[Footnote 1: Lecture to the Royal Dublin Society by Dr. Oliver J.
Lodge, April 2, 1884.]
Within the last few years a singular interest has arisen in the
subject of dust, smoke, and fog, and several scientific researches
into the nature and properties of these phenomena have been recently
conducted. It so happened that at the time I received a request from
the secretary of this society to lecture here this afternoon I was in
the middle of a research connected with dust, which I had been
carrying on for some months in conjunction with Mr. J.W. Clark,
Demonstrator of Physics in University College, Liverpool, and which
had led us to some interesting results. It struck me that possibly
some sort of account of this investigation might not be unacceptable
to a learned body such as this, and accordingly I telegraphed off to
Mr. Moss the title of this afternoon's lecture. But now that the time
has come for me to approach the subject before you, I find myself
conscious of some misgivings, and the misgivings are founded upon this
ground: that the subject is not one that lends itself easily to
experimental demonstration before an audience. Many of the experiments
can only be made on a small scale, and require to be watched closely.
However, by help of diagrams and by not confining myself too closely
to our special investigation, but dealing somewhat with the wider
subject of dust in general, I may hope to render myself and my subject
intelligible if not very entertaining.
First of all, I draw no distinction between "dust" and "smoke." It
would be possible to draw such a distinction, but it would hardly be
in accordance with usage. Dust might be defined as smoke which had
settled, and the term smoke applied to solid particles still suspended
in the air. But at present the term "smoke" is applied to solid
particles produced by combustion only, and "dust" to particles owing
their floating existence to some other cause. This is evidently an
unessential distinction, and for the present I shall use either term
without distinction, meaning by dust or smoke, solid particles
floating in the air. Then "fog"; this differs from smoke only in the
fact that the particles are liquid instead of solid. And the three
terms dust, smoke, and fog, come to much the same thing, only that the
latter term is applied when the suspended particles are liquid. I do
not think, however, that we usually apply the term "fog" when the
liquid particles are pure water; we call it then mostly either mist or
cloud. The name "fog," at any rate in towns, carries with it the idea
of a hideous, greasy compound, consisting of smoke and mist and
sulphur and filth, as unlike the mists on a Highland mountain as a
country meadow is unlike a city slum. Nevertheless, the finest cloud
or mist that ever existed consists simply of little globules of water
suspended in air, and thus for our present purpose differs in no
important respect from fog, dust, and smoke. A cloud or mist is, in
fact, fine water-dust. Rain is coarse water-dust formed by the
aggregation of smaller globules, and varying in fineness from the
Scotch mist to the tropical deluge. It has often been asked how it is
that clouds and mists are able to float about when water is so much
heavier (800 times heavier) than air. The answer to this is easy. It
depends on the resistance or viscosity of fluids, and on the smallness
of the particles concerned. Bodies falling far through fluids acquire
a "terminal velocity," at which they are in stable equilibrium--their
weight being exactly equal to the resistance--and this terminal
velocity is greater for large particles than for small; consequently
we have all sorts of rain velocity, depending on the size of the
drops; and large particles of dust settle more quickly than small.
Cloud-spherules are falling therefore, but falling very slowly.
To recognize the presence of dust in air there are two principal
tests; the first is, the obvious one of looking at it with plenty of
light, the way one is accustomed to look for anything else; the other
is a method of Mr. John Aitken's, viz., to observe the condensation of
water vapor.
Take these in order. When a sunbeam enters a darkened room through a
chink, it is commonly said to be rendered visible by the motes or dust
particles dancing in it; but of course really it is not the motes
which make the sunbeam visible, but the sunbeam the motes. A dust
particle is illuminated like any other solid screen, and is able to
send a sufficient fraction of light to our eyes to render itself
visible. If there are no such particles in the beam--nothing but
clear, invisible air--then of course nothing is seen, and the beam
plunges on its way quite invisible to us unless we place our eyes in
its course. In other words, to be visible, light must enter the eye.
(A concentrated beam was passed through an empty tube, and then
ordinary air let in.)
