742 THE POPULAR SCIENCE MONTHLY. 



Electricity is the same now as it has ever been, yet it was once 

 spoken of as a fluid, then as a force, now as an energy readily con- 

 vertible into caloric or mechanical energy ; and in what light it will 

 be considered fifty years hence no one can predict. 



Now, what I desire to enforce here is, that amid all these changes 

 and revolutions of theories, so called, it is simply man, the inter- 

 preter, that has erred, and not Nature ; her laws are the same ; we 

 simply have not been able to read them correctly, and perhaps never 

 will be. 



What, it may be asked, are we to do, then ? Must we cease 

 theorizing; ? Not at all. The lesson to be learned from this is to be 

 more modest in our generalizations ; to generalize as far as our care- 

 fully-made-out facts will permit us, and no further ; check the imagina- 

 tion, and let it not run riot and shipwreck us upon some metaphysical 

 quicksand. 



The fact is, it becomes a question whether there is such a thing as 

 pure theory in science. No true scientific theory deserves the name 

 that is not based on verified hypothesis ; in fact, it is but a concise in- 

 terpretation of the deductions of scientific facts. Dumas has well said 

 that theories are like crutches, the strength of them is, to be tested 

 by attempting to walk with them. And I might further add, that very 

 often scientists, who are without sure-footed facts to carry them along, 

 take to these crutches. 



It is common to speak of the theory of gravitation, when there is 

 nothing purely hypothetical in connection with the manner in which it 

 was studied ; in it we only see a clear generalization of observed laws 

 which govern the mutual attraction of bodies. If at any time New- 

 ton did assume an hypothesis, it was only for the purpose of facilitat- 

 ing his calculations : " Newton's passage from the falling of an apple 

 to the falling of a moon was at the outset a leap of the imagination ; " 

 but it was this hypothesis, verified by mathematics, which gave to the 

 so-called theory of gravitation its present status. 



In regard to light, we are in the habit of connecting with it a pure 

 hypothesis, viz., the impressions of light being produced by emission 

 from luminous bodies, or by the undulation of an all-pervading, at- 

 tenuated medium ; and these hypotheses are to be regarded as probable 

 so long as the phenomena of light are explained by them, and no 

 longer. The failure to explain one single well-observed fact is suffi- 

 cient to cast doubt upon or subvert any pure hypothesis, as has been 

 the case with the emission theory of light, and may be the fate of the 

 undulatory theory, which, however, up to the present time, serves in 

 all cases. 



It is not my object to criticise the speculations of any one or more 

 of the modern scientists who have carried their investigations into 

 the world of the imagination ; in fact, it could not be done in a dis- 

 course so limited as this, and one only intended as a prologue to the 



