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NATURE 
THURSDAY, MARCH 31, 1870 
THE SIZE OF ATOMS 
TT HE idea of an atom has been so constantly associated 
with incredible assumptions of infinite strength, 
absolute rigidity, mystical actions at a distance, and 
indivisibility, that chemists and many other reasonable 
naturalists of modern times, losing all patience with it, 
have dismissed it to the realms of metaphysics, and 
made it smaller than “ anything we can conceive.” But if 
atoms are inconceivably small, why are not all chemical 
actions infinitely swift? Chemistry is powerless to deal 
with this question, and many others of paramount im- 
portance, if barred, by the hardness of its fundamental 
assumptions, from contemplating the atom as a real por- 
tion of matter occupying a finite space, and forming a 
not immeasurably small constituent of any palpable body. 
More than thirty years ago naturalists were scared by 
a wild proposition of Cauchy’s, that the familiar prismatic 
colours proved the “sphere of sensible molecular ac- 
tion” in transparent liquids and solids to be comparable 
with the wave-length of light. The thirty years which 
have interyened have only confirmed that proposition. 
They have produced a large number of capable judges ; 
and it is only incapacity to judge in dynamical questions 
that can admit a doubt of the substantial correctness of 
Cauchy’s conclusion. But the “sphere of molecular 
action” conveys no very clear idea to the non-mathe- 
matical mind. The idea which it conveys to the mathe- 
matical mind is, in my opinion, irredeemably false. 
For I have no faith whatever in attractions and repulsions 
acting at a distance between centres of force according 
to various laws. What Cauchy’s mathematics really 
proves is this: that in palpably homogeneous bodies such 
as glass or water, contiguous portions are not similar 
when their dimensions are moderately small fractions of 
the wave-length. Thus in water, contiguous cubes, each 
of one one-thousandth of a centimetre breadth, are sen- 
sibly similar. But contiguous cubes of one ten-millionth 
of a centimetre must be very sensibly different. Soin a 
solid mass of brickwork, two adjacent lengths of 20,000 
centimetres each, may contain, one of them nine hundred 
and ninety-nine bricks and two half bricks, and the other 
“one thousand bricks: thus two contiguous cubes of 20,000 
centimetres breadth may be considered as sensibly similar. 
But two adjacent lengths of forty centimetres each might 
contain, one of them one brick and two half bricks, and 
the other two whole bricks ; and contiguous cubes of forty 
centimetres would be very sensibly dissimilar. In short, 
optical dynamics leaves no alternative but to admit that 
the diameter of a molecule, or the distance from the 
centre of a molecule to the centre of a contiguous mole- 
cule in glass, water, or any other of our transparent 
liquids and solids, exceeds a ten-thousandth of the wave- 
length, or a two-hundred-millionth of a centimetre. 
By experiments on the contact electricity of metals 
made eight or ten years ago, and described in a letter to 
Dr. Joule, which was published in the Proceedings of the 
Literary and Philosophical Society of Manchester, I found 
that plates of zinc and copper connected with one another 
by a fine wire attract one another, as would similar pieces 
of one metal connected with the two plates of a galvanic 
element, having about three-quarters of the electro-motive 
force of a Daniel’s element. 
Measurements published in the Proceedings of the Royal 
Society for 1860 showed that the attraction between 
parallel plates of one metal held at a distance apart 
small in comparison with their diameters, and kept con- 
nected with such a galvanic element, would experience an 
attraction amounting to two ten-thousand-millionths of a 
gramme weight per area of the opposed surfaces equal to 
the square of the distance between them. Leta plate of 
zinc anda plate of copper, each a centimetre square and a 
hundred-thousandth of a centimetre thick, be placed with 
a corner of each touching a metal globe of a hundred- 
thousandth of a centimetre diameter. Let the plates, 
kept thus in metallic communication with one another be 
at first wide apart, except at the corners touching the 
little globe, and let them then be gradually turned round 
till they are parallel and at a distance of a hundred-thou- 
sandth of a centimetre asunder. In this position they 
will attract one another with a force equal in all to two 
grammes weight. By abstract dynamics and the theory 
of energy, it is readily proved that the work done by the 
changing force of attraction during the motion by which 
we have supposed this position to be reached, is equal to 
that of a constant force of two grammes weight acting 
through a space of a hundred-thousandth of a centimetre ; 
that is to say, to two hundred-thousandths of a centimetre- 
gramme. Now let a second plate of zinc be brought by a 
similar process to the other side of the plate of copper; a 
second plate of copper to the remote side of this second 
plate of zinc, and so on till a pile is formed consisting of 
50,001 plates of zinc and 50,000 plates of copper, sepa- 
rated by 100,000 spaces, each plate and each space one 
hundred-thousandth of a centimetre thick. The whole 
work done by electric attraction in the formation of 
this pile is two centimetre-grammes. 
The whole mass of metal is eight grammes. Hence 
the amount of work is a quarter of a centimetre-gramme 
per gramme of metal. Now 4,030 centimetre-grammes 
of work, according to Joule’s dynamical equivalent of 
heat is the amount required to warm a gramme of zinc or 
copper by one degree centigrade. Hence the work done 
by the electric attraction could warm the substance by 
only zstsp Of a degree. But now let the thickness of 
each piece of metal and of each intervening space be a 
hundred-millionth of a centimetre instead of a hundred- 
thousandth. The work would be increased a millionfold 
unless a hundred-millionth of a centimetre approaches 
the smallness of a molecule. The heat equivalent would 
therefore be enough to raise the temperature of the 
material by 62°. This is barely, if at all, admissible, 
according to our present knowledge, or, rather, want of 
knowledge, regarding the heat of combination of zinc and 
copper. But suppose the metal plates and intervening 
spaces to be made yet four times thinner, that is to say, 
the thickness of each to be four-hundred-millionth of a 
centimetre. The work and its heat equivalent will be 
increased sixteen-fold. It would therefore be 990 times 
as much as that required to warm the mass by Io cent., 
which is very much more than can possibly be produced 
by zinc and copper in entering into molecular combina- 
tion. Were there in reality anything like so much heat 
