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THE POPULAR SCIENCE MONTHLY.

the nature of that agent as well as of dynamic philosophy—so Dr. Lardner went over the ground of physics in his five-years' lectures in this country in complete obliviousness of the new point of view that had even then been assumed by investigators of his own and other countries.

But Prof. Tyndall belongs to the later era: he has done his share in bringing it about, and is among its ablest representatives. Besides his original contributions to the more recent phases of science, by his genius for lucid and eloquent statement he has done perhaps more than any other man to put the new doctrines into popular and attractive form. In his classical volume entitled "Heat as a Mode of Motion," he takes the point of view definitely assumed by Rumford, and has worked out the science of thermotics on a modern basis and in harmony with the later views of the nature of force or energy. As all who have read his works are aware, Tyndall is more than a mere specialist; he is a broad thinker—a philosopher of science. No man is more painstaking or scrupulous in elaborating isolated facts with accuracy, but that does not content him, nor is he satisfied with the narrow theories that have been applied to them; but he strives after those wider and deeper explanations by which diverse phenomena are brought into harmonized relations. The various physical forces are interesting to him in their pure phenomenal workings, but they have a larger interest as clews to the constitution of matter. Physics has two great departments. Molar Physics treats of the movements and mechanical properties of masses, as the revolutions and attractions of the celestial orbs, or the laws of motion in terrestrial bodies. Molecular Physics, on the other hand, deals with the subtler forces of magnetism, heat, light, electricity, and affinity, by which the inner nature of matter is affected and its profoundest changes brought about. It is this division or aspect of physics that has mainly engaged the attention of Prof. Tyndall. His first scientific reputation was made by researches in the field of magnetism, and his original papers upon this subject have recently appeared in an elaborate volume. Glacial phenomena have also been favorite objects of study with him. Involving as they do the molecular mutations of water, through the vaporous, liquid, and solid conditions, on a grand and impressive scale, they afford a fine exemplification of the play of molecular forces of which Prof. Tyndall has availed himself, both to extend our knowledge of the subject and to enlist the interest of the public in some of the most beautiful and wonderful operations of Nature. The first book by which Prof. Tyndall became widely known was his "Glaciers of the Alps." now long out of print; and his latest work, to be immediately published, is on the Forms of "Water," in clouds, rain, rivers, ice, and glaciers. Much of his time during the last dozen years has been devoted to the revision and extension of his early opinions upon these subjects. The courses of lectures which he is to give in this country will be eminently valuable as reflecting the latest views that have been formed in a field of science that has undergone a great change within a recent period. We shall be able to listen to the authentic teachings of a master in science, and one who is, moreover, a master in the art of popular exposition.

 

THE PRESIDENTIAL ADDRESSES.

In our present number will be found the addresses of the presidents of the two scientific associations held in August, the one in Dubuque, Iowa, and the other in Brighton, England. They are entitled to consideration from the positions of their respective authors, the weight and dignity of the bodies addressed, and the interest of