q Mar. 6, 187 3] 
same direction as the electricity of the magnetising 
current. - 
It follows from this, by strict dynamical reasoning, that 
the medium under the action of magnetic force must be 
ina state of rotation—that is to say, that small portions of 
the medium, which we may call molecular vortices, are 
rotating, each on its own axis, the direction of this axis 
being that of the magnetic force. 
Here, then, we have an explanation of the tendency of 
the lines of magnetic force to spread out laterally and to 
shorten themselves. It arises from the centrifugal force 
of the molecular vortices. The mode in which electro- 
motive force acts in starting and stopping the vortices is 
more abstruse, though it is of course consistent with 
dynamical principles. 
We have thus found that there are several different 
kinds of work to be done by the electro-magnetic medium 
if it exists. We have also seen that magnetism has an 
intimate relation to light, and we know that there is a 
theory of light which supposes it to consist of the vibra- 
tions of a medium, How is this luminiferous medium 
related to our electro-magnetic medium ? 
It fortunately happens that electro-magnetic measure- 
ments have been made from which we can calculate by 
dynamical principles the velocity of propagation of small 
magnetic disturbances in the supposed electro-magnetic 
medium. 
This velocity is very great, from 288 to 314 millions of 
metres per second, according to different experiments. 
Now the velocity of light, according to Foucault’s experi- 
ments, is 298 millions of metres per second.- In fact, the 
different determinations of either velocity differ from each 
other more than the estimated velocity of light does from 
the estimated velocity of propagation of small electro- 
magnetic disturbance. But if the luminiferous and the 
electro-magnetic media occupy the same place, and trans- 
mit disturbances with the same velocity, what reason have 
we to distinguish the one from the other? By considering 
them as the same, we avoid at least the reproach of filling 
space twice over with different kinds of zther. 
Besides this, the only kind of electro-magnetic disturb- 
ance which can be propagated through a non-conducting 
medium is a disturbance transverse to the direction of 
propagation, agreeing in this respect with what we know 
of that disturbance which we call light. Hence, for all 
we know, light also may be an electro-magnetic disturb- 
ance in a non-conducting medium. If we admit this, the 
electro-magnetic theory of light will agree in every respect 
with the undulatory theory, and the work of Thomas 
Young and Fresnel will be established on a firmer basis 
than ever, when joined with that of Cavendish and Cou- 
lomb by the keystone of the combined sciences of light 
and electricity—Faraday’s great discovery of the electro- 
magnetic rotation of light. 
The vast interplanetary and interstellar regions will no 
longer be regarded as waste places in the universe, which 
the Creator has not seen fit to fill with the symbols of the 
manifold order of His kingdom. We shall find them to 
be already full of this wonderful medium ; so full, that no 
human power can remove it from the smallest portion of 
space, or produce the slightest flaw in its infinite conti- 
nuity. It extends unbroken from star to star, and when 
a molecule of hy‘rogen vibrates in the dogstar, the 
medium receives the impulses of these vibrations; and 
after carrying them in its immense bosom for three years, 
delivers them in due course, regular order, and full tale 
into the spectroscope of Mr. Huggins, at Tulse Hill. 
Butthe medium has other functions and operations besides 
bearing light from man to man, and from world to world, 
and giving evidence of the absolute unity of the metric 
system of the universe. Its minute parts may have rota- 
tory as well as vibratory motions, and the axes of rotation 
form those lines of magnetic force which extend in un- 
broken continuity into regions which no eye has seen, 
NATURE 
> c 
‘: 
343 
and which by their action on our magnets, are telling us 
in language not yet interpreted what is going on in the 
hidden under-world from minute to minute and from cen- 
tury to century. 
And these lines must not be regarded as mere mathe- 
matical abstractions. They are the directions in which 
the medium is exertinga tension like that of a rope, or 
rather like that of our own muscles. The tension of the 
medium in the direction of the earth’s magnetic force is 
in this country one grain weight on eight square feet. In 
some of Dr, Joule’s experiments, the medium has exerted 
a tension of 200 lbs, weight per square inch. 
But the medium, in virtue of the very same elasticity by 
which itis able to transmit the undulations of light, is also 
able to act as a spring. When properly wound up, it 
exerts a tension, different from the magnetic tension, by 
which it draws oppositely electrified bodies together, pro- 
duces effects through the length of telegraph wires, and 
when of sufficient intensity, leads to the rupture and ex- 
plosion called lightning. 
These are some of the already discovered properties of 
that which has often been called vacuum, or nothing at 
all. They enable us to resolve several kinds of action at 
a distance into actions between contiguous parts of a con- 
tinuous substance. Whether this resolution is of the 
nature of explication or complication, I must leave to the 
metaphysicians. 
ON LEAF-ARRANGEMENT* 
ASSUMING, as generally known, the main facts of 
Leaf-arrangement—the division into the whorled 
and spiral types, and in the latter more especially the 
establishment of the convergent series of fractions, 4, 3, 
2,3,7¢> 8) 4% H, B, 85, &c., as representatives of a 
corresponding series of spiral leaf-orders among plants— 
we have to ask what is the meaning that lies hidden in 
this law ? 
Mr. Darwin has taught us to regard the different 
species of plants as descended from some common 
ancestor ; and therefore we must suppose that the diffe- 
rent leaf-orders now existing have been derived by 
different degrees of modification from some common 
ancestral leaf-order. 
One spiral order may be made to pass into another by 
a twist of the axis that carries the leaves. This fact in- 
dicates the way in which all the spiral orders may have 
been derived from one original order, namely, by means 
of different degrees of twist in the axis. 
We naturally look to the simplest of existing leaf- 
orders, the two ranked alternate order 4, as standing 
nearest to the original ; for it is manifest that the orders 
at the other extreme of the series, the condensed arrange- 
ment of scales on fir-cones, of florets in heads of Com- 
posite, of leaves in close-lying plantains, &c., are special 
and highly developed instances, to meet special needs of 
protection and congregation: they are, without doubt, 
the latest feat of phyllotactic development ; and we may 
be sure that the course of change has been from the 
simple to the complex, not the reverse. This point will 
be illustrated by experiment below. 
But first, what are the uses of these orders?—and at 
what period of the leaf’s life does the advantage of leaf- 
order operate? The period must be that at which the 
leaf-order is most perfect ; not therefore when the twig is 
mature, with long internodes between the leaves ; but 
while the twig and its leaves are yet zz the bud; for it is 
in the bud (and similar crowded forms) that the leaf-order 
is in perfection, undisturbed by contortions or inequalities 
of growth ; but, as the bud develops into the twig, the 
leaves become separated, the stem often gets a twist, the 
leaf-stalks are curved and wrung to present the blades 
* Abstract of paper read by Mr. Hubert Airy, M,A., M.D., before the 
Royal Society, February 27, 1873. 
