PROPERTIES OF MATTER IN THE GASEOUS STATE. 
771 
pressure of 4 inches, and then steadily increased as the pressure fell, until for very 
small pressures the fibre moved through about 1,000 divisions on the micrometer. 
With hydrogen, Table XIX. shows that the results were positive from the pressure 
of the atmosphere and for small pressures were somewhat larger than with air. 
Although only one series of such observations is recorded in the table, the experi¬ 
ments were repeated several times with each gas. Also a flame was used instead of 
a heater, and the results were consistent throughout. 
Elevation of the heater. 
49. The effect of having the heater at different elevations was carefully studied, for 
it was obvious that this would affect the air currents in the tube. It was found, 
however, that the elevation of the heater did not produce any effect on the direction 
in which the fibre moved at pressures of less than 6 or 8 inches of mercury for air, and 
less than 20 inches for hydrogen. For pressures greater than these, considerable 
alterations in the elevation of the heater did produce very slight modifications in the 
motion of the fibre. 
Bending of the fibre . 
50. The possibility of the results being due to a tendency of the fibre to bend with 
the warmth was also considered. Observations were taken at different points up the 
fibre and on different sides ; and the results were such as to lead to the conclusion 
that the bending of the fibre did not produce any material effect. 
Spider line , 
51. A spider line was also used : it was not found possible to suspend this freely 
in the tube. It was attached top and bottom to a wire frame, but it was quite loose 
between the points of attachment, so that it could swing to either side. 
Considerable difficulty was found in observing the spider line, as it was lost sight of 
the instant it was the least out of focus ; but the general result of the observation was, 
that at higher pressures both for air and hydrogen the motion was negative or to the 
heater ; but at pressures of less than about 8 inches it was decidedly positive, the fibre 
being driven away from the heater as far as its frame would allow. 
From the fact that the fibre of silk had shown positive motion so nearly up to the 
pressure of the atmosphere it might have been anticipated that the spider line, on 
account of its much greater thinness, would have shown positive motion even at 
pressures considerably above that of the atmosphere. But the reasoning of Art. 46 
respecting the differences of temperature to be maintained and the effect of the air 
currents, obviously applies with greater force to the spider line than to the fibre of 
