"Take a strong iron beam, like one of those used so generally in building nowadays. It is set up in space, ten metres high, between two walls which support its ends. It is 'solid.' In the middle of it is placed a weight of one, two, or ten thousand kilograms; but it does not even show this enormous weight,--a level would hardly find a depression in it. And yet this beam is composed of particles which do not touch each other, which are in perpetual vibration, which separate under the influence of heat, and are drawn together by cold. Tell me, if you please, in what the solidity of this bar of iron consists. Its material atoms? Assuredly not, since they do not touch. That solidity lies in molecular attraction,--that is to say, in an immaterial force. "Speaking absolutely, solidity does not exist. Take up a heavy iron cannon-ball: this ball is composed of invisible molecules which do not touch each other. The continuity which the surface seems to have, and the apparent solidity of the ball are, then, pure illusions. To the mind which would analyze it, its inner structure is an eddying swarm of little gnats, like those darting about in the air on a summer day. Then suppose we heat this apparently solid ball: it will melt; heat it more, it will evaporate,--but without changing its nature for all that; gas or liquid, it will still be iron. "We are in a house. All these walls, these floors, these carpets, this furniture, the marble mantelpiece, are also composed of particles which do not touch each other; and all these particles which constitute these objects are in constant motion, circulating around each other. "Our body is in the same condition. It is formed by a perpetual circulation of molecules; it is a flame which is ceaselessly consumed and renewed; it is a stream on whose banks one sits down, expecting to see the same water again, but the perpetual course of things always brings fresh water. Each globule of our blood is a world (and we have five millions per cubic millimetre). Constantly, without let or hindrance, in our arteries and veins, in our flesh, in our brain, all circulates,--all moves, all hurries along in a vital whirl as rapid, proportionately, as that of the heavenly bodies. Molecule by molecule, our brain, our skull, our eyes, our nerves, our entire flesh ceaselessly renews itself, and so rapidly that in a few months our entire body is reconstituted. * * * * * "From estimates founded on molecular attraction it has been calculated that in a tiny drop of water taken up on the point of a pin, a drop invisible to the naked eye, measuring one thousandth of a cubic millimetre, there are more than two hundred and twenty-five million molecules.