244 FRAGMENTS OF SCIENCE. 



deal with the subject in mass; they had no time to look 

 after details. But the desire for more exact knowledge is 

 increasing; facts are flowing in which, while they leave 

 untouched the intrinsic wonders of geology, are gradually 

 supplanting by solid truths the uncertain speculations 

 which beset the subject in its infancy. Geologists now 

 aim to imitate, as far as possible, the conditions of nature, 

 and to produce her results; they are approaching more and 

 more to the domain of physics, and I trust the day will 

 soon come when we shall interlace our friendly arms across 

 the common boundary of our sciences, and pursue our 

 respective tasks in a spirit of mutual helpfulness, encourage- 

 ment and goodwill. 



[I would now lay more stress on the lateral yielding, 

 referred to in the note at the bottom of page 241, accom- 

 panied as it is by tangential sliding, than I was prepared to 

 do when this lecture was given. This sliding is, I think, 

 the principal cause of the planes of weakness, both in 

 pressed wax and slate rock. J. T. 1871.] 



CHAPTER XIII. 



OK PARAMAGNETIC AND DIAMAGNETIC FORCES.* 



THE NOTION of an attractive force, which draws bodies 

 toward the center of the earth, was entertained by Anax- 

 agoras and his pupils, by Democritus, Pythagoras, and 

 Epicurus; and the conjectures of these 'ancients were 

 renewed by Galileo, Huyghens, and others, who stated 

 that bodies attract each other as a magnet attracts iron. 

 Kepler applied the notion to bodies beyond the surface of 

 the earth, and affirmed the extension of this force to the 

 most distant stars. Thus it would appear, that in the 

 attraction of iron by a magnet originated the conception of 

 the force of gravitation. Nevertheless, if we look closely 

 at the matter, it will be seen that the magnetic force 

 possesses characters strikingly distinct from those of the 

 force which holds the universe together. The theory of 

 gravitation is, that every particle of matter attracts 



* Abstract of a discourse delivered in the Royal Institution, 

 February 1, 1856. 



