a movement of translation. A wheel, if frictionless like a molecule, could revolve on its bearings forever; if it were small enough, its motion would forever escape observation. Were it dropped from its bearings, through however short a distance, to a horizontal plane, part of its energy would be at once expressed in its advancing in a line long enough for detection. The question behind attraction and repulsion is, How shall two distant bodies move on their axes, or in their orbits, so as to act on a chain of intervening bodies with the effect that the two shall approach or recede from each other? This problem does not seem to present insuperable difficulty to the inventiveness which has built so many models illustrating the architecture of the molecule, showing how, in all probability, the links subsist between the atoms of an alcohol or an ether.
One after another various forms of energy once called potential have been brought into line with energy actual, have been reasonably explained as meaning nothing more or less than motion; is it not time that old conceptions of motion should be expanded so as to include the phenomena of gravity as well as all the others once deemed to consist in mere "advantage of position"? Gravity can be imagined as a special molecular motion in its propagation either instantaneous or too swift for existing means of measurement. This supposition may be an unwelcome one, but what is the alternative? Whereas the physicist of to-day holds that the chemical energy of such an element as carbon, the elasticity of a coiled spring or of a confined body of gas, and the quality we call temperature, all denote real activities, nevertheless? the lifting of a weight, into which any of these activities can bo readily transformed, is not represented by motion at all, but by an ultimate and unnamed something else. Whether is it better to cherish a conception in its inherited form or to try to broaden it as the facts demand? For the inclusion of gravity among the phases of veritable motion there is cumulative suggestion. When in every other phase of energy there is either detection of motion in what seeems rest, or an assumption of motion the validity of which is proved in the fulfillment of the predictions to which it leads, the hint is clear. It is that gravity, too, will be demonstrated as motion by future means of inquiry which may as far transcend our present resources as these surpass the methods of the men of science who, not so very long ago, could bring forward reasons for believing phlogiston to be a substance and electricity to be a fluid.
The advance of knowledge thus far has been a process of identification. Heat, chemical affinity, electricity, magnetism, and all the other forms of palpable energy are now held to differ from one another only as do the circles, spirals, and straight lines de-
