552 
NATURE 
[| March 31, 1870 
of combination as this, a mixture of zinc and copper 
powders would, if melted in any one spot, run together, 
generating more than heat enough to melt each through- 
out ; just as a large quantity of gunpowder if ignited in 
any one spot burns throughout without fresh application 
of heat. Hence plates of zinc and copper of a three- 
hundred-millionth of a centimetre thick, placed close 
together alternately, form a near approximation to a 
chemical combination, if indeed such thin plates could be 
made without splitting atoms. 
The theory of capillary attraction shows that when a 
bubble—a soap-bubble for instance—is blown larger and 
larger, work is done by the stretching of a film which 
resists extension as if it were an elastic membrane with a 
constant contractile force. This contractile force is to be 
reckoned as a certain number of units of force per unit 
of breadth. Observation of the ascent of water in capil- 
lary tubes shows that the contractile force of a thin film of 
water is about sixteen milligrammes weight per millimetre 
of breadth. Hence the work done in stretching a water 
film to any degree of thinness, reckoned in millimetre- 
milligrammes, is equal to sixteen times the number of 
square millimetres by which the area is augmented, pro- 
vided the film is not made so thin that there is any sen- 
sible diminution of its contractile force. In an article 
“On the Thermal effect of drawing out a Film of 
Liquid,” published in the Proceedings of the Royal 
Society for April 1858, I have proved from the second 
law of thermodynamics that about half as much more 
energy, in the shape of heat, must be given to the film 
to prevent it from sinking in temperature while it is 
being drawn out. Hence the intrinsic energy of a mass 
of water in the shape of a film kept at constant tempera- 
ture increases by twenty-four milligramme-millimetres for 
every square millimetre added to its area. 
Suppose then a film to be given witha thickness of a milli- 
metre, and suppose its area to be augmented ten thousand 
and one fold: the work done per square millimetre of the 
original film, that is to say per milligramme of the mass, 
would be 240,000 millimetre-milligrammes. The heat 
equivalent of this is more than half a degree centigrade of 
elevation of temperature of the substance. The thickness 
to which the film is reduced on this supposition is very 
approximately a ten-thousandth of a millimetre. The 
commonest observation on the soap-bubble (which in 
contractile force differs no doubt very little from pure 
water) shows that there is no sensible diminution of con- 
tractile force by reduction of the thickness to the ten- 
thousandth of a millimetre ; inasmuch as the thickness 
which gives the first maximum brightness round the 
black spot seen where the bubble is thinnest, is only about 
an eight-thousandth of a millimetre. 
The very moderate amount of work shown in the pre- 
ceding estimates is quite consistent with this deduction. 
But suppose now the film to be further stretched, until its 
thickness is reduced to a twenty-millionth of a millimetre. 
The work spent in doing this is two thousand times more 
than that which we have just calculated. The heat 
equivalent is 1,130 times the quantity required to raise 
the temperature of the liquid by one degree centigrade. 
This is far more than we can admit as a possible amount 
of work done in the extension of a liquid film, A smaller 
amount of work spent on the liquid would convert it into 
vapour at ordinary atmospheric pressure. The conclusion 
is unavoidable, that a water-film falls off greatly in its 
contractile force before it is reduced to a thickness of 
a twenty-millionth of a millimetre. It is scarcely pos- 
sible, upon any conceivable molecular theory, that there 
can be any considerable falling off in the contractile force 
as long as there are several molecules in the thickness. 
It is therefore probable that there are not several mole- — 
cules in a thickness of a twenty-millionth of a millimetre 
of water. 
The kinetic theory of gases suggested a hundred 
years ago by Daniel Bernouilli has, during the last 
quarter of a century, been worked out by Herapath, 
Joule, Clausius, and Maxwell, to so great perfection that — 
we now find in it satisfactory explanations of all non-_ 
chemical properties of gases. However difficult it may 
be even to imagine what kind of thing the molecule is, — 
we may regard it as an established truth of science that a~ 
gas consists of moving molecules disturbed from recti- 
lineal paths and constant velocities by collisions or mutual 
influences, so rare that the mean length of proximately 
rectilineal portions of the path of each molecule is many 
times greater than the average distance from the centre 
of each molecule to the centre of the molecule nearest it at 
any time. If, for a moment, we suppose the molecules to be 
hard elastic globes all of one size, influencing one another 
only through actual contact, we have for each molecule 
simply a zigzag path composed of rectilineal portions, with 
abrupt changes of direction. On this supposition Clausius 
proves, by a simple application of the calculus of proba- 
bilities, that the average length of the free path of a 
particle from collision to collision bears to the diamieter 
of each globe, the ratio of the whole space in which the 
globes move, to eight times the sum of the volumes of the - 
globes. It follows that the number of the globes in unit 
volume is equal to the square of this ratio divided by 
the volume of a sphere whose radius is equal to that 
average length of free path. But we cannot believe 
that the individual molecules of gases in general, or 
even of any one gas, are hard elastic globes. Any two 
of the moving particles or molecules must act upon 
one another somehow, so that when they pass very 
near one another they shall produce considerable de- 
flexion of the path and change in the velocity of each. 
This mutual action (called force) is different at dif 
ferent distances, and must vary, according to varia- 
tions of the distance, so as to fulfil some definite 
law. If the particles were hard elastic globes acting 
upon one another only by contact, the law of force 
would be—zero force when the distance from centre to 
centre exceeds the sum of the radii, and infinite repul- 
sion for any distance less than the sum of the radii. 
This hypothesis, with its “hard and fast” demarcation 
between no force and infinite force, seems to require 
mitigation. Without entering on the theory of vortex — 
atoms at present, I may at least say that soft elastic 
solids, not necessarily globular, are more promising than 
infinitely hard elastic globes. And, happily, we are not 
left merely to our fancy as to what we are to accept 
for probable in respect to the law of force. If the par- 
ticles were hard elastic globes, the average time from 
