amount of electricity belonging to the monad atom be taken as a unit, then that of the dyad atom is two, of the triad atom, three, and so on. Hence, then, thanks first to Faraday and now to Helmholtz, chemists have now a new and unlooked-for confirmation of one of their most important doctrines from the science of electricity."
Popular Lectures ok Scientific Subjects. By H. Helmholtz. Translated by E. Atkinson, Ph.D. Second Series. New York: D. Appleton & Co. Pp. 265. Price, $1.50.
The first series of Helmholtz's lectures met with the success which has induced Professor Atkinson to translate an additional volume of them. It is gratifying to know that the translator feels himself justified in this, as it shows a growing popular appreciation of solid intellectual work in science. The contents of this volume are considerably varied, and represent the action of Helmholtz's mind upon widely different subjects. The first paper is an in memoriam address on Professor Gustave Magnus, who died in 1869. The essay is not a mere biographical notice or an ordinary eulogy, but is rather an analysis of the character and the scientific labors of Magnus in connection with the state of knowledge and circumstances of his time, so that the paper becomes in some respects an interesting portion of scientific history.
The second paper is "On the Origin and Significance of Geometrical Axioms," and it was a lecture delivered in Heidelberg in 1870. This discussion is not child's play, but many will be attracted to master it because it breaks into the field of speculation with regard to the different dimensions of space.
Artists will be interested in the abstracts of five lectures "On the Relation of Optics to Painting," which were delivered in Cologne, Berlin, and Bonn. After the introductory he takes up successively the subjects, form, shade, color, and harmony of color. His point of view is neither that of the practical artist nor of the student of pictures and schools of painting, but it is that of the physiological optician who is master of a subject. He shows in various ways how a knowledge of the mode of perception of the organ of vision may be of importance to the artist.
Perhaps the most striking of all the papers is the lecture "On the Origin of the Planetary System." So much is said about the nebular hypothesis of Kant and Laplace in these evolutionary times, that many will be glad to see the subject summed up within a moderate compass, and by an authoritative hand. No man is better prepared by his broad scientific erudition and his thorough mastery of mathematical and experimental physics than Professor Helmholtz to report on the present state of knowledge regarding the origin of the planetary system. But it was very far from the author's intention to make a mere popular statement of what former inquirers have arrived at. As one of the founders of the doctrine of the conservation of forces, he may be said to have been an original contributor to the nebular theory; and he is very pointed in his remarks on the grave scientific significance of the inquiry. He says, "Science is not only entitled, but is indeed beholden, to make such an investigation. For her it is a definite and important question—the question, namely, as to the existence of limits to the validity of the laws of nature, which rule all that now surrounds us; the question whether they have always held in the past, and whether they will always hold in the future; or whether, on the supposition of an everlasting uniformity of natural laws, our conclusions from present circumstances as to the past, and as to the future, imperatively lead to an impossible state of things; that is, to the necessity of an infraction of natural laws, of a beginning which could not have been due to processes known to us. Hence, to begin such an investigation as to the possible or probable primeval history of our present world, is considered as a question of science—no idle speculation, but a question as to the limits of its methods, and as to the extent to which existing laws are valid."
Professor Helmholtz is of opinion that our planetary system must sooner or later