In considering cases of Potential Energy, it is wise to realise that our knowledge about Gravitation is altogether too vague to make the case of a raised weight useful. And our knowledge of solid elasticity, though not so insignificant, is small enough to make the case of a bent bow or wound spring not very easy for fundamental contemplation. A case of chemical Energy, like gun-cotton, is in much the same predicament. But a typical and satisfactory example of Potential Energy is the case of a vessel of compressed air. Here is Energy stagnant enough, and violent enough when released, and one that can be locked up apparently to all eternity, and yet released by the pulling of a trigger. It represents, however, a case of which we know something concerning the internal mechanism; and we have learnt that in this case the force statically exerted on the walls of a vessel is really a kinetic bombardment of the molecules. In other words, we recognise in this case that Potential Energy is ultimately resolvable into Kinetic. It may be so in the other cases. And on Kelvin’s Kinetic Theory of Elasticity, which he showed a tendency in later life to abandon, all strain or stress in Ether may be ultimately due to its ultramicroscopic vortex circulation. But none of this is yet proven. * * * * * The general argument of your lecture deals with the ease with which certain general propositions are accepted as it were intuitively, without real conclusive evidence. I am entirely with you. And the way we feel secure about general laws, when adequate evidence for them is really impossible, has often struck me as remarkable. Even when facts appear to go against them, we question the facts, and find after all that in so doing we have been right. FOOTNOTES: [12] See Lecture VI. [13] See note at the end of the Lecture. [14] I got this view of Boyle’s relation to modern chemistry from Ostwald’s work. [15] In this chapter, especially in that part of it which deals with beliefs of conservation, I am greatly indebted to Meyerson’s “Identité et Réalité.” This acute and learned work is not written from the same point of view as that which I have adopted; but this in no way diminishes the amount of my obligation to its author. PART IV _SUMMARY AND CONCLUSION_ LECTURE X SUMMARY AND CONCLUSION I