December 27, 1895.] 



SCIENCE. 



863 



convenient, but even indispensable, if it is 

 the result of our conviction that the most 

 essential elements of electric and magnetic 

 phenomena, that is, electric and magnetic 

 forces and the substances which are the seat 

 of activity of these forces, cannot be de- 

 scribed completely in terms of the concepts 

 of dynamics of ponderable matter. We 

 acknowledge that such is our conviction as 

 soon as we admit that the electric and mag- 

 netic forces are manifestations of the vari- 

 ous states of a substance which is not ordi- 

 nary matter and which, therefore, does 

 not form a part of that past experience of 

 ours which led to the formulation of the 

 fundamental axioms of Dynamics. Modern 

 electrical research believes in the existence 

 of a substance which has the relation just 

 mentioned to the electric and magnetic 

 forces, and it has succeeded in identifying 

 this substance with the lumeniferous ether ; 

 it sanctions, therefore, a temporary division 

 of Dynamics into Dynamics of Ponderable 

 Matter and Dynamics of Ether. 



In order to approach this subject more 

 closely, it is necessary now to state briefly 

 how our present ideas of electric and mag- 

 netic forces developed gradually out of 

 our ideas of ordinary mechanical forces. 

 This is, in my opinion, the most efficient 

 method of distinguishing between the es- 

 sential and the non-essential elements of 

 Physics of Ether. Without this distinction 

 there is no reliable way of assigning the 

 proper weight to the various features of the 

 tendencies of modern electrical research. 



MODERN DYNAMICS AND THE DOCTRINE OF DI- 

 RECT ACTION AT A DISTANCE. 



Pressures and tensions are the oldest and 

 most intelligible pictures of our ideas of the 

 concept force. But there was a time when 

 these pictures seemed to give us but one 

 phase of this mental concept. They failed 

 to explain the motion of machines and of 

 projectiles, although it was recognized long 



before the Science of Statics had reached 

 its stage of perfection in which we find it 

 nearly three hundred years ago, that in 

 these phenomena force was an essential 

 element. It should also be observed that 

 there was a time when the action of a me- 

 chanical force between bodies that had no 

 visible material connection was not sus- 

 pected. Thus Thales, of Miletus, believed 

 to have detected in the attraction of a piece 

 of amber when electrified by friction the 

 presence of an awakened universal soul. 

 The attraction of a loadstone gave rise to 

 most remarkable superstitions. Even Gil- 

 bert, the foremost physician and physicist 

 of the Elizabethan age, suspected in the 

 attractions and repulsions between magnets 

 the manifestations of an occult virtue, and 

 whenever he attempted to explain this vir- 

 tue he necessarily dragged in his ideas of 

 force as illustrated by pressures and ten- 

 sions in material bodies. 'Horror vacui' and 

 the tendency of bodies to seek their proper 

 place were for nearly two thousand years 

 the only explanations of the action of 

 gravitating force. The modern concept of 

 force as a concise statement of a law of 

 motion was first introduced into the Science 

 of Mechanics by the genius of Galileo Gali- 

 lei in the seventeenth century. This new, 

 so-called dynamical concept offeree, discov- 

 ered by Galileo as a result of his experi- 

 ments on the motion of a freely falling body 

 is really nothing more nor less than a con- 

 cise statement that a freely falling body in- 

 creases its vertical velocity uniformly with 

 the time of descent, and that, therefore, we 

 can say: the gravitational force acting upon 

 it is constant. Lagrange speaks as follows 

 of this discovery : "It forms to-day the per- 

 manent and the most essential part of the 

 glory of this great man. His discoveries of 

 the satellites of Jupiter, of the phases of 

 Venus, of sunspots, and so forth, required 

 telescopes and patience only ; but it was a 

 stroke of extraordinary genius to disen- 



