“ NEW FORCE ” OF M. J. THORE. 
467 
point seen at Experiments 66 to 69 when working with M. Thoke’s apparatus ; in each 
case the negative rotation is slight before the neutral point is reached, while the 
positive rotation observed after neutrality increases rapidly in each case, until it 
eventually far exceeds the original movement. 
In conclusion, I think I may consider as established by these experiments the 
following results :— 
1. The broad facts of rotation as observed by M. Thqre are abundantly confirmed. 
2. The numerous experiments in which the face and hot water are tested under the 
same circumstances, and especially Experiments 46 and 47, prove that the action is 
due to radiation alone. 
3. Blackening the cylinder increases the actiou. This is especially shown in 
Experiments 11, 12, 13, 14, and 15. 
4. The action is slightly increased by blackening both cylinder and pillar. 
5. The remarkable fact observed by M. Thore that a fine fibre of silk brought 
near the suspended cylinder produces rotation has been verified in Experiments 27 
and 28, and with other fine fibres in Experiments 19 to 26. 
6. That the rotation is produced by a reaction between the cylinder and pillar, and 
not between the cylinder and the source of radiation, is shown in Experiments 54 to 75. 
7. The hypothesis that the rotations are produced by air-currents is disproved 
partially by the experiments in which the effect of an ascending current of air is 
shown to be almost without action, and it is entirely disproved by Experiments 66 to 
75, and 86 to 96, in which the movements become more energetic in proportion as the 
space in which they occur is exhausted of air. 
8. The rotation takes place negatively in dense air, and positively in high vacua. 
It is proved beyond a doubt by Experiments 66 to 75 and 86 to 96 that the positive 
rotations are due to the same cause which produces rotation of the radiometer : i.e., to 
molecular pressure caused by radiation falling on the blackened surfaces.* In all 
cases there is noticed strong action in very high vacua, diminishing as the vacuum 
gets less perfect, until a point is reached where there is no action. Below this neutral 
point movement recommences, but in the opposite direction to that observed at high 
exhaustions. This negative movement is common to M. Thore’s phenomena, and to 
the whole series of phenomena investigated in my researches on “ Repulsion resulting 
from Radiation." The explanation of tire negative motion is, however, not clearly made 
out. But, from the strict parallelism between the two sets of phenomena, I have no 
doubt that the explanation which will account for the one will be equally adequate to 
account for the other. 
In my sixth paper on “ Repulsion resulting from Radiation,”! pars. 415, 416, I 
described apparatus in which negative rotation was produced at an exhaustion of 
117 M, and positive rotation at an exhaustion of 0’18 M. This phenomenon is, 
* ‘Phil. Trans.,’ 1876, pp. 375-376; ‘Roy. Soc. Proc.,’ vol. 25, 1876, p. 308. 
t ‘ Phil. Trans.,’ 1879, pp. 101-103. 
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