INTRODUCTION. 3 



and thereby constituting physical astronomy as one vast 

 application of the laws of force. By this magnificent 

 generalization, it was shown that the laws of gravitation has shown 

 and of motion are the same throughout celestial space : f^^^ ""V . 

 and a variety of facts which have been ascertained since natural 

 ITewton's time, including the wonderful discoveries of spec- through 

 trum analysis, have established, with almost equal certainty, «pace. 

 the identity of chemical laws throughout the universe. 



While astronomy has shown the uniformity of natural Geology 

 laws throughout space, geology has shown the constancy ^j^^ gp^.^^ 

 of natural laws through time. The change which has stancy of 

 transformed geology from a mere mass of conjectures into j^V 

 a true science, essentially consists in explaining geological through 

 phenomena by the same causes that we see in action 

 at present, in the world as it is around us. The revo- 

 lution in geology which is chiefly associated with the 

 name of Lyell is an exact parallel to the revolution in 

 astronomy which was effected by Newton. Before N'ew- 

 ton's time, the forces which govern the celestial motions 

 were believed to be different in kind from those of which 

 we see the effects on the earth : Newton showed them to 

 be the same. Before Lyell's time, the facts of geology 

 were usually referred to imaginary " catastrophes," pro- 

 duced by causes of which it could only be said that they 

 were unlike anything now in action : Lyell and his fellow- 

 workers have shown how to account for them by the slow 

 and continued action of those forces which we still see at 

 work around us, in the air, in the waters, and in the 

 volcanic fires.-^ 



The latest of these great scientific generalizations is still Thether- 

 in the same direction. It consists in the establishment of ™i'c'^ theory 

 what is called the thermo-dynamic theory ; that is to say, has shown 

 in proving that heat consists in molecular motion, and that force to be 

 the laws of heat are only a particular case of the laws of *''^\® °" °-^^ 



. ,.. scales. 



force. By this generalization, second m importance only 



1 I shall, however, have to state in the chapter on " the Motive Powers 

 of the Universe," my reasons for believing that some of these forces, though 

 the same in kind, acted in former tiroes with greater intensity than they 

 do at present. 



b2 



