338 
PROFESSOR TYNDALL ON THE ACTION OF RAYS 
chanically into the experimental tube. Precautions were therefore taken to prevent any 
such transfer. They produced little or no mitigation. I did not imagine at the time 
that the dust of the external air could find such free passage through the caustic potash 
and the sulphuric-acid tubes. But the motes really came from without. They also passed 
with freedom through a variety of ethers and alcohols placed in the flask F. In fact, it 
requires long-continued action on the part of an acid first to wet the motes and afterwards 
to destroy them. By carefully passing the air through the flame of a spirit-lamp or 
through a platinum tube heated to bright redness, the floating matter was sensibly 
destroyed. It was therefore combustible, in other words, organic matter*. I tried to 
intercept it by a large respirator of cotton-wool tied round the end of the tube tt. 
Close pressure was necessary to render the wool effective. A plug of the wool rammed 
pretty tightly into the tube 1 1' was finally found competent to hold back the motes. 
They appeared from time to time afterwards and gave me much trouble ; but they were 
invariably traced in the end to some defect in the purifying-apparatus, — to some crack 
or flaw in the sealing-wax used to render the tubes air-tight. Without due care, more- 
over, liquid particles may also be carried mechanically over. To prevent the entrance 
of such into the experimental tube, the narrow conduit Avhich connects it with the flask 
F is plugged with clean asbestos. Thus through proper care, but not without a great 
deal of searching out of disturbances, the experimental tube, even when filled with pure 
am or vapour, contains nothing competent to scatter the light. The space within it has 
the aspect of an absolute vacuum. 
An experimental tube in this condition I call optically empty. 
Here follows one of the numerous experiments executed in relation to this subject. A 
platinum tube 9 inches long, 04 of an inch wide, and having within it a roll of platinum 
gauze, was placed in a gas-furnace where it could be intensely heated. One end of this 
tube was connected with the entry stopcock of the experimental tube S S', fig. 1, the 
other end was open to the air of the laboratory. The air was sent first through the 
platinum tube cold, then through the same tube heated to various degrees of redness, 
into the experimental tube, where it was subjected to the scrutiny of the concentrated 
electric beam. Here are the results. 
Quantity of air. 
State of platinum tube. 
State of experimental tube. 
15 in. of mercury. 
Cold. 
Full of floating particles. 
15 
Bed-hot. 
Optically empty. 
15 
Cold. 
Full of floating particles. 
15 
Bed-hot. 
Optically empty. 
15 
Intensely heated. 
An exceedingly fine cloud, which 
discharged perfectly polarized 
blue light in a direction at right 
angles to the illuminating beam. 
* Mr. Dancer las recently examined microscopically the dust of Manchester, and found it to consist almost 
wholly of organic particles. 
