The Principle of Gravitation, demonstrated by Newton, explains a vast number of facts relating to the motions of the Heavenly bodies:--the {160} Undulatory Theory of Light, largely developed by the labours of Fresnel, renders equally clear and systematic an almost endless number and variety of optical phenomena; Oersted's law of Electro-magnetism similarly explains and renders consistent a multitude of facts respecting the movements of magnets and electric conductors, which would otherwise be confusing to remember and impossible to satisfactorily explain. And the great mental value of these comprehensive ideas to mankind, consists largely in relieving the memory and diminishing mental confusion, by co-ordinating a large number of different facts and apparently inconsistent phenomena by means of a general conception which embraces the whole of them. Thus a knowledge of the Principle of Gravitation informs us that both the ascent of a balloon in the air, and the descent of a stone in water, are alike due to the same force of gravity; and that of Chemical Affinity proves to us that the apparently unlike phenomena of slow rusting of iron and vivid combustion of phosphorus are essentially alike and due to the same cause. All bodies, whether living or dead, and all forms of energy, appear to be absolutely subject to the great laws of Causation, Progress, Conservation, &c., no one can escape them; the man who transgresses the Law of Progress by being too much in advance of his epoch, is punished as certainly as he who lags behind it; all must advance together, and at approximately the prescribed rates. {161} The real source of all that is good in new scientific knowledge arises from its verified and verifiable character, its high degree of certainty, and its capacity of withstanding all the tests which can be applied to it. By the term "scientific knowledge" in this case I mean that only that which has been verified, and I purposely exclude all matters of hypothesis, mere opinion or belief. Scientific research is the chief basis of national progress, not only because it is continually disclosing new truths to us, but also because the truths it reveals are frequently of the most definite kind.