The memorable investigations of Leslie and Rumford, and the subsequent
classical reasearches of Melloni, dealt, in the main, with the
properties of radiant heat; while in my investigations, radiant heat,
instead of being regarded as an end, was employed as a means of
exploring molecular condition. On this score little could be said until
the gaseous form of matter was brought under the dominion of experiment.
This was first effected in 1859, when it was proved that gases and
vapors, notwithstanding the open door which the distances between their
molecules might be supposed to offer to the heat waves, were, in many
cases, able effectually to bar their passage. It was then proved that
while the elementary gases and their mixtures, including among the
latter the earth's atmosphere, were almost as pervious as a vacuum to
ordinary radiant heat, the compound gases were one and all absorbers,
some of them taking up with intense avidity the motion of the
ether-waves.
A single illustration will here suffice. Let a mixture of hydrogen and
nitrogen, in the proportion of three to fourteen by weight, be inclosed
in a space through which are passing the heat rays from an ordinary
stove. The gaseous mixture offers no measurable impediment to the rays
of heat. Let the hydrogen and nitrogen now unite to form the compound
ammonia. A magical change instantly occurs. The number of atoms present
remains unchanged. The transparency of the compound is quite equal to
that of the mixture prior to combination. No change is perceptible to
the eye, but the keen vision of experiment soon detects the fact that
the perfectly transparent and highly attenuated ammonia resembles pitch
or lampblack in its behavior to the rays of heat.
There is probably boldness, if not rashness, in the attempt to make
these ultra-sensible actions generally intelligible, and I may have
already transgressed the limits beyond which the writer of a familiar
article cannot profitably go. There may, however, be a remnant of
readers willing to accompany me, and for their sakes I proceed. A
hundred compounds might be named which, like the ammonia, are
transparent to light, but more or less opaque--often, indeed, intensely
opaque--to the rays of heat from obscure sources. Now the difference
between these latter rays and the light rays is purely a difference of
period of vibration. The vibrations in the case of light are more rapid,
and the ether waves which they produce are shorter, than in the case of
obscure heat. Why, then, should the ultra-red waves be intercepted by
bodies like ammonia, while the more rapidly recurrent waves of the whole
visible spectrum are allowed free transmission? The answer I hold to be
that, by the act of chemical combination, the vibrations of the
constituent atoms of the molecules are rendered so sluggish as to
synchronize with the motions of the longer waves. They resemble loaded
piano strings, or slowly descending water jets, requiring notes of low
pitch to set them in motion.
The influence of synchronism between the "radiant" and the "absorbent"
is well shown by the behavior of carbonic acid gas. To the complex
emission from our heated stove, carbonic acid would be one of the most
transparent of gases. For such waves olefiant gas, for example, would
vastly transcend it in absorbing power. But when we select a radiant
with whose waves the atoms of carbonic acid are in accord, the case is
entirely altered. Such a radiant is found in a carbonic oxide flame,
where the radiating body is really hot carbonic acid. To this special
radiation carbonic acid is the most opaque of gases.
And here we find ourselves face to face with a question of great
delicacy and importance. Both as a radiator and as an absorber, carbonic
acid is, in general, a feeble gas. It is beaten in this respect by
chloride of methyl, ethylene, ammonia, sulphurous acid, nitrous oxide,
and marsh gas. Compared with some of these gases, its behavior, in fact,
approaches that of elementary bodies. May it not help to explain their
neutrality? The doctrine is now very generally accepted that atoms of
the same kind may, like atoms of different kinds, group themselves to
molecules. Affinity exists between hydrogen and hydrogen and between
chlorine and chlorine, as well as between hydrogen and chlorine. We have
thus homogeneous molecules as well as heterogeneous molecules, and the
neutrality so strikingly exhibited by the elements may be due to a
quality of which carbonic acid furnishes a partial illustration. The
paired atoms of the elementary molecules may be so out of accord with
the periods of the ultra red waves--the vibrating periods of these atoms
may, for example, be so rapid--as to disqualify them both from emitting
those waves, and from accepting their energy. This would practically
destroy their power, both as radiators and absorbers. I have reason to
know that a distinguished authority has for some time entertained this
hypothesis.
