340 
NATURE 
[ Fan. 27, 1870 
once revealed by a powerfully condensed beam. Two tubes were 
placed in succession in the path of the dust : the one containing 
fragments of glass wetted with concentrated sulphuric acid ; the 
other, fragments of marble wetted with a strong solution of 
caustic potash. To my astonishment it passed through both. 
The air of the Royal Institution, sent through these tubes at a 
rate sufficiently slow to dry it and to remove its carbonic acid, 
carried into the experimental tube a considerable amount of 
mechanically-suspended matter, which was illuminated when the 
beam passed through the tube. The effect was substantially the 
same when the air was permitted to bubble through the liquid 
acid and through the solution of potash. 
Thus, on the 5th of October, 1868, successive charges of air 
were admitted through the potash and sulphuric acid into tHe 
exhausted experimental tube. Prior to the admission of the air 
the tube was oftically empty; it contained nothing competent to 
scatter the light. After the air had entered the tube, the conical 
track of the electric beam was in all cases clearly revealed. This 
indeed was a daily observation at the time to which I now refer. 
I tried to intercept this floating matter in various ways; and 
on the day just mentioned, prior to sending the air through the 
drying apparatus, I carefully permitted it to pass over the tip of 
a spirit-lamp flame. The floating matter no longer appeared, 
having been burnt up by the flame. It was therefore organic 
matter, \Nhen the air was sent too rapidly through the flame, a 
fine blue cloud was found in the experimental tube. This was 
the smoke of the organic particles. I was by no means prepared 
for this result ; for I had thought, with the rest of the world, 
that the dust of our air was, in great part, inorganic and non- 
combustible. 
Mr. Valentin had the kindness to procure for me a small gas- 
furnace, containing a platinum tube, which could be heated to 
vivid redness. The tube also contained a roll of platinum gauze, 
which, while it permitted the air to pass through it, ensured the 
practical contact of the dust with the incandescent metal. The 
air of the laboratory was permitted to enter the experimental 
tube, sometimes through the cold, and sometimes through the 
heated tube of platinum. The rapidity of admission was also 
varied. In the first column of the following table the quantity 
of air operated on is expressed by the number of inches which 
the mercury gauge of the air-pump sank when the air entered. 
In the second column the condition of the platinum tube is 
mentioned, and in the third the state of the air which entered 
the experimental tube. 
Quantity of Air. State of Platinum Tube. State of Experimental Tube. 
15 inches Cold Full of particles. 
39 a 10 Red-hot . . Optically empty. 
iS) ss a9 Cold. = . Pullof particles. 
TS iss 0 Red-hot Optically empty. 
S45 7) ee (Cold) =) Mulliof particles: 
ug, fp . . . Red-hot . . Optically empty. 
The phrase ‘‘ optically empty”? shows that when the condi- 
tions of perfect combustion were present, the floating matter 
totally disappeared. It was wholly burnt up, leaving not a trace 
of residue. From spectrum analysis, however, we know that 
soda floats in the air; these organic dust particles are, I believe, 
the va/¢s that support it, and when they are removed it sinks and 
vanishes. 
When the passage of the air was so rapid as to render imper- 
fect the combustion of the floating matter, instead of optical 
emptiness a fine blue cloud made its appearance in the experi- 
mental tube. The following series of results illustrate this 
point t= 
Quantity Platinum Tube. Experimental Tube. 
15 inches, slow Cold . . . Fullof particles. 
T5455 20 Red-hot Optically empty. 
TS ase quick om . . A blue cloud. 
Se », + Intenselyhot . A fine blue cloud. 
The optical character of these clouds was totally different from 
that of the dust which produced them. At right angles to the 
illuminating beam they discharged perfectly polarised light. The 
cloud could be utterly quenched by a transparent Nicol’s prism, 
and the tube containing it reduced to optical emptiness. 
