things upon the earth's surface,was once utterly different from anything that human tradition can remember, and it was accordingly quite natural that they should suppose that things have always been about as they are. The human mind can not transcend experience. The man who has always lived in a comparatively unchanged environment will, of course, never believe in a different state of things until taught by some fresh experience. How long it was before it was brought home to men that the testimony of the unaided senses needs to be corrected by systematic observation and reasoning! From this point of view, as indeed from some others also, the revolution in astronomical theory effected by Copernicus was one of the greatest events in human history. Its philosophic consequences were profound. In teaching men the necessity of going back of superficial appearances, and subjecting their crude opinions to some kind of critical test, it was an object lesson of unsurpassed value. Along with this abrupt shifting of man's apparent position in the universe, came the astonishing results of oceanic discovery, enlarging fourfold the dimensions of the known world and bringing the mind into contact with organic and inorganic nature in various new and unsuspected forms. Then came the Newtonian astronomy, in which a generalization from terrestrial physics was extended into the celestial spaces and quantitatively verified. There was an immense enlargement of the mental horizon, and the problems immediately connected with it were enough to occupy the attention of all the foremost mathematical minds for more than a century. It made man a denizen of the solar system as well as of his own particular planet; and in these latter days, since the law of gravitation has been extended to the sidereal heavens and spectrum analysis has begun to deal with nebula?, there is abundant proof that properties of matter and processes with which we are familiar on this earth are to be found in some of the remotest bodies which the telescope can reach, and it is thus forcibly impressed upon us that all are parts of one stupendous whole.
This enlargement of the mental horizon, from Newton to Kirchhoff, had reference to space. A similar enlargement with reference to time was an indispensable preliminary to any correct understanding of how the world is made and what is going on in it. But, before much headway could be made in geology, it was necessary that physics and chemistry, the sciences which generalize the properties of matter, in the mass and in the molecule, should be to some extent apprehended; and it is almost startling to think how modern all this is—scarcely more than a hundred years since Priestley discovered oxygen, since it became possible to tell what goes on when you burn a log of wood on the hearth! and not so very much longer since Black discovered latent heat and gave us
