MR. W. CROOKES ON REPULSION RESULTING FROM RADIATION. 127 
459. In the next experiments the candle was placed 400 millims. off, and the screen 
and blacked disk were separated 3, 6, and 12 millims. :— 
rr 
C. 
S. 
F. 
millims. 
millims. 
millims. 
2-0 
400 
3 
16- 
5 5 
6 
11- 
55 
55 
12 
9-5 
1-0 
3 
24- 
55 
55 
6 
16- 
12 
11- 
0-2 
3 
60- 
55 
55 
6 
43- 
12 
38- 
0-01 
8 
107- 
55 
55 
6 
86- 
55 
55 
12 
60- 
Fig. 30 shows the curves given by these figures, using, as in the last diagram, T and 
F for abscissae and ordinates. 
Fig. 30. 
460. An examination of these tables and diagrams shows that the law of increase of 
the force with the diminution of the distance between the disks does not remain 
uniform at all rarefactions. At the lowest exhaustions the mean free path of the mole¬ 
cules of the attenuated gas is less than 1 millim., as rendered evident by the force of 
repulsion diminishing rapidly as the distance increases. At exhaustions higher than 
9 millims. this condition alters ; and as the gauge approaches barometric height, the 
pressure tends to become uniform through considerable distances, the mean path of the 
molecules now being comparable with the greatest distance separating the surfaces 
between which they act. As the distance between the disks increases, the tangential 
action between them and the sides of the glass comes more into play, and tends to 
interfere with and augment the direct pressure between the two surfaces. 