We must, however, refresh ourselves by occasional contact with the solid
ground of experiment, and an interesting problem now lies before us
awaiting experimental solution. Suppose two hundred men to be scattered
equably throughout the length of Pall Mall. By timely swerving now and
then, a runner from St. James's Palace to the Athenaeum Club might be
able to get through such a crowd without much hinderance. But supposing
the men to close up so as to form a dense file crossing Pall Mall from
north to south; such a barrier might seriously impede, or entirely stop,
the runner. Instead of a crowd of men, let us imagine a column of
molecules under small pressure, thus resembling the sparsely distributed
crowd. Let us suppose the column to shorten, without change in the
quantity of matter, until the molecules are so squeezed together as to
resemble the closed file across Pall Mall. During these changes of
density, would the action of the molecules upon a beam of heat passing
among them at all resemble the action of the crowd upon the runner?
We must answer this question by direct experiment. To form our molecular
crowd we place, in the first instance, a gas or vapor in a tube 38
inches long, the ends of which are closed with circular windows,
air-tight, but formed of a substance which offers little or no
obstruction to the calorific waves. Calling the measured value of a heat
beam passing through this tube 100, we carefully determine the
proportionate part of this total absorbed by the molecules in the tube.
We then gather precisely the same number of molecules into a column 10.8
inches long, the one column being thus three and a half times the length
of the other. In this case also we determine the quantity of radiant
heat absorbed. By the depression of a barometric column, we can easily
and exactly measure out the proper quantities of the gaseous body. It is
obvious that one mercury inch of vapor, in the long tube, would
represent precisely the same amount of matter--or, in other words, the
same number of molecules--as 31/2 inches in the short one; while 2
inches of vapor in the long tube would be equivalent to 7 inches in the
short one.
The experiments have been made with the vapors of two very volatile
liquids, namely, sulphuric ether and hydride of amyl. The sources of
radiant heat were, in some cases, an incandescent lime cylinder, and in
others a spiral of platinum wire, heated to bright redness by an
electric current. One or two of the measurements will suffice for the
purposes of illustration. First, then, as regards the lime light; for 1
inch of pressure in the long tube, the absorption was 18.4 per cent. of
the total beam; while for 3.5 inches of pressure in the short tube, the
absorption was 18.8 per cent., or almost exactly the same as the former.
For 2 inches pressure, moreover, in the long tube, the absorption was
25.7 per cent.; while for 7 inches in the short tube it was 25.6 per
cent. of the total beam. Thus closely do the absorptions in the two
cases run together--thus emphatically do the molecules assert their
individuality. As long as their number is unaltered, their action on
radiant heat is unchanged. Passing from the lime light to the
incandescent spiral, the absorptions of the smaller equivalent
quantities, in the two tubes, were 23.5 and 23.4 per cent.; while the
absorptions of the larger equivalent quantities were 32.1 and 32.6 per
cent., respectively. This constancy of absorption, when the density of a
gas or vapor is varied, I have called "the conservation of molecular
action."
