NAGE URE 
499 
THURSDAY, MARCH 17, 1870 
ON FLOATING MATTER AND BEAMS OF 
LIGHT 
EAMS of light may be employed to reveal the 
existence of floating matter in the air; or the 
floating matter may be employed to reveal the track of 
the beams. 
When the beam is intense it becomes an extremely 
powerful searcher and revealer of the state of the air. 
Thus examined, the air of a room which in diffuse day- 
light appears absolutely pure is seen to be loaded with 
suspended matter. Many of the fine clouds developed 
in my experiments on the action of light upon vapours 
disappear utterly in diffuse daylight ; while when the 
room is darkened and the light of an intense beam con- 
fined to the clouds themselves, they appear highly 
luminous. The eye is the real re-agent here. Rendered 
sensitive by darkness, and receiving light from the floating 
matter alone, the amount of light competent to produce a 
sensible effect is incalculably small. The power of the 
light to make an impression is moreover increased by the 
extension given to the body which emits it. 
The mobility of these actinic clouds is in some cases 
quite extraordinary. The differences of temperature 
introduced by the act of decomposition often cause the 
clouds to assume forms of astonishing complexity and 
beauty. The clouds which thus shape themselves by 
internal action are also exceedingly sensitive to external 
action. Supposing a thin actinic cloud to fill the experi- 
mental tube, the whole of it being flooded with the light 
of a beam passing longitudinally through the tube, an 
instant’s contact of the tip of a spirit lamp flame with the 
under surface of the tube causes the cloud to break 
upwards in a violent current, and to whirl itself into 
the most beautiful vortices right and left of the vertical 
line. The rapidity with which the heat passes through 
the thick glass and sets the cloud in motion is sur- 
prising. The warmth of the finger suffices to produce 
an effect feebler than, but substantially the same, as that 
produced by the spirit lamp flame. 
In fact, the floating matter of the air, properly illumi- 
nated, might be converted into a thermoscope of sur- 
passing delicacy. A little brown paper smoke was diffused 
in an ordinary glass shade; the track of the beam 
through it was much whiter than through the air. Hence 
the invasion of the smokeless air could be instantly seen 
by the darkness it produced. On introducing the hand 
at the open base of the shade, a violent uprush of air 
immediately occurred. The smoke was violently whirled 
about, and the course of the whirlwinds distinctly marked 
by the relative action on the light of the smoky and the 
unsmoky air. I was not prepared to see so small a 
difference of temperature produce so large and prompt 
an effect. 
Nor is it necessary to introduce dark extraneous air 
to render the currents through nebulous matter visible. 
The current produced in the actinic cloud by the spirit 
flame forms a dark vertical septum between the two 
adjacent parts of the cloud. When the current curls to 
form a cyclone, the septum curls also, producing dark 
spirals through the illuminated nebule. 
The late Principal Forbes often referred to the floating 
scum on water slowly flowing through a channel, the 
lateral parts of which are retarded by friction. Such 
scum, or froth, arranges itself in distinct striz, separated 
from each other by comparatively free intervals. It is 
practically impossible to establish differential motions, 
either in solids or liquids, without producing some effect 
of this kind. Fibrous iron shows it ; while in the atmo- 
sphere differential currents produce cirrus clouds. I have 
often watched the way in which the suspended matter of 
the turbid Arve at Chamouni traced itself through the 
water. Notwithstanding the tossing endured from the 
source of the Arveyron downwards, the mixture of mud 
and liquid was by no means perfect. In fact, every new 
obstacle which introduced differential motion introduced 
also the strize, and destroyed all uniformity of mixture. 
Five or six weeks ago I had a square chamber con- 
structed, the upper half of which is glazed, its floor 
consisting of transverse rails, over which is placed a thick 
mat of cotton wool. The chamber has a brass chimney, 
in which a rose burner can be lighted. An upward current 
is thus established in the chimney, the air below entering 
through the cotton-wool to supply the place of that 
discharged by the flame. When the chamber is filled 
with the ordinary laboratory air, a beam sent through it 
tracks its course on the floating matter. When the fame 
is ignited, the air enters through the cotton-wool ; but the 
consequence is not a uniform enfeeblement of the light of 
the beam. Perfectly dark strize pass through the luminous 
track, and they sometimes bend and whirl so as to form 
gracefully curved streams of darkness. Even air urged 
from the nozzle of a bellows through the luminous track, 
shows a tendency to form those striz, though it, like the 
water of the Arve at Chamouni, is filled with the same 
floating matter as that of the air through which it is 
urged. 
On a recent occasion the following effects were 
described, and an attempt was then made to explain 
them :— 
In a 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 I 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? This 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 
hydrogen 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 ques- 
tion. 
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 éz sifu ; 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 illustrate the invisi- 
bility 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, 
