From the Preface. ...Few tasks are as difficult as that of conveying to a general audience a true impression of the results of scientific inquiry. One must avoid the Scylla of too great certainty and also the Charybdis of too great uncertainty. Even the proper words to use are a matter of doubt; and the difficulty is not lightened by the fact that, through the daily press and the popular magazines, many of the discoveries of science have been given exposition - in many cases, by people entirely ignorant of the subject. The plan adopted, after most serious consideration, was to accept the general theory of molecules and atoms as proposed by Sir J. J. Thomson and the properties of electrons as deduced by H. A. Lorentz, and to attempt to explain how from these one may deduce the general and even specific properties of matter. This method obviously is one suited only for a general audience; and even there it has its dangers. One is liable to produce the impression that our theories are verified, whereas they are but hypotheses still; but this is better, perhaps, than to leave the conviction that nothing is certain. It is difficult to make any body of listeners, however great their general intelligence, realize that in the end the great purpose of scientific investigation is the pursuit of Truth, the attainment of knowledge. Hypotheses rise and fall; the facts of experiment remain. The temptation is great to stop here and there and emphasize what is not known, what is not proved; and the real usefulness of the lectures is lessened of course by the fact that this was done so rarely. However successful the attempt has been, the main purpose of the lectures was to make clear to a body of people, not students of physics, some of the results of investigators in unifying our knowledge of the world around us.