[Footnote 4: A resolute scrutiny of the experiments, recently executed with reference to this question, is sure to yield instructive results.] [Footnote 5: Sir William Thomson.] [Footnote 6: In the ‘Prefatory Letter’ to his ‘Lay Sermons,’ Mr. Huxley speaks of ‘microscopists, ignorant alike of Philosophy and Biology.’ With reference to one conspicuous member of this class, a doctor of medicine, lately professor in a London college famous for its orthodoxy, both Mr. Huxley and myself have long practised, and shall, I trust, continue to practise, the tolerance recommended above.] [Footnote 7: An Address to the Mathematical and Physical Section of the British Association assembled at Norwich on August 19, 1868.] [Footnote 8: From an article headed ‘Physics and Metaphysics,’ in the _Saturday Review_ for August 4, 1860.] LONDON: PRINTED BY SPOTTISWOODE AND CO., NEW-STREET SQUARE AND PARLIAMENT STREET =TRANSCRIBER’S NOTES= Obvious typographical errors have been silently corrected. Punctuation, hyphenation, and spelling were made consistent when a predominant preference was found in the original book; otherwise they were not changed.