Here, too, we question whether Sir William Thomson will be content with this definition of the process by which he has been guided to his most recent advance in molecular physics. In the splendid series of inductions verified step after step by rigorous experiment and observation, and kept in exactest continuity by the chain of mathematical evolution, is imagination the faculty to which are to be given the chief honours of this conquest of a new realm of physics? This would surely be to force upon us a new and arbitrary classification or analysis of the powers of the intellect. If we follow Professor Tyndall himself through the masterly train of reasoning whereby he leads us to the laws of reflection and transmission of light as the cause of the azure of the firmament, is what we admire the leap of imagination, or the firmly balanced and duly graduated tread of a mind trained in the discipline of logic, and careful to plant every step on the assured ground of fact or experience? It is simply a misnomer to apply the name of imagination to the process or the faculty to which this onward march into the realm of unexplored nature is really due. As well describe as a triumph of the imagination the connected and organized plan of the great strategist which has drawn round Paris a living cordon of 300,000 men. _From a Lecture addressed to Teachers at the South Kensington Museum, April 30, 1861, by_ J. TYNDALL. Here, then, is an exhibition of power which we can call forth or cause to disappear at pleasure. We magnetize our strip of steel by drawing it along the pole of a magnet; we can demagnetize it, or reverse its magnetism, by properly drawing it along the same pole in the opposite direction. What, then, is the real nature of this wondrous change? What is it that takes place among the atoms of the steel when the substance is magnetized? The question leads us beyond the region of sense, and into that of imagination. This faculty, indeed, is the divining rod of the man of science. Not, however, an imagination which catches its creations from the air, but one informed and inspired by facts, capable of seizing firmly on a physical image as a principle, of discerning its consequences, and of devising means whereby these forecasts of thought may be brought to an experimental test. If such a principle be adequate to account for all the phenomena, if from an assumed cause the observed facts necessarily follow, we call the assumption a theory, and once possessing it, we can not only revive at pleasure facts already known, but we can predict others which we have never seen. Thus, then, in the prosecution of physical science, our powers of observation, memory, imagination, and inference, are all drawn upon. We observe facts and store them up; imagination broods upon these memories, and by the aid of reason tries to discern their interdependence. The theoretic principle flashes, or slowly dawns upon the mind, and then the