[D] I am particularly indebted to my friend and colleague Professor Ruecker, F.R.S., for the many acute criticisms and suggestions on my remarks respecting the ultimate problems of physics, with which he has favored me, and by which I have greatly profited. [E] I am aware that this proposition may be challenged. It may be said, for example, that, on the hypothesis of Boscovich, matter has no extension, being reduced to mathematical points serving as centres of 'forces.' But as the 'forces' of the various centres are conceived to limit one another's action in such a manner that an area around each centre has an individuality of its own extension comes back in the form of that area. Again, a very eminent mathematician and physicist--the late Clerk Maxwell--has declared that impenetrability is not essential to our notions of matter, and that two atoms may conceivably occupy the same space. I am loth to dispute any dictum of a philosopher as remarkable for the subtlety of his intellect as for his vast knowledge; but the assertion that one and the same point or area of space can have different (conceivably opposite) attributes appears to me to violate the principle of contradiction, which is the foundation not only of physical science, but of logic in general. It means that A can be not-A. [F] 'Molecule' would be the more appropriate name for such a particle. Unfortunately, chemists employ this term in a special sense, as a name for an aggregation of their smallest particles, for which they retain the designation of 'atoms.' [G] 'At present more organic analyses are made in a single day than were accomplished before Liebig's time in a whole year.'--Hofmann, _Faraday Lecture_, p. 46. [H] In the preface to his _Mecanique Chimique_ M. Berthelot declares his object to be 'ramener la chimie tout entirere ... aux memes principes mecaniques qui regissent deja les diverses branches de la physique.'