282 Prof. Maxwell on the Theory of Molecular Vortices 
position that the surrounding medium is put into such a state 
that at every point the pressures are different in different direc- 
tions, the direction of least pressure being that of the observed 
lines of force, and the difference of greatest and least pressures 
being proportional to the square of the intensity of the force at 
that point. 
Such a state of stress, if assumed to exist in the medium, and 
to be arranged according to the known laws regulating lines of 
force, will act upon the magnets, currents, &c. im the field with 
precisely the same resultant forces as those calculated on the 
ordinary hypothesis of direct action at a distance. This is true 
independently of any particular theory as to the cause of this 
state of stress, or the mode in which it can be sustained in the 
medium. We have therefore a satisfactory answer to the ques- 
tion, “Is there any mechanical hypothesis as to the condition of 
the medium indicated by lines of force, by which the observed 
resultant forces may be accounted for?” The answer is, the 
lines of force indicate the direction of minimum pressure at every 
point of the medium. 
The second question must be, ‘“ What is the mechanical cause 
of this difference of pressure in different directions?” We have 
supposed, in the first part of this paper, that this difference of 
pressures is caused by molecular vortices, having their axes 
parallel to the lines of force. 
We also assumed, perfectly arbitrarily, that the direction of 
these vortices is such that, on looking along a line of force from 
south to north, we should see the vortices revolving in the direc- 
tion of the hands of a watch. 
We found that the velocity of the circumference of each vortex 
must be proportional to the intensity of the magnetic force, and 
that the density of the substance of the vortex must be propor- 
tional to the capacity of the medium for magnetic induction. 
We have as yet given no answers to the questions, ‘‘ How are 
these vortices set in rotation?” and “ Why are they arranged 
according to the known laws of lines of force about magnets and 
currents?” These questions are certainly of a higher order of 
difficulty than either of the former; and I wish to separate the 
suggestions I may offer by way of provisional answer to them, 
from the mechanical deductions which resolved the first question, 
and the hypothesis of vortices which gave a probable answer to 
the second. 
We have, in fact, now come to inquire into the physical con- 
nexion of these vortices with electric currents, while we are still 
in doubt as to the nature of electricity, whether it is one sub- 
stance, two substances, or not a substance at all, or in what way 
it differs from matter, and how it is connected with it, 
