In again revising this book I have conformed to the earlier plan of writing what I should have said if the lectures had been delivered in 1920 instead of 1908. The original statement has been amplified rather than modified. It has lost, long since, the appearance of challenge to existing theories which at first it may have presented.<br><br>But the subject has now grown entirely beyond the power of being fully encompassed by the original very simple and popular mode of treatment. I have thought it best, therefore, not to compress the original part unduly, as it still may serve a useful purpose to those not familiar with scientific conceptions, but to add, as a second part, a more briefly written and less elementary account of the later developments, particularly those that bear upon the problem of the constitution of the atom. It is to be hoped that even those who are not chemists or physicists, who have followed the exposition in the first part, may not be entirely unable to profit by the second part. Though, naturally, the new subject-matter, by reasons of its more general and often more speculative character - much of it still being in the making - cannot but be more difficult to understand than the original work, which dealt with distinct and easily understood steps in the progress of knowledge, made once and for all time.