SEPTEMBER II, 1913] 
NATURE 39 
ever, the effective rigidity of molecules must be com- 
plete, otherwise the sharing of energy must ultimately 
occur. They do not seem able to be set vibrating by 
anything less than a certain minimum stimulus; and 
that is the basis for the theory of quanta. 
Quantitative applications of Plamck’s theory, to 
elucidate the otherwise shaky stability of the astro- 
nomically constituted atom, have been made; and the 
agreement between results so calculated and_ those 
observed, including a determination of series of 
spectrum lines, is very remarkable. One of the latest 
contributions to this subject is a paper by Dr. Bohr 
in The Philosophical Magazine for July this year. 
To show that I am not exaggerating the modern 
tendency towards discontinuity, I quote, from M. 
Poincaré’s ‘‘Derniéres Pensées,”’ a proposition which 
he announces in italics as representing a form of 
Prof. Planck’s view of which he apparently approves :— 
‘A physical system is susceptible of a finite number 
only of distinct conditions; it jumps from one of these 
conditions to another without passing through a con- 
tinuous series of intermediate conditions.” 
Also this from Sir Joseph Larmor’s preface to 
Poincaré’s ‘‘ Science and Hypothesis ’’ :— 
“Still more recently it has been found that the 
good Bishop Berkeley’s logical jibes against the New- 
tonian ideas of fluxions and limiting ratios cannot be 
adequately appeased in the rigorous mathematical 
conscience, until our apparent continuities are resolved 
mentally into discreet aggregates which we only 
partially apprehend. The irresistible impulse to 
atomise everything thus proves to be not merely a 
disease of the physicist : a deeper origin, in the nature 
of knowledge itself, is suggested.”’ 
One very valid excuse for the prevalent attitude 
is the astonishing progress that has been made in 
actually seeing or almost seeing the molecules, and 
studying their arrangement and distribution. 
The laws of. gases have been found to apply to 
emulsions and to fine powders in suspension, of 
which the Brownian movement has long been known. 
This movement is caused by the orthodox molecular 
bombardment, and its average amplitude exactly 
represents the theoretical mean free path calculated 
from the ‘‘molecular weight’’ of the relatively 
gigantic particles. The behaviour of these micro- 
scopically visible masses corresponds closely and 
quantitatively with what could be predicted for them 
as fearfully heavy atoms, on the kinetic theory of 
gases; they may indeed be said to constitute a gas 
with a gram-molecule as high as 200,000 tons; and, 
what is rather important as well as interesting, they 
tend visibly to verify the law of equipartition of 
energy even in so extreme a case, when that law is 
properly stated and applied. 
_Still more remarkable—the application of X-rays to 
display the arrangement of molecules in crystals, and 
ultimately the arrangement of atoms in molecules, 
as initiated by Prof. Laue with Drs. Friedrich and 
Knipping, and continued by Prof. Bragg and jis 
son and by Dr. Tutton, constitute a series of re- 
searches of high interest and promise. By this means 
many of the theoretical anticipations of our country- 
man, Mr. William Barlow, and—working with him 
—Prof. Pope, as well as of those distinguished 
crystallographers von Groth and von Fedorow, have 
been confirmed in a striking way. These brilliant 
researches, which seem likely to constitute a branch 
of physics in themselves, and which are being con- 
tinued by Messrs. Moseley and C. G. Darwin,- and 
‘by Mr. Keene and others, may be called an apotheosis 
of the atomic theory of matter. 
_ One other controversial topic I shall touch upon 
in the domain of physics, though I shall touch upon 
NO. 2289, VOL. 92] 
it lightly, for it is not a matter for easy reference 
as yet. If the Principle of Relativity in an extreme 
sense establishes itself, it seems as if even time would 
become discontinuous and be supplied in atoms, as 
money is doled out in pence or centimes instead of 
continuously; in which case our customary existence 
will turn out to be no more really continuous than 
the events on a kinematograph screen, while that 
great agent of continuity, the ether of space, will 
be relegated to the museum of historical curiosities. 
In that case differential equations will cease to 
represent the facts of nature, they will have to be 
replaced by finite differences, and the most funda- 
mental revolution since Newton will be inaugurated. 
Now in all the debateable matters of which I have 
indicated possibilities I want to urge a conservative 
attitude. I accept the new experimental results on 
which some of these theories—such as the principle 
of relativity—are based, and am profoundly interested 
in them, but I do not feel that they are so revolu- 
tionary as their propounders think. I see a way to 
retain the old and yet embrace. the new, and I 
urge moderation in the uprooting and removal of 
landmarks. 
And of these the chief is Continuity. I cannot 
imagine the exertion of mechanical force across empty 
space, no matter how minute; a continuous medium 
seems to me essential. I cannot admit discontinuity 
in either space or time, nor can I imagine any sort 
of experiment which would justify such a hypothesis. 
For surely we must realise that we know nothing 
experimental of either space or time, we cannot 
modify them in any way. We make experiments on 
bodies, and only on bodies, using ‘‘body"’ as an 
exceedingly general term. 
We have no reason to postulate anything but con- 
tinuity for space and time. We cut them up into 
conventional units for convenience’ sake, and those 
units we can count; but there is really nothing atomic 
or countable about the things themselves. We can 
count the rotations of the earth, or the revolutions of 
an electron, or the vibrations of a pendulum, or the 
waves of light. All these are concrete and tractable 
physical entities; but space and time are ultimate 
data, abstractions based on experience. We know 
them through motion, and through motion only, and 
motion is essentially continuous. We ought clearly to 
discriminate between things themselves and our mode 
of measuring them. Our measures and perceptions 
may be affected by all manner of incidental and trivial 
causes, and we may get confused or hampered by our 
own movement; but there need be no such complica- 
tion in things themselves, any more than a landscape 
is distorted by looking at it through an irregular 
window-pane or from a travelling coach. It is an 
ancient and discarded fable that complications intro- 
duced by the motion of an observer are real com- 
plications belonging to the outer universe. 
Very well, then, what about the ether, is that in 
the same predicament? Is that an abstraction, or 
a mere convention, or is it a concrete physical entity 
on which we can experiment? 
Now it has to be freely admitted that it is exceed- 
ingly difficult to make experiments on the ether. It 
does not appeal to sense, and we know no means 
of getting hold of it. The one thing we know 
metrical about it is the velocity with which it can 
transmit transverse waves. That is clear and definite, 
and thereby to my judgment it proves itself a 
physical agent; not, indeed, tangible or sensible, but 
yet concretely real. 
But it does elude our laboratory grasp. If we 
rapidly move matter through it, hoping to grip it 
and move it too, we fail; there is no mechanical 
connection. And even if we experiment on light, 
