We may notice another instance of an exploded hypothesis, which is generally mentioned only to be ridiculed, and which undoubtedly is both false in the extent of its assertion, and unphilosophical in its expression; but which still, in its day, was not without merit. I mean the doctrine of _Nature's horrour of a vacuum_ (_fuga vacui_), by which the action of siphons and pumps and many other phenomena were explained, till Mersenne and Pascal taught a truer doctrine. This hypothesis was of real service; for it brought together many facts which really belong to the same class, although they are very different in their first aspect. A scientific writer of modern times[13\2] appears to wonder that men did not at once divine the weight of the air, from which the phenomena formerly ascribed to the _fuga vacui_ really result. 'Loaded, {85} compressed by the atmosphere,' he says, 'they did not recognize its action. In vain all nature testified that air was elastic and heavy; they shut their eyes to her testimony. The water rose in pumps and flowed in siphons at that time, as it does at this day. They could not separate the boards of a pair of bellows of which the holes were stopped; and they could not bring together the same boards without difficulty, if they were at first separated. Infants sucked the milk of their mothers; air entered rapidly into the lungs of animals at every inspiration; cupping-glasses produced tumours on the skin; and in spite of all these striking proofs of the weight and elasticity of the air, the ancient philosophers maintained resolutely that air was light, and explained all these phenomena by the horrour which they said nature had for a vacuum.' It is curious that it should not have occurred to the author while writing this, that if these facts, so numerous and various, can all be accounted for by _one_ principle, there is a strong presumption that the principle is not altogether baseless. And in reality is it not true that nature _does_ abhor a vacuum, and does all she can to avoid it? No doubt this power is not unlimited; and moreover we can trace it to a mechanical cause, the pressure of the circumambient air. But the tendency, arising from this pressure, which the bodies surrounding a space void of air have to rush into it, may be expressed, in no extravagant or unintelligible manner, by saying that nature has a repugnance to a vacuum. [Note 13\2: Deluc, _Modifications de l'Atmosphère_, Partie 1.] That imperfect and false hypotheses, though they may thus explain _some_ phenomena, and may be useful in the progress of science, cannot explain _all_ phenomena;--and that we are never to rest in our labours or acquiesce in our results, till we have found some view of the subject which _is_ consistent with _all_ the observed facts;--will of course be understood. We shall afterwards have to speak of the other steps of such a progress.