The other test, that of Mr. Aitken, depends on the condensation of
steam. When a jet of steam finds itself in dusty air, it condenses
around each dust particle as a nucleus, and forms the white visible
cloud popularly called steam. In the absence of nuclei Mr. Aitken has
shown that the steam cannot condense until it is highly
supersaturated, and that when it does it condenses straight into
rain--that is, into large drops which fall. The condensation of steam
is a more delicate test for dust than is a beam of light. A curious
illustration of the action of nuclei in condensing moisture has just
occurred to me, in the experiment--well known to children--of writing
on a reasonably clean window-pane with, say, a blunt wooden point, and
then breathing on the glass; the condensation of the breath renders
the writing legible. No doubt the nuclei are partially wiped away by
the writing, and the moisture will condense into larger drops with
less light-scattering power along the written lines than over the
general surface of the pane where the nuclei are plentiful, and the
drops therefore numerous and minute. Mr. Aitken points out that if the
air were ever quite dustless, vapor could not condense, but the air
would gradually get into a horribly supersaturated condition, soaking
all our walls and clothes, dripping from every leaf, and penetrating
everywhere, instead of falling in an honest shower, against which
umbrellas and slate roofs are some protection. But let us understand
what sort of dust it is which is necessary for this condensing
process. It is not the dust and smoke of towns, it is not the dust of
a country road; all such particles as these are gross and large
compared with those which are able to act as condensers of moisture.
The fine dust of Mr. Aitken exists everywhere, even in the upper
regions of the atmosphere; many of its particles are of
ultra-microscopic fineness, one of them must exist in every raindrop,
nay, even in every spherule of a mist or cloud, but it is only
occasionally that one can find them with the microscope. It is to such
particles as these that we owe the blue of the sky, and yet they are
sufficiently gross and tangible to be capable of being filtered out of
the air by a packed mass of cotton-wool. Such dust as this, then, we
need never be afraid of being without. Without it there could be no
rain, and existence would be insupportable, perhaps impossible; but it
is not manufactured in towns; the sea makes it; trees and wind make
it; but the kind of dust made in towns rises only a few hundred yards
or so into the atmosphere, floating as a canopy or pall over those
unfortunate regions, and sinks and settles most of it as soon as the
air is quiet, but scarcely any of it ever rises into the upper regions
of the atmosphere at all.
Dust, then, being so universally prevalent, what do I mean by
dust-free spaces? How are such things possible? And where are they to
be found? In 1870 Dr. Tyndall was examining dusty air by means of a
beam of light in which a spirit-lamp happened to be burning, when he
noticed that from the flame there poured up torrents of apparently
thick black smoke. He could not think the flame was really smoky, but
to make sure he tried, first a Bunsen gas flame and then a hydrogen
flame. They all showed the same effect, and smoke was out of the
question. He then used a red-hot poker, a platinum wire ignited by an
electric current, and ultimately a flask of hot water, and he found
that from all warm bodies examined in dusty air by a beam of light the
upstreaming convection currents were dark. Now, of course smoke would
behave very differently. Dusty air itself is only a kind of smoke, and
it looks bright, and the thicker the smoke the brighter it looks; the
blackness is simply the utter absence of smoke; there is nothing at
all for the light to illuminate, accordingly we have the blankness of
sheer invisibility. Here is a flame burning under the beam, and, to
show what real smoke looks like, I will burn also this spirit lamp
filled with turpentine instead of alcohol. _Why_ the convention
currents were free from dust was unknown; Tyndall thought the dust was
burnt and consumed; Dr. Frankland thought it was simply evaporated.
In 1881 Lord Rayleigh took the matter up, not feeling satisfied with
these explanations, and repeated the experiment very carefully. He
noted several new points, and hit on the capital idea of seeing what a
cold body did. From the cold body the descending current was just as
dark and dust-free as from a warm body. Combustion and evaporation
explanations suffered their death-blow. But he was unable to suggest
any other explanation in their room, and so the phenomenon remained
curious and unexplained.
In this state Mr. Clark and I took the matter up last summer, and
critically examined all sorts of hypotheses that suggested themselves,
Mr. Clark following up the phenomena experimentally with great
ingenuity and perseverance. One hypothesis after another suggested
itself, seemed hopeful for a time, but ultimately had to be discarded.
Some died quickly, others lingered long. In the examination of one
electrical hypothesis which suggested itself we came across various
curious phenomena which we hope still to follow up.[2] It was some
months before what we now believe to be the true explanation began to
dawn upon us. Meanwhile we had acquired various new facts, and first
and foremost we found that the dark plane rising from a warm body was
only the upstreaming portion of a dust-free _coat_ perpetually being
renewed on the surface of the body. Let me describe the appearance and
mode of seeing it by help of a diagram. (For full description see
_Philosophical Magazine_ for March, 1884.)
[Footnote 2: For instance, the electric properties of crystals
can be readily examined in illuminated dusty air; the dust grows
on them in little bushes and marks out their poles and neutral
regions, without any need for an electrometer. Magnesia smoke
answers capitally.]
Surrounding all bodies warmer than the air is a thin region free from
dust, which shows itself as a dark space when examined by looking
along a cylinder illuminated transversely, and with a dark background.