But it may be urged that the change of density, in these experiments,
has not been carried far enough to justify the enunciation of a law of
molecular physics. The condensation into less than one-third of the
space does not, it may be said, quite represent the close file of men
across Pall Mall. Let us therefore push matters to extremes, and
continue the condensation till the vapor has been squeezed into a
liquid. To the pure change of density we shall then have added the
change in the state of aggregation. The experiments here are more easily
described than executed; nevertheless, by sufficient training,
scrupulous accuracy, and minute attention to details, success may be
insured. Knowing the respective specific gravities, it is easy, by
calculation, to determine the condensation requisite to reduce a column
of vapor of definite density and length to a layer of liquid of definite
thickness. Let the vapor, for example, be that of sulphuric ether, and
let it be introduced into our 38 inch tube till a pressure of 7.2 inches
of mercury is obtained. Or let it be hydride of amyl, of the same
length, and at a pressure of 6.6 inches. Supposing the column to
shorten, the vapor would become proportionally denser, and would, in
each case, end in the production of a layer of liquid exactly one
millimeter in thickness.[1] Conversely, a layer of liquid ether or of
hydride of amyl, of this thickness, were its molecules freed from the
thrall of cohesion, would form a column of vapor 38 inches long, at a
pressure of 7.2 inches in the one case, and of 6.6 inches in the other.
In passing through the liquid layer, a beam of heat encounters the same
number of molecules as in passing through the vapor layer: and our
problem is to decide, by experiment, whether, in both cases, the
molecule is not the dominant factor, or whether its power is augmented,
diminished, or otherwise overridden by the state of aggregation.
[Footnote 1: The millimeter is 1-25th of an inch.]
Using the sources of heat before mentioned, and employing diathermanous
lenses, or silvered minors, to render the rays from those sources
parallel, the absorption of radiant heat was determined, first for the
liquid layer, and then for its equivalent vaporous layer. As before, a
representative experiment or two will suffice for illustration. When the
substance was sulphuric ether, and the source of radiant heat an
incandescent platinum spiral, the absorption by the column of vapor was
found to be 66.7 per cent. of the total beam. The absorption of the
equivalent liquid layer was next determined, and found to be 67.2 per
cent. Liquid and vapor, therefore, differed from each only 0.5 per
cent.; in other words, they were practically identical in their action.
The radiation from the lime light has a greater power of penetration
through transparent substances than that from the spiral. In the
emission from both of these sources we have a mixture of obscure and
luminous rays; but the ratio of the latter to the former, in the lime
light is greater than in the spiral; and, as the very meaning of
transparency is perviousness to the luminous rays, the emission in which
these rays are predominant must pass most freely through transparent
substances. Increased transmission implies diminished absorption; and
accordingly, the respective absorption of ether vapor and liquid ether,
when the lime light was used, instead of being 66.7 and 67.2 per cent.,
were found to be
Vapor....................33.3 per cent.
Liquid...................33.3 "
no difference whatever being observed between the two states of
aggregation. The same was found true of hydride of amyl.
This constancy and continuity of the action exerted on the waves of heat
when the state of aggregation is changed, I have called "the thermal
continuity of liquids and vapors." It is, I think, the strongest
illustration hitherto adduced of the conservation of molecular action.
Thus, by new methods of search, we reach a result which was long ago
enunciated on other grounds. Water is well known to be one of the most
opaque of liquids to the waves of obscure heat. But if the relation of
liquids to their vapors be that here shadowed forth, if in both cases
the molecule asserts itself to be the dominant factor, then the
dispersion of the water of our seas and rivers, as invisible aqueous
vapor in our atmosphere, does not annul the action of the molecules on
solar and terrestrial heat. Both are profoundly modified by this
constituent; but as aqueous vapor is transparent, which, as before
explained, means pervious to the luminous rays, and as the emission from
the sun abounds in such rays, while from the earth's emission they are
wholly absent, the vapor screen offers a far greater hinderance to the
outflow of heat from the earth toward space than to the inflow from the
sun toward the earth. The elevation of our planet's temperature is
therefore a direct consequence of the existence of aqueous vapor in our
air. Flimsy as that garment may appear, were it removed terrestrial life
would probably perish through the consequent refrigeration.
I have thus endeavored to give some account of a recent incursion into
that ultra-sensible world mentioned at the outset of this paper. Invited
by my publishers, with whom I have now worked in harmony for a period of
twenty years, to send some contribution to the first number of their new
Magazine, I could not refuse them this proof of my good will.