139. After inertia, which is not accounted for by any of the hypotheses as to the ultimate nature of matter which we have just given, the most general property of matter which we recognise is that of universal gravitation, in virtue of which portions of matter, if situated at a distance from one another, are possessed of potential energy. We are apt to hold exaggerated notions of the immense power of gravity; but a little consideration will show us that it is in reality one of the most trivial of the forces to which matter is directly or indirectly subject. Think for a moment of the fundamental experiments in electricity and magnetism, known to men for far more than 2000 years,—the lifting of light bodies in general by rubbed amber, and of iron filings by a loadstone. To produce the same effects by gravitation-attraction,—at least if the attracting body had the moderate dimensions of a hand-specimen of amber or loadstone,—we should require it to be of so dense a material as to weigh at the very least 1,000,000,000 lbs., instead of (as usual) a mere fraction of a pound. Hence it is at once obvious that the imposing nature of the force of gravity, as usually compared with other attractive forces, is due not to its superior qualitative magnitude, but to the enormous masses of the bodies which exercise it. In fact, the excessively delicate Torsion-balance of Michell was absolutely requisite to demonstrate, much more to measure, the mutual attraction between a large and a small leaden sphere. And (unless the third of the hypotheses as to the nature of matter above given be correct, in which case the _form_ of our statement would require modification) small or even moderately large pieces of matter are held together entirely by cohesion, gravitation being absolutely insensible; though in a huge mass like the earth, the force exerted by one hemisphere on the other (_i.e._ the force which would be called into play to prevent its being split in two) depends mainly upon gravitation, in comparison with whose enormous amount even a cohesive force of 500 lbs. weight per square inch over a circular surface of 4000 miles radius sinks into utter insignificance![46] 140. One only of the many hypotheses which have been advanced to explain the cause of gravitation has succeeded in passing the first preliminary tests. Of course, the assumption of action at a distance may be made to account for anything; but it is impossible (as Newton long ago pointed out in his celebrated letters to Bentley) for any one ‘who has in philosophical matters a competent faculty of thinking’ for a moment to admit the possibility of such action.