872 FRAGMENTS OF SCIENCE 



call forth at pleasure or cause to disappear. We magne- 

 tize our strip of steel by drawing it along the pole of a 

 magnet; we can demagnetize it, or reverse its magnetism, 

 by properly drawing it along the same pole in the oppo- 

 site direction. What, then, is the real nature of this 

 wondrous change? What is it that takes place among the 

 atoms of the steel when the substance is magnetized? The 

 question leads us beyond the region of sense, and into 

 that of imagination. This faculty, indeed, is the divining- 

 rod of the man of science. Not, however, an imagination 

 which catches its creations from the air, but one informed 

 and inspired by facts; capable of seizing firmly on a 

 physical image as a principle, of discerning its conse- 

 quences, and of devising means whereby these forecasts 

 of thought may be brought to an experimental test. If 

 such a principle be adequate to account for all the phe- 

 nomena — ^if, from an assumed cause, the observed acts 

 necessarily follow, we call the assumption a theory, and, 

 once possessing it, we can not only revive at pleasure 

 facts already known, but we can predict others which we 

 have never seen. Thus, then, in the prosecution of phys- 

 ical science, our powers of observation, memory, imagina- 

 tion, and inference, are all drawn upon. We observe 

 facts and store them up; the constructive imagination 

 broods upon these memories, tries to discern their inter- 

 dependence and weave them to an organic whole. The 

 theoretic principle flashes or slowly dawns upon the mind; 

 and then the deductive faculty interposes to carry out the 

 principle to its logical consequences. A perfect theory 

 gives dominion over natural facts; and even an assump- 

 tion which can only partially stand the test of a compar- 

 ison with facta may be of eminent use in enabling us to 



