OF THE COSMOS. INTRODUCTION. 23 



short preface to the second edition of his Optics, he thought 

 it necessary to declare explicitly that he by no means re- 

 garded gravitation as an " essential property of bodies" : ( 43 ) 

 while more than a century before, in 1600, Gilbert had 

 viewed magnetism as a force inherent in all matter. So 

 much did the most profound of thinkers, Newton himself, 

 who ever leaned so strongly to experience, hesitate in respect 

 to the " ultimate mechanical cause" of motion. 



The establishment of a science of Nature, from the laws 

 of gravity up to the formative impulse in animated bodies, 

 as one organic Whole, is no doubt a brilliant problem, and 

 one worthy of the human intellect ; but the imperfect state 

 of so many parts of our knowledge places insuperable diffi- 

 culties in the way of its solution. The impossibility of 

 complete experimental knowledge, in a boundless sphere of 

 observation, renders the problem of explaining all the 

 changes of matter from the powers of matter itself an " in- 

 determinate problem.'" What is perceived is far from 

 exhausting what is perceivable. If, to recall only the pro- 

 gress of the time nearest to our own, we compare the 

 imperfect knowledge of nature possessed by Gilbert, Eobert 

 Boyle, and Hales with the present, and if we remember 

 that the rate of progress is a rapidly increasing one, we may 

 have some idea of the periodical endless transformations 

 which still await all the physical sciences. New substances 

 and new powers will be discovered. Even though many 

 natural processes, as those of light, heat, and electro-mag- 

 netism, being reduced to movement (undulations), have 

 become accessible to mathematical treatment, yet there 

 remain the often referred to, and perhaps unconquerable, 

 problems of the cause of chemical diversity of substance, 



