CHAPTER I. 



TRACES OF UNITY IN THE VARIOUS MODES 

 OF PHYSICAL FORCE. 



IN a library where books of science are arranged with 

 some regard to their merits a place of honour not far 

 from that assigned to Goethe's " Die Metamorphose der 

 Pflanzen " must be assigned to Grove's " Correlation 

 of the Physical Forces." Each work opens out, as it 

 were, several stages of the way along which I am bent 

 upon travelling as far as I can. Each work is the pro- 

 duction of a master-mind in such engineering, and, to 

 say the least, it is difficult to think that the time will ever 

 come in which a nearer or easier road will be opened 

 out through the same regions. 



The many stages of the road in which Mr. Justice 

 Grove is the engineer is, not in the clouds, but on the 

 hard ground. Throughout their entire length the one 

 ruling idea is to discard the hypothesis of subtle, occult, 

 imponderable entities of any sort, and to resolve the 

 various modes of physical force which form the subject 

 of inquiry motion, heat, light, electricity, magnetism, 

 chemical affinity and the rest into correlative and 

 mutually dependent changes in ponderable matter, 

 which changes are themselves resolvable into modes of 

 motion. No one of these forces, taken abstractedly, can 

 be held to be the essential cause of the others, each one 

 producing or being converted into another, or all the 



