OF HIGH REFRANGIBILITY UPON GASEOUS MATTER. 
(339 
Here the character of the floating matter is very clearly shown. The non-combus- 
tible or inorganic part of it was too minute to be revealed by the concentrated beam. 
The tube after its combustion appeared “ optically empty.” 
The “ blue cloud ” just referred to was due to the too rapid passage of the air through 
the intensely heated tube, which prevented the perfect combustion of the floating matter. 
It was to all intents and purposes the smoke of the particles*. The subject is further 
illustrated by the following series of experiments : — 
Quantity of air. 
State of platinum tube. 
State of experimental tube. 
15 inches. 
Cold. 
Full of particles. 
15 
5 5 
Dull red. 
Optically empty. 
15 
55 
Intensely heated. 
Optically empty. 
30 
55 
Intensely heated. 
Optically empty. 
15 
55 
(admitted quickly). 
55 
A perfectly polarized blue cloud. 
15 
55 
(quickly). 
Barely visible redness. 
Particles. 
15 
55 
(quickly). 
Intensely heated. 
Blue cloud. 
15 
55 
(slower). 
55 
A very fine blue cloud. 
15 
55 
(very slow). 
55 
Optically empty. 
15 
55 
Cold. 
Full of particles. 
15 
55 
(quickly). 
Bed-hot. 
Blue cloud. 
The polarization of light by such clouds as the blue ones here mentioned will receive 
due attention subsequently. 
A remarkably fine experiment may be thus made : — Placing a spirit-lamp underneath 
the cylindrical beam of the electric lamp as it marks its track upon the illuminated 
dust of the atmosphere, torrents of what would be infallibly mistaken for black smoke 
rise from the flame into the beam. A Bunsen’s flame produces the same effect. But 
the action of a red-hot poker placed underneath the beam is precisely similar; the 
action of a hydrogen flame, moreover, where smoke is out of the question, is not to be 
distinguished from that of the spirit-lamp flame. The apparent smoke rises even 
when the flame or the poker is placed at a good distance below the beam. The 
action is really due to the destruction of the floating matter by contact with the 
heated body. It sends upwards streams of air from which every thing competent to 
scatter the light has been removed. This air, in passing through the beam, jostles aside 
the illuminated particles, the space it occupies being Hack in contrast with the adjacent 
luminosity. The experiment is capable of various instructive modifications, and may 
of course be executed with sunlight. 
It is needless to dwell upon the possible influence of the floating organic matter of the 
air upon health. Its quantity, when illuminated by a powerful and strongly concentrated 
beam, appears sometimes to be enormous. One recoils from the idea of placing the mouth 
at the intensely illuminated focus and inhaling the swimming dirt revealed there. Nor 
* la subsequent experiments I found that this “ smoke ” arose in great part from the action of the heated 
air upon the india-rubber joint which connected the platinum tube with the experimental tube. 
