504 THE POPULAR SCIENCE MONTHLY. 



a movement of translation. A wheel, if frictionless like a mole- 

 cule, 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, show- 

 ing 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 poten- 

 tial have been brought into line with energy actual, have been 

 reasonably explained as meaning nothing more or less than mo- 

 tion ; is it not time that old conceptions of motion should be ex- 

 panded so as to include the phenomena of gravity as well as all 

 the others once deemed to consist in mere " advantage of posi- 

 tion " ? 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 be 

 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 demon- 

 strated 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 iden- 

 tification. 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- 



