friends on the matter. Devoting, as Professor Birks does, his chapter entitled "The Transformation of Force and Motion," to the incongruities which result when the doctrine of the persistence of force is joined with the doctrine of potential energy, as commonly received, it was doubtless convenient to assume, spite of the direct evidence to the contrary, that I accept this doctrine, and am implicated in all the consequences. But there can be but one opinion respecting the honesty of making the assumption. Let me add that my rejection of this doctrine is not without other warrant than my own. Since the issue of the last edition of this work, containing the passages I have referred to, Mr. James Croll, no mean authority as a mathematician and physicist, has published in the "Philosophical Magazine" for October, 1876, page 241, a paper in which he shows, I think conclusively, that the commonly accepted view of potential energy can not be sustained, but that energy invariably remains actual. I learn from him that he had in 1867 indicated briefly this same view.
The remaining case, above adverted to as calling for comment, concerns my motive for suppressing a certain passage in the chapter on "Ultimate Scientific Ideas," and substituting another passage. Before proceeding to state the reasons for this substitution, and to disprove the inferences which Professor Birks draws from it, I may remark that it is usual in literary criticism to judge an author by the latest expression of his views. It is commonly thought nothing but fair that if he has made an error (I say this hypothetically, for in this case I have no error to acknowledge), he should be allowed the benefit of any correction he makes. Professor Birks, however, apparently thinks that, moved by the high motive of "doing God service," he is warranted in taking the opposite course—perhaps thinks, indeed, that he would fail of his duty did any regard for generous dealing prevent him from making a point against an opponent of his creed.
But now, saying no more about the ethics of criticism, I pass to the substantial question. In the first place, I have to point out that in the passage suppressed I have not said that which Professor Birks alleges. He represents me as asserting that "gravitation is a necessary result of the laws of space" (p. 227). I have asserted no such thing. He says, "There can be no a priori necessity that every particle should act on every other at all at every distance" (p. 222). I have nowhere said, or even hinted, that there is any such a priori necessity. The notion that "gravitation results by a fatal necessity from the laws of space," which he ascribes to me (p. 229), is one which I should repudiate as utterly absurd, and one which is not in the remotest way implied by anything I have said. What I have said is that "light, heat, gravitation, and all central forces, vary inversely as the squares of the distances," and that "this law is not simply an empirical one, but one deducible mathematically from the relations of space." Now, what is here said to be "deducible mathematically from the rela-
