But notions, hitherto unknown, may be introduced, which, without casting doubts upon pristine truths, may cause new ones to enter their domain, and change, or even upset, our established notions of things. The facts may be unforeseen, but they will never be contradictory. The history of sciences teaches us, that their bulwarks have never been overthrown by the inroad of a new science. At one time no notion of tubercular infection existed. We now know that it is transmitted by microbes. This is a new notion, teeming with important conclusions, but it does not invalidate the clinical table of pulmonary phthisis drawn up by physicians of other days. The discovery of Hertzian waves has in nowise shaken Ampère’s laws. Newton’s and Fresnel’s optics have not been changed into a tissue of errors because Rœntgen rays and luminous vibrations are able to penetrate opaque bodies. It appears that radium can throw out unremittingly, without any appreciable chemical molecular phenomena, great quantities of calorific energy; nevertheless, we may be quite sure, that the law of conservation of energy and thermo-dynamic principles will remain as true now as ever. Likewise, if the facts called ‘occult’ become established, as seems more and more probable, we need not feel anxious as to the fate of classical science. New and unknown facts, however strange they may be, will not do away with old established facts. To take an example from Dr. Maxwell’s work, let us admit that the phenomenon of _raps_—that is to say, sonorous vibrations in wood or other substances—is a real phenomenon, and that, in certain cases, there are sounds which no mechanical force known to us can explain, would the science of physics be overthrown? It would be a new force thrown out on to wood, etc., exercising its power on matter, but the old forces would none the less preserve their activity, and it is even likely that the transmission of vibrations by means of this new force would be found to be in obedience to the same laws as those governing the transmission of other vibrations;—the temperature, the pressure, the density of air or wood would continue to exercise their usual influence. There would be nothing new, save the existence of a force until then unknown. Now, is there any savant worthy of the name who can affirm, that there are no forces, hitherto unknown, at work in the world? However impregnable Science may be when establishing facts, it is miserably subject to error when claiming to establish negations.