 @ 
PRESIDENTS ADDRESS—SECTION A. 4] 
discharge the function of communicating motion from one body 
to another, and its fertility in leading to discoveries in the 
realm of light, electricity, magnetism, and heat; the extra- 
ordinary analogies and identities which exist among the phe- 
nomena of those elements of natural science, viewed in the light 
of that conception, all of which afford abundant evidence, too, of 
the organic unity of the idea of energy; these have for the 
immediate purposes of the physical sciences justified its adoption. 
Newton felt that such a medium must be assumed, but he re- 
frained from expressing himslf definitely in regard to it, in the 
absence of a sufficient body of experimental evidence (/). Com- 
plex mechanical conceptions of this medium have been promul- 
gated in order to render its functions picturable. But a medium 
regarded as subject to stress and strain and possessed of dynamic 
stability, in order that it may fulfil its functions in the respect 
indicated, involves again the original difficulty of choosing be- 
tween action at a distance, or supersensible existences, as ulti- 
mate hypotheses, representative of reality. Recently observed 
phenomena, such as fluorescence, cathodic, Lenard, and Réntgen 
radiations, the Zeeman phenomena, &c., all go to show what 
great complexity exists in the relations of ether and matter, and 
how difficult it is to assign definitely the functions of either in a 
manner that is mechanically intelligible. The erudite papers of Mr. 
Larmor on this subject in recent numbers of the Philosophical 
Transactions of the Royal Society (7) afford impressive evidence 
of this. 
The introduction into the ether of hypothetical structures 
conceived in the likeness of various mechanical models, gyrostatic 
cells, or other similar motions, may result in a representational 
mechanism, the reactions of which are analogous to those con- 
ceived as actually occurring. They may, too, fulfil the require- 
ments of deduction when handled by those whose powerful 
mathematical resources are adequate to the task. But the old 
conceptual difficulties are not eliminated. If the structural 
character be regarded as real, the original difficulties are reintro- 
duced. Such mechanisms are intelligible only by retaining 
the original ideas of matter and energy ; as ultimate explanations 
they wholly beg the question. 
Denial of Actio in Distans.—The third method of explain- 
ing motion is to at once admit the idea of action at a distance. 
It is a curious fact that no attempt to get rid of this idea by 
mechanical contact interpretations of the universe has been com- 
pletely successful. The implication of such action may have 
been pushed back so far that it is not readily discerned ; we are 
(7) Sed haee paucis exponi non possunt; neque adest sufficiens copia experimentorum, 
quibus leges actionum hujus spiritus accurate determinari et monstram debent. Prin- 
cipia Lib. III. De Mundi Systemate, p. 530. 
(m) A dynamical theory of the electric and luminiferous medium. Phil. Trans. 185A, 
Pp. 719-822. 186 A, pp. 695-743. 190 A, pp. 205-300. 