At high temperatures the coat is thick; at very low temperatures it is
absent, and dust then rapidly collects on the rod. On a warm surface
only the heavy particles are able to settle--there is evidently some
action tending to drive small bodies away. An excess of temperature of
a degree or two is sufficient to establish this dust-free coat, and it
is easy to see the dust-free plane rising from it. The appearances may
also be examined by looking along a cylinder _toward_ the source of
light, when the dust-free spaces will appear brighter than the rest. A
rod of electric light carbon warmed and fixed horizontally across a
bell-jar full of dense smoke is very suitable for this experiment, and
by means of a lens the dust-free regions may be thus projected on to a
screen. Diminished pressure makes the coat thicker. Increased pressure
makes it thinner. In hydrogen it is thicker, and in carbonic acid
thinner, than in air. We have also succeeded in observing it in
liquids--for instance, in water holding fine rouge in suspension, the
solid body being a metal steam tube. Quantitative determinations are
now in progress.
[Illustration: Fig. 1 and Fig. 2]
Fig. 1 shows the appearance when looking along a copper or carbon rod
laterally illuminated; the paths of the dust particles are roughly
indicated. Fig. 2 shows the coat on a semi-cylinder of sheet copper
with the concave side turned toward the light.
It is difficult to give the full explanation of the dust free spaces
in a few words, but we may say roughly that there is a molecular
bombardment from all warm surfaces by means of which small suspended
bodies get driven outward and kept away from the surface. It is a sort
of differential bombardment of the gas molecules on the two faces of a
dust particle somewhat analogous to the action on Mr. Crookes'
radiometer vanes. Near cold surfaces the bombardment is very feeble,
and if they are cold enough it appears to act toward the body, driving
the dust inward--at any rate, there is no outward bombardment
sufficient to keep the dust away, and bodies colder than the
atmosphere surrounding them soon get dusty. Thus if I hold this piece
of glass in a magnesium flame, or in a turpentine or camphor flame, it
quickly gets covered with smoke--white in the one case, black in the
other. I take two conical flasks with their surfaces blackened with
camphor black, and filling one with ice, the other with boiling water,
I cork them and put a bell jar over them, under which I burn some
magnesium wire; in a quarter of an hour or so we find that the cold
one is white and hoary, the hot one has only a few larger specks of
dust on it, these being of such size that the bombardment was unable
to sustain their weight, and they have settled by gravitation. We thus
see that when the air in a room is warmer than the solids in it--as
will be the case when stoves, gas-burners, etc., are used--things will
get very dusty; whereas when walls and objects are warmer than the
air--as will be the case in sunshine, or when open fireplaces are
used, things will tend to keep themselves more free from dust. Mr.
Aitken points out that soot in a chimney is an illustration of this
kind of deposition of dust; and as another illustration it strikes me
as just possible that the dirtiness of snow during a thaw may be
partly due to the bombardment on to the cold surface of dust out of
the warmer air above. Mr. Aitken has indeed suggested a sort of
practical dust or smoke filter on this principle, passing air between
two surfaces--one hot and one cold--so as to vigorously bombard the
particles on to the cold surface and leave the air free.
But we have found another and apparently much more effectual mode of
clearing air than this. We do it by discharging electricity into it.
It is easily possible to electrify air by means of a point or flame,
and an electrified body has this curious property, that the dust near
it at once aggregates together into larger particles. It is not
difficult to understand why this happens; each of the particles
becomes polarized by induction, and they then cling together end to
end, just like iron filings near a magnet. A feeble charge is often
sufficient to start this coagulating action. And when the particles
have grown into big ones, they easily and quickly fall. A stronger
charge forcibly drives them on to all electrified surfaces, where they
cling. A fine water fog in a bell jar, electrified, turns first into a
coarse fog or Scotch mist, and then into rain. Smoke also has its
particles coagulated, and a space can thus be cleared of it. I will
illustrate this action by making some artificial fogs in a bell-jar
furnished with a metal point. First burn some magnesium wire,
electrify it by a few turns of this small Voss machine, and the smoke
has become snow; the particles are elongated, and by pointing to the
charged rod indicate the lines of electrostatic force very
beautifully; electrify further, and the air is perfectly clear. Next
burn turpentine, and electrify gently; the dense black smoke
coagulates into black masses over an inch long; electrify further, and
the glass is covered with soot, but the air is clear. Turpentine smoke
acts very well, and can be tried on a larger scale; a room filled with
turpentine smoke, so dense that a gas-light is invisible inside it,
begins to clear in a minute or two after the machine begins to turn,
and in a quarter of an hour one can go in and find the walls thickly
covered with stringy blacks, notably on the gas-pipes and everything
most easily charged by induction. Next fill a bell-jar full of steam,
and electrify, paying attention to insulation of the supply point in
this case. In a few seconds the air looks clear, and turning on a beam
of light we see the globules of water dancing about, no longer fine
and impalpable, but separately visible and rapidly falling. Finally,
make a London fog by burning turpentine and sulphur, adding a little
sulphuric acid, either directly as vapor or indirectly by a trace of
nitric oxide, and then blowing in steam. Electrify, and it soon
becomes clear, although it lakes a little longer than before; and on
removing the bell-jar we find that even the smell of SO2 has
disappeared, and only a little vapor of turpentine remains. Similarly
we can make a Widnes fog by sulphureted hydrogen, chlorine, sulphuric
acid, and a little steam. Probably the steam assists the clearing when
gases have to be dealt with. It may be possible to clear the air of
tunnels by simply discharging electricity into the air--the
electricity being supplied by Holtz machines, driven say by small
turbines--a very handy form of power, difficult to get out of order.