The particles floating in the air of London being thus proved 
to be organic, I sought to burn them up at the focus of a concave 
reflector. One of the powerfully convergent mirrors employed 
in my experiments on combustion by dark rays was here made 
use of, but I failed in the attempt. Doubtless the floating par- 
ticles are in part transparent to radiant heat, and are so far incom- 
bustible by such heat. Their rapid motion through the focus 
also aids their escape. They do not linger there sufficiently long 
to be consumed. A flame it was evident would burn them up, 
but I thought the presence of the flame would mask its own 
action among the particles. : 
Ina cylindrical beam, which powerfully illuminated the dust 
of the laboratory, was placed an ignited spirit-lamp. Mingling 
with the flame, and round its rim, were seen wreaths of darkness 
resembling an intensely black smoke, On lowering the flame 
below the beam the same dark masses stormed upwards. They 
were at times blacker than the blackest smoke that 1 have ever 
seen issuing from the funnel of a steamer, and their resemblance 
to smoke was so perfect as to lead the most practised observer to 
conclude that the apparently pure flame of the alcohol lamp 
required but a beam of sufficient intensity to reveal its clouds of 
liberated carbon. 
But is the blackness smoke? The question presented itself in 
a moment. A red-hot poker was placed underneath the beam, 
and from it the black wreaths also ascended. A large hydro- 
gen flame was next employed, and it produced those whirling 
masses of darkness far more copiously than either the spirit- 
flame or poker. Smoke was therefore out of the question. 
What, then, was the blackness? It was simply that of stellar 
space ; that is to say, blackness resulting from the absence from 
the track of the beam of all matter competent to scatter its 
light. When the flame was placed below the beam the 
floating matter was destroyed 7 situ; and the air, freed from 
this matter, rose into the beam, jostled aside the illuminated 
particles, and substituted for their light the darkness due to its 
own perfect transparency. Nothing could more forcibly illus- 
trate the invisibility of the agent which renders all things visible. 
The beam crossed, unseen, the black chasm formed by the 
transparent air, while at both sides of the gap the thick- 
strewn particles shone out like a luminous solid under the 
powerful illumination. 
3ut here a difficulty meets us. It is not necessary to burn 
the particles to produce a stream of darkness. Without actual 
combustion, currents may be generated which shall exclude the 
floating matter, and therefore appear dark amid the surrounding 
brightness. I noticed this effect first on placing a red-hot copper 
ball below the beam, and permitting it to remain there until its 
temperature had fallen below that of boiling water. The dark 
currents, though much enfeebled, were still produced. They 
may also be produced by a flask filled with hot water. 
To study this effect a platinum wire was stretched across 
the beam, the two ends of the wire being connected with 
the two poles of a voltaic battery. To regulate the strength 
of the current a rheostat was placed in the circuit. 
Beginning with a feeble current the temperature of the 
wire was gradually augmented, but before it reached the 
heat of ignition, a flat stream of air rose from it, which 
when looked at edgeways appeared darker and sharper than one 
of the blackest lines of Fraunhofer in the solar spectrum. Right 
and left of this dark vertical band the floating matter rose up- 
wards, bounding definitely the non-luminous stream of air. What 
is the explanation? Simply this. The hot wire rarefied the air 
in contact with it, but it did not equally lighten the floating 
matter. The convection current of pure air therefore passed 
upwards among the particles, dragging them after it right and 
left, but forming between them an impassable black partition. 
In this way we render an account of the dark currents produced 
by bodies at a temperature below that of combustion. 
Oxygen, hydrogen, nitrogen, carbonic acid, so prepared as to 
exclude all floating particles, produce the darkness when poured 
or blown into the beam. Coal-gas does the same. An ordinary 
glass shade placed in the air with its mouth downwards permits 
the track of the beam to be seen crossing it. Let coal-gas or 
hydrogen enter the shade by a tube reaching to its top, the gas 
gradually fills the shade from the top downwards. As soon as it 
occupies the space crossed by the beam, the luminous track is 
instantly abolished. Lifting the shade so as to bring the com- 
mon boundary of gas and air above the beam, the track flashes 
forth. After the shade is full, if it be inverted, the gas passes 
upwards like a black smoke among the illuminated particles. 
The air of our London rooms is loaded with this organic dust, 
nor is the country air free from its pollution. However ordinary 
daylight may permit it to disguise itself, a sufficiently powerful 
beam causes the air in which the dust is suspended to appear as 
a semi-solid rather than as a gas. Nobody could, in the first 
instance, without repugnance place the mouth at the illuminated 
