Search

they do strain the sacred writings, who there interpret these words for the measured quantities." But Cajori's "improvement" would have Newton say, "On this account, those violate the accuracy of language, which ought to be kept precise, who interpret these words for the measured quantities."16 For those who may wonder how Cajori could have altered those innocent words sacris litteris to "the accuracy of language, which ought to be kept precise," may I remind them that Cajori's alterations of Motte do not generally arise from a verbatim et litteratim confrontation of Newton's Latin and Motte's English versions. And in this case, Cajori's source was neither Newton nor Motte. He chose to replace Motte's correct and literal rendition by Thomas Thorp's, only changing the latter's "which ought to be kept sacred" to "which ought to be kept precise." 7 Each reader may frame his own joke on this travesty of "accuracy of language" which-all would agree-really "ought to be kept precise." 16 Motte-Cajori version, p. 11. though the prospectus, of which there are two copies in the library of the old Ashmolean 17 Only Vol. I of Thomas Thorp's transla- Museum in Oxford, announced that he had tion of the Principia was ever published, al- translated all three books. See n. 8 supra.

The electromagnetic theory of light which was formulated by Maxwell 2) in 1865 has since, especially through the experiments of various physicists on the specific inductive capacity of nonconducting media, acquired a high degree of credibility. Although I consider it exceedingly probable that a change in our conceptions as regards electric phenomena will lead also in some degree to an alteration in the theory of Maxwell, I still hold that its main principle, i.e., the hypothesis that vibrations of light are movements of the same character as electric currents, can hardly be questioned. It appeared therefore to me to be desirable to make a further investigation of the results of this theory which have as yet been only partially compared with experience. By this means it will not only be possible to estimate the value of the theory, but if it is correct there is also a chance that the investigation of light phenomena may advance to some extent our knowledge of the action of electricity.

It is a great honor to me to have been requested to address you on this memorable occasion and I have many good reasons for being interested in to-day's proceedings. In the first place, I have been so kindly and warmly welcomed by the scientific men of the institute that I feel almost as if I belonged to them and as if I also were going to have a share in the facilities for scientific research that have now been put at their disposal.

The illustrious founder of our society will forever occupy a prominent place in the history of physical science, not only for his experimental researches in electricity and his invention of the lightning rod, but also for his theoretical views. Franklin tried to explain all electrical phenomena that were known in his time by means of a single electric fluid, which he supposed to be present in certain definite quantities in all ponderable bodies, when in their natural or unelectrified state, and in larger or smaller quantities in positively or negatively charged bodies. The rival doctrine was that of two electric fluids, which in the days before Maxwell served as the foundation of the mathematical theory of electricity and which was adopted by those physicists who, like Riemann, Weber and Clausius, sought to account for electrostatic and electrodynamic phenomena by one fundamental law for the mutual action of electric particles.

The title chosen for this lecture is, strictly speaking, not quite appropriate. The experimental investigations, for which I shall ask your attention, envisaged for the most part the testing of suppositions which had already been formulated previous to experimental investigation, and though it is of the greatest importance for our knowledge of this subject that they should be confirmed, we can properly only speak of „foundations“ if we add that many of these have been laid after the edifice had already risen a long way above ground. A method of treatment which, it need not be demonstrated, is unavoidable, will we not be too slow in progress.

When I had the honour to be invited to deliver this May Lecture I, of course, thought that it would be appropriate to choose for my subject some of the phenomena that occur in metallic substances. However, as I know much less than you of the constitution of metals, and of the properties which depend on it, I felt that I should have to confine myself to questions of a somewhat general character. Fortunately, in the remarks which I am going to make about the motion of electricity in metals, my ignorance — so, at least, I hope — will not become too apparent.

When once the two kinds of electricity and their respective actions were known, it was only natural to apply to them the terms positive and negative. By attaching algebraic signs to the charges e and e′ of two bodies one could sum up the rules for their mutual action in a simple formula; the force at a distance r can be represented by ee′/r 2, where one can tell from the sign of the product ee′ whether one has to deal with a repulsion or with an attraction. The words positive and negative were also in accordance with the empirical fact that out of a neutral state two equal and opposite electric charges can arise and that vice-versa two such charges can, on combining, disappear altogether. And yet many phenomena occur showing differences which are not adequately rendered by opposite algebraic signs only. The well-known figures of Lichtenberg, for example, show that the two electricities do not spread in the same way over the surface of a plate of resin. Everyone knows the different luminous phenomena visible at the two sets of sharp points of a Holtz machine, long pencils at the one side, at the other small spots of light, and in the same way the difference between the positive and the negative light in a discharge-tube is apparent at once. Moreover, the phenomena of electrolysis show that the two electricities are attached to certain substances — the positive one to the metallic atoms, the negative one to the other part of the electrolyte.