2 6o THE POPULAR SCIENCE MONTHLY. 



men adopted, developed, and diffused the dangerous doctrine of atoms 

 and molecules which found its consummation in this city of Manches- 

 ter at the hands of the immortal John Dalton. Now, the grand old 

 pagans whom I have named, and their followers up to the time of 

 Newton, had pictured their atoms as falling and flying through space, 

 hitting each other, and clinging together by imaginary claws and 

 hooks. They entirely missed the central idea that the atoms and 

 molecules could come together, not by being fortuitously knocked 

 against each other, but by their own mutual attractions. This is one 

 of the great steps taken by Newton. He familiarized the world with 

 the conception of molecular force. 



But the matter does not end here ; experience had given us the 

 key to further mysteries. In the case of electricity and magnetism a 

 double exercise of force had been observed repulsion had been always 

 seen to accompany attraction. Electricity and magnetism were ex- 

 amples of what are called polar forces; and, in the case of magnetism, 

 experience itself pushed the mind irresistibly beyond the bounds of 

 experience, compelling it to conclude that the polarity of the magnet 

 was resident in its molecules. I hold a strip of steel by its centre, be- 

 tween my finger and thumb. One half of the strip attracts, and the 

 other. half repels the north end of a magnetic needle. I break the 

 strip in the middle, and what occurs ? The middle point or equator 

 of the magnetism has shifted to the centre of the new stri]^. This 

 half, which a moment ago attracted throughout its entire length the 

 north pole of a magnetic needle, is now divided into two new halves, 

 one of which wholly attracts, and the other of which wholly repels, 

 the north pole of the needle. Thus the half when broken off proves 

 to be as perfect a magnet as the whole. You may break this half, and 

 go on breaking till further breaking becomes impossible through the 

 very smallness of the fragments ; still you find at the end that the 

 smallest fragment is endowed with two poles, and is, therefore, a per- 

 fect magnet. But you cannot stop here : you imagine where you 

 cannot experiment / and reach the conclusion entertained by all scien- 

 tific men, that the magnet which you can see and feel is an assemblage 

 of molecular magnets which you cannot see and feel, but which must 

 be intellectually discerned. 



I shall endeavor to show you some of the actions of this polar force, 

 at the same time asking you to remember that my main object here 

 to-night is to show you the growth of scientific ideas, and to illustrate 

 the manner in which the scientific investigator uses his thoughts and 

 his hands in the investigation of Nature. 



Scientific ideas, as already stated, spring out of experience, but 

 they extend beyond the boundary of experience. And, indeed, in 

 this power of ideal extension consists for the most part the differences 

 between scientific men. The man who cannot break the bounds of 

 experience, but holds on to the region of sensible facts, may be an ex- 



