Sbptembke 21, 1900.] 



SCIENCE. 



429 



strain in the surrounding sether which is 

 part of its essence, and cannot be con- 

 sidered apart from it ; each of them essen- 

 tially pervades the entire space, though on 

 account of its invariable character we con- 

 sider it as a unit. Thus we appear to be 

 debarred from imagining the sether to act 

 as an elastic connection which is merely 

 the agent of transmission of a pull from 

 the one nucleus to the other, because there 

 are already stresses belonging to and con- 

 stituting an intrinsic part of the terminal 

 electrons, which are distributed all along 

 the medium. Our action criterion of a dy- 

 namical system, in fact, allows us to reason 

 about an electron as a single thing, not- 

 withstanding that its field of energy is 

 spread over the whole medium ; it is only 

 in material solid bodies, and in problems 

 in which the actual sphere of physical ac- 

 tion of the molecule is small compared 

 with the smallest element of volume that 

 our analysis considers, that the familiar 

 idea of transmission of force by simple 

 stress can apply. Whatever view may 

 ultimately command itself, this question is 

 one that urgently demands decision. A 

 very large amount of effort has been ex- 

 pended by Maxwell, Helmholtz, Heaviside, 

 Hertz and other authorities in the attempt 

 to express the mechanical phenomena of 

 electrical action in terms of a transmitting 

 stress. The analytical results up to a cer- 

 tain point have been promising, most strik- 

 ingly so at the beginning, when Maxwell 

 established the mathematical validity of 

 the way in which Faraday was accustomed 

 to represent to himself the mechanical in- 

 teractions across space, in terms of a ten- 

 sion along the lines of force equilibrated by 

 an equal pressure preventing their expan- 

 sion sideways. According to the views 

 here developed, that ideal is an impossible 

 one ; if this could be established to general 

 satisfaction the field of theoretical discus- 

 sion would be much simplified. 



This view that the atom of matter is, so 

 far as regards physical actions, of the na- 

 ture of a structure in the sether involving 

 an atmosphere of sethereal strain all around 

 it, not a small body which exerts direct 

 actions at a distance on other atoms accord- 

 ing to extraneous laws of force, was practi- 

 cally foreign to the eighteenth century, 

 when mathematical physics was modelled 

 on the Newtonian astronomy and domi- 

 nated by its splendid success. The scheme 

 of material dynamics, as finally compactly 

 systematized by Lagrange, had therefore no 

 direct relation to such a view, although it 

 has proved wide enough to include it. The 

 remark has often been made that it is prob- 

 ably owing to Faraday's mathematical 

 instinct, combined with his want of ac- 

 quaintance with the existing analysis, that 

 the modern theory of the sether obtained a 

 start from the electric side. Through his 

 teaching and the weight of his authority, 

 the notion of two electric currents exerting 

 their mutual forces by means of an inter- 

 vening medium, instead of by direct at- 

 traction across space, was at an early period 

 firmly grasped in this country. In 1845 

 Lord Kelvin was already mathematically 

 formulating, with most suggestive success, 

 continuous elastic connections, by whose 

 strain the fields of activity of electric cur- 

 rents or of electric distributions could be 

 illustrated ; while the exposition of Max- 

 well's interconnected scheme, in the earlier 

 form in which it relied on concrete models 

 of the electric action, goes back almost to 

 1860. Corresponding to the two physical 

 ideals of isolated atoms exerting attraction 

 at a distance, and atoms operating by atmos- 

 pheres of sethereal strain, there are, as 

 already indicated, two different develop- 

 ments of dynamical theory. The original 

 Newtonian equations of motion determined 

 the course of a system by expressing the 

 rates at which the velocity of each of its 

 small parts or elements is changing. This 



