and other Forces. 2538 
according to the views which I maintain, these are phenomena 
which ultimately may receive explanations by means of ztherial 
undulations and currents, and therefore ought not to be put in 
the same category. 
As the article I have been referring to contains a theory of 
magnetic phenomena wholly different from that which I have 
advanced, it may be worth while to point out a difference in 
principle between the fundamental hypotheses of the two theories. 
Professor Maxwell assumes the existence of a magnetic medium, 
which is not fluid, but “ mobile,” and which acts along lines of 
magnetic force by stress combined with hydrostatic pressure ; in 
other words, there is greater pressure in the equatorial, than in 
the axial, direction of the magnetic field. To account for this 
difference of pressure, it is assumed that “ molecular vortices” 
circulate about axes parallel to the lines of magnetic force. Why 
they are called ‘‘ molecular” is not expressly stated im this arti- 
cle; but it may be gathered from other of the author’s writings, 
that he conceives the matter of the vortices to consist of mole- 
ecules which by their motions may come into collision, the result- 
ing dynamic effect depending on the number and frequency of 
such collisions. It is not my intention to criticise these hypo- 
theses, which have been adduced merely for the sake of remarking 
that, as they are of a particular character, and have been framed 
apparently with special reference to observed laws of magnetic 
phenomena, the results of a mathematical investigation founded 
on them can hardly amount to more than an empirical expression 
of those laws. After all that can be done by this kind of re- 
search, an independent and @ priori theory of the same kind as 
that which I have proposed, if not the very one, is still needed. 
The hypotheses on which my investigations have been founded 
are these only. The physical forces are modes of action of the 
pzessure of the ether, which is a continuous fluid medium, having 
the property of pressing in proportion to its density, and filling 
all space not occupied by the discrete atoms of sensible bodies, 
which atoms are inert, spherical, and of different, but constant, 
magnitudes. It may be remarked that these hypotheses include 
no ideas that are not intelligible by sensation and experience, 
and therefore conform to the rule of philosophy according to 
which cur knowledge of natural operations must ultimately rest 
on such ideas. Also they are strictly related to antecedent and 
existing physical science. The prominent terms, ether and atom, 
which had their origin in ancient speculations, have obtained. re- 
markable significance in modern science,—the first, by explana- 
tions of the phenomena of light, and the other, by aiding us to 
conceive of chemical analyses. The hypotheses under the above 
form are mainly due to Newton, who gave a definition of atoms, 
