MR. W. CROOKES OK MOLECULAR PHYSICS IN HIGH VACUA. 
651 
found to be positively electrified. The outside of the glass tube, both near the 
negative pole and near the positive pole, was also positively electrified. 
Eig. 11. 
The above experiments were tried with 6300 cells, a resistance equal to 800,000 
ohms being interposed. The current through the tube was 0‘00383 weber. These 
measurements were taken by Mr. De La Rue, to whom I am greatly indebted for 
permission to experiment with his magnificent battery, and who himself kindly 
assisted me in making the arrangements.—August 12, 1879.] 
607. These phenomena agree with the explanation above given (604). Experi¬ 
ments recorded in a previous paper (556 to 571) have proved that the velocity of the 
molecules is greater as the vacuum gets higher, and that in consequence the trajectory 
of the molecules under the deflecting action of a magnet is flatter at high than at low 
vacua. The space covered by the penumbra may be taken as representing the size of 
the shadow when the positive charge of the idle pole is so small as to have little 
deflecting action on the molecular rays. The deflecting action being constant (like 
that of the permanent magnet in the experiments just quoted), a different trajectory 
corresponds to different velocities; therefore at a low vacuum, where the penumbra 
is not much larger than the original shadow, the molecules pass the object or idle pole 
a, slowly, and are deflected to b. At a higher exhaustion the molecular velocity is 
greater, and the deflecting force having less time to act they go to c; whilst at the 
highest vacuum, when the velocity of the molecules is very great, they are scarcely 
deflected at all, and proceed to d. On allowing the idle pole a to remain insulated, 
its positive charge increases during rarefaction in the same degree as does the nega¬ 
tive charge of the molecular stream; the velocity and the deflecting force, therefore, 
keep about equal, and the result is that the shadow e remains now nearly constant at 
all exhaustions. 
It is not unlikely that, at the highest exhaustions, when the penumbra is very large, 
the negative electricity of the molecular stream overcomes the slight positive charge 
of the uninsulated idle pole (604) and gives it a negative charge, which causes the 
stream to be repelled outwards instead of attracted inwards. 
