658 
MR. W. CROOKES OK MOLECULAR PHYSICS IK HIGH VACUA. 
622. These experiments show that the law is not the same at high as at low ex¬ 
haustions. At high exhaustions the magnet acts the same on the molecules whether 
they are coming to the magnet or going from it, the direction of rotation being entirely 
governed by the magnetic pole presented to them, as shown in cases a and b where 
the north pole rotates the molecular stream in a direct sense, although in one case the 
top electrode is positive and in the other negative. Cases c and d are similar ; here 
the magnetic pole being changed, the direction of rotation changes also. The direction 
of rotation impressed on the molecules by a magnetic pole is opposite to the direction 
of the electric current circulating round the magnet. 
623. The magnetic rotations in low vacua are not only fainter than in high vacua, 
but they depend as much on the direction in which the induction spark passes through 
the rarefied atmosphere, as upon the pole of the magnet presented to it. The luminous 
discharge connecting the positive and negative electrode carries a current, and the 
rotation is governed by the mutual action of the magnet on the perfectly flexible 
conductor formed by the discharge. 
In high vacua, however, the law is not the same, for in cases b and d similar arrange¬ 
ments produce opposite rotations in high and in low vacua. The deflection exerted by 
a magnet on the molecular stream in a high vacuum may be compared to the action of 
a strong wind blowing across the line of fire from a mitrailleuse. The deflection is 
independent of the to-and-fro direction of the bullets, and depends entirely upon the 
direction of the wind. 
624. I have already mentioned that platinum will fuse in the focus of converging 
molecular rays projected from a concave pole (582). If a brush of very fine iridio- 
platiniun wire, which has a much higher fusing point than platinum, be used to receive 
the molecular bombardment, a brilliant light is produced, which might perhaps be 
utilised. By drawing the focus to the side of a glass tube by means of a magnet, a 
series of phenomena take place. The brilliant green phosphorescence first developed 
by the impact of the focus rapidly fades out owing to the heat. The centre of the 
focus becomes dark brown, and if the coil is now stopped this brown spot remains 
permanent as a superficial stain on the glass, and the glass has lost its power of 
phosphorescing at this jfiace. If, however, the focus is kept steadily on one spot a 
sparkling appearance is noticed, and a superficial disintegration takes place as if the 
glass were fusing. Thick tubes now crack at this stage, but thin bulbs soften by the 
heat, when the pressure of the atmosphere forces the glass in and bursts through a fine 
hole which comes in the centre of the indentation. 
625. The dark brown spot which forms in the focus is only a concentration of a 
stain which gradually forms on anything which is long submitted to molecular bom¬ 
bardment when aluminium and probably other poles are used. It has nothing to do 
with a reduction of the constituents of the glass, for it forms equally well on the 
polished surface of a diamond. It is not aluminium, for it is not dissolved by boiling 
in strong potash solution or in chlorhydric acid ; and is only slowly attacked by aqua- 
