PROPERTIES OF MATTER IN THE GASEOUS STATE. 
7G9 
on the assumption that the pressure at which the effect would be apparent, increases 
proportionally as the size of the vanes diminishes, it was clear that in order to obtain 
the repulsive effect at the pressure of the atmosphere the size of the vanes must be 
reduced several thousand times. 
The only means of obtaining such small vanes was to suspend a fibre of silk or a 
spider line. A single fibre of silk has a diameter of vowo^ 1 °f an inch (about), which 
is less than ToQoth the breadth of the vanes of the light mill on which my previous 
experiments had been made. But in order that the pressures at which the results 
would be sensible might be inversely proportional to the size of the vanes, the vanes 
should preserve the same shape ; whereas the vanes in the light mill were square, 
while the fibre of silk wns only narrow in one direction, which would be considerably 
to the disadvantage of the fibre of silk. More than this : it appeared probable that 
the thinness and transparency of the fibre, together with the cooling action of the air, 
would only allow an extremely small difference of temperature to be maintained on its 
opposite faces by radiant heat falling on one side ; wdiereas air currents in the tubes, 
which would tend to carry the fibre with them, would be caused by the greater 
temperature of the glass on that side of the tube on which was the hot body, and 
these, which would be quite independent of the size of the fibre or vane, would 
exercise, proportionally, as great an effect on the fibre as on the larger vanes. 
For the foregoing reasons a result was hardly probable, but as a preliminary step 
I suspended a fibre in a test tube ‘7 inch in diameter and 5 inches long; I then 
brought a gas flame near to the tube to see if it would cause any motion in the 
fibre, the pressure of the air within the tube being that of the atmosphere. 
The result was that the hair moved very slightly and somewhat uncertainly towards 
the flame. 
As I had more than suspected that such would be the result at the pressure of the 
atmosphere, and as I had no means at hand for exhausting the tube, I postponed 
further experiments in this direction in order to take up the more promising investi¬ 
gation with the porous plates. When, however, I had concluded this, and succeeded 
almost beyond my expectation, I returned to the experiments on the fibre with the 
intention of exhausting the tube and using hydrogen as well as air. 
Subsequent experiments. 
47. These experiments were commenced on July 24, 1878. 
A single fibre of unspun silk, having a thickness of '0005 of an inch, was suspended 
in a test tube 1 inch in diameter and 7 inches long. The tube was closed with an 
india-rubber cork, through which passed a small glass tube to allow of exhaustion ; 
this tube was connected with the vacuum gauge and the mercury pump, also with 
drying tubes for admitting dry air or hydrogen. A microscope with micrometer eye¬ 
piece reading ToEToofh an ^ nc ^ 1 (the same as had formed the cathetometer in the 
previous experiments) was arranged for the observation of the motion of the fibre. 
5 F 2 
