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 anything as to the nature of objects which so successfully elude giving any direct testimony appreciable to our senses? Seeing this difficulty, it can hardly be surprising if some should have doubted the accuracy of the results at which philosophers have arrived with regard to the ultimate constitution of matter. But there are methods of discovering truth which are in certain cases capable of more subtle work than the direct indication of our senses. It is these indirect processes which have taught us much regarding that invisible world which lies around. In certain respects we may contrast the subject about which we are now to be engaged with those great themes which more usually occupy the attention of astronomers. In the case of the heavenly bodies the mind is taxed by the effort to conceive distances so tremendous, masses so enormous, and periods so protracted that we often despair of obtaining adequate notions of magnitudes which altogether transcend our ordinary experience. We are now to make an appeal to the imagination in a precisely opposite direction. We are to speak of masses so minute, of distances so short, and of periods so infinitesimal, that it is utterly impossible for us to parallel them by the phenomena with which our senses make us directly acquainted.

At both extremes, however, we employ the same weapons for the study of the phenomena of nature. Mathematical investigation submits to no restriction either in the greatness of the space over which its command extends, or in the minuteness of the portions which must obey its laws. The principles of dynamics are equally applicable whether the periods of time which they contemplate shall be millions of years, as they often