A picture by Ribera, representing St. Luke taking the likeness of the Virgin, was sold recently in Paris for 21,000f.
French landscape art has lost one of its chief illustrators in the person of Constant Troyon, who died in the latter part of March, aged about fifty-two. He has been called the creator of the modern French school of landscape, and delighted in cheerful aspects of nature, which he rendered with masterly skill. Rural life, with its pleasing accessories of winding streams, picturesque low banks, groups of cattle, and shady hamlets, formed the favorite subjects of his pencil; and though his style was not always exact, he succeeded in infusing an unusual degree of physical life into his pictures, without ever degenerating into mere naturalism. As a colorist he excelled all contemporary animal and landscape painters, and used his brush with a freedom rivalling that of Delacroix. He died insane, and is said to have left a fortune of 1,200,000 francs. Some of his pictures are owned in New York.
A painting by Murillo, from the collection of the late Marquis Aguado, representing the death of Santa Clara, has been sold to the Royal Gallery of Madrid for 75,000 francs.
NEW PUBLICATIONS.
THE CORRELATION AND CONSERVATION OF FORCES: A SERIES OF EXPOSITIONS, by Prof. Grove, Prof. Helmholtz, Dr. Mayer, Dr. Faraday, Prof. Liebig, and Dr. Carpenter. With an Introduction and brief Biographical Notices of the chief Promoters of the new views. By Edward L. Youmans, M.D. 12mo., pp. xlii., 438. New York: D. Appleton & Company.
Religious writers have repeatedly deplored the materialistic tendency of modern scientific research, and in many cases, no doubt, the complaint is a just one. But we must not forget that the bad tendency is in the philosophical system which is sought to be built upon the facts of discovery, not in the facts themselves. Every development, of truth, every fresh unveiling of the mechanism of the universe, must of necessity redound to the greater glory of God. And it seems to us that no scientific theory which has been broached for many years speaks more gloriously of the disposing and over-ruling hand of an all-wise Creator than the one to which the volume now before us is devoted. If there could be any place for comparison in speaking of the exercise of omnipotence, we might say that the new view of the nature and mode of action of the physical forces represents creation as a far more marvellous act than the old one did.
We speak of the correlation and conservation of force as a "new" theory because it is only lately that it has attracted much attention beyond the higher scientific circles, and indeed it would perhaps be going too far to say that it is yet firmly established. It has been developing however for a number of years, and the most distinguished experts in physical science have for some time accepted it with remarkable unanimity. In the book whose title we have given above, Dr. Yournans has brought together eight of the most valuable essays in which the theory has been maintained or explained by its founders and chief supporters. He has made his selection with excellent judgment, and prefixed to the whole a clear and well-written introduction, by the aid of which any reader of ordinary education will be able to appreciate what follows. The longest and most important essay is that by Professor Grove on "The Correlation of Physical Forces."
Force is defined by Professor Grove as that active principle inseparable from matter which induces its various changes. In other words, it is the agent or producer of change or motion. The modifications of this general agent—heat, light, electricity, magnetism, chemical affinity, gravity, cohesive attraction, etc.—are called the physical forces. In many cases, where one of these is excited all the others are set in motion: thus when sulphuret of antimony is electrified, at the moment of electrization it becomes magnetic; at the same time it is heated; if the heat is raised to a certain intensity, light is produced; the compound is decomposed, and chemical action is thereby brought into play; and so on. Moreover, we cannot magnetize a body without electrizing it, and vice-versa. This necessary reciprocal production is what is understood by the term "correlation of forces"—or in other words, we may say that any one of the natural forces may be converted into another mode of force, and may be reproduced by the same force. A striking example of the conversion of heat into electricity is furnished by an experiment of Seebeck's. Two dissimilar metals are brought together and heated at the point of contact. A current of electricity flows through the metals, having a definite direction according to the metals employed; continues as long as an increasing temperature is pervading the metals; ceases when the temperature is stationary; and flows backward when the heat begins to decrease. The immediate convertibility of heat into light is not yet established beyond question, although these two forces exhibit many curious analogies with each other. But heat through the medium of electricity may easily be turned into light, chemical affinity, magnetism, etc. Electricity directly produces heat, as in the ignited wire, the electric spark, and the [{426}] voltaic arc. The last-named phenomenon—the flame which plays between the terminal points of a powerful voltaic battery produces the most intense heat with which we are acquainted; so intense, in fact, that it cannot be measured, as every sort of matter is dissipated by it. For instance, it actually distils or volatilizes iron, a metal which by ordinary means is fusible only at a very high temperature. The voltaic arc also produces the most intense light that we know of. Instances of the conversion of electricity into magnetism and chemical action are familiar to everybody. The reciprocal relations of light with other modes of force are thus far very imperfectly known. Professor Grove however describes an experiment by which light is made to produce simultaneously chemical action, electricity, magnetism, heat, and motion. The conversion of light into chemical force in photography is another exemplification of the law of correlation, and Bunsen and Roscoe have experimentally shown that certain rays of light are extinguished or absorbed in doing chemical work. A familiar example of the change of light into heat is seen in the phenomena of what is termed the absorption of light. Place different colored pieces of cloth on snow exposed to sunshine: black will absorb the most light, and will also develop the most heat, as may be seen by its sinking deepest in the snow; white, which absorbs little or no light, will not sink at all.