechanical theory by the energistic. 



I lay at this point great weight on the declaration which I 

 make that we are not dealing with some fresh novelty, first given 

 in our time. No, we have been in possession of the truth for half 

 a century without knowing it. If there ever was a place for the 

 expression "mysteriously evident," it is here; we can read it 

 every day, and do not understand it. 



When fifty-three years ago Julius Robert Mayer first dis- 

 covered the equivalence of the different natural forces, or, as we 

 should say now, of the various forms of energy, he made a great 

 advance toward an ultimate solution of the problem. But, ac- 

 cording to a constantly recurring law in universal thought, a new 

 fact is never accepted in its primary clearness and simplicity. 

 The receiver, who has not inwardly felt the advance, but has 

 taken it in from without, strives first of all things to connect the 

 new as well as he can with what is already existing. Hence the 

 new thought is obscured, and, although not exactly falsified, is 

 robbed of the best of its force. So strong is this peculiarity of 

 thought that it does not even leave the discoverer free ; as even 

 the powerful mind of Copernicus was competent to let the sun and 

 earth change places in their motions, but not to comprehend the 

 simple motions of the other planets ; to account for these he ad- 

 hered to the received theory of epicycles. So it was with Mayer. 

 Hence, as it nearly always is, the work of the next generation 

 consists, not in harvesting the results of the new knowledge, but 

 rather in removing again, piece by piece, the involuntary additions 

 that do not belong to the subject, till at last the fundamental 

 thought can come out in its whole plain identity. 



A similar development can be perceived, even in our case. 

 After J. R. Mayer had defined the law of equivalence, his theory 