Or possibly some hydro-electric arrangement might be devised for the
locomotive steam to do the work. I even hope to make some impression
on a London fog, discharging from lightning conductors or captive
balloons carrying flames, but it is premature to say anything about
this matter yet. I have, however, cleared a room of smoke very quickly
with a small hand machine.
It will naturally strike you how closely allied these phenomena must
be to the fact of popular science that "thunder clears the air." Ozone
is undoubtedly generated by the flashes, and may have a beneficial
effect, but the dust-coagulating and dust-expelling power of the
electricity has a much more rapid effect, though it may not act till
the cloud is discharged. Consider a cloud electrified slightly; the
mists and clouds in its vicinity begin to coagulate, and go on till
large drops are formed, which may be held up by electrical action, the
drops dancing from one cloud to another and thus forming the very
dense thunder cloud. The coagulation of charged drops increases the
potential, as Prof. Tait points out, until at length--flash--the cloud
is discharged, and the large drops fall in a violent shower. Moreover,
the rapid excursion to and fro of the drops may easily have caused
them to evaporate so fast as to freeze, and hence we may get hail.
While the cloud was electrified, it acted inductively on the earth
underneath, drawing up an opposite charge from all points, and thus
electrifying the atmosphere. When the discharge occurs this
atmospheric electrification engages with the earth, clearing the air
between, and driving the dust and germs on to all exposed surfaces. In
some such way also it may be that "thunder turns milk sour," and
exerts other putrefactive influences on the bodies which receive the
germs and dust from the air.
But we are now no longer on safe and thoroughly explored territory. I
have allowed myself to found upon a basis of experimental fact, a
superstructure of practical application to the explanation of the
phenomena of nature and to the uses of man. The basis seems to me
strong enough to bear most of the superstructure, but before being
sure it will be necessary actually to put the methods into operation
and to experiment on a very large scale. I hope to do this when I can
get to a suitable place of operation. Liverpool fogs are poor affairs,
and not worth clearing off. Manchester fogs are much better and more
frequent, but there is nothing to beat the real article as found in
London, and in London if possible I intend to rig up some large
machines and to see what happens. The underground railway also offers
its suffocating murkiness as a most tempting field for experiment, and
I wish I were able already to tell you the actual result instead of
being only in a position to indicate possibilities. Whether anything
comes of it practically or not, it is an instructive example of how
the smallest and most unpromising beginnings may, if only followed up
long enough, lead to suggestions for large practical application. When
we began the investigation into the dust-free spaces found above warm
bodies, we were not only without expectation, but without hope or idea
of any sort, that anything was likely to come of it; the phenomenon
itself possessed its own interest and charm.
And so it must ever be. The devotee of pure science never has
practical developments as his primary aim; often he not only does not
know, but does not in the least care whether his researches will ever
lead to any beneficial result. In some minds this passive ignoring of
the practical goes so far as to become active repulsion; so that some
singularly biased minds will not engage in anything which seems likely
to lead to practical use. I regard this as an error, and as the sign
of a warped judgment, for after all man is to us the most important
part of nature; but the system works well nevertheless, and the
division of labor accomplishes its object. One man investigates nature
impelled simply by his own genius, and because he feels he cannot help
it; it never occurs to him to give a reason for or to justify his
pursuits. Another subsequently utilizes his results, and applies them
to the benefit of the race. Meanwhile, however, it may happen that the
yet unapplied and unfruitful results evoke a sneer, and the question:
"Cui bono?" the only answer to which question seems to be: "No one is
wise enough to tell beforehand what gigantic developments may not
spring from the most insignificant fact."
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