244 FRAGMENTS OP 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. 
ON 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 
Eossesses characters strikingly distinct from those of the 
:>rce 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. 
