MOLECULAR PRESSURE, AND THE TRAJECTORY OF MOLECULES. 155 
On reversing the battery current passing round the magnet, the above-named 
deflections are obtained in the opposite direction. 
548. The indicator fly is brought into position h (fig. 15, plan), and the induction 
spark passed, the pole b being negative. The shadow entirely envelopes the indicator, 
and no movement is produced. The current from 3 cells is then sent round the electro¬ 
magnet. This twists the shadow round to position l e, m e, and thereby brings half the 
indicator into the stream of molecules. Instantly the fly rotates with great speed in 
the normal direction, the same as it did at i. The indicator is now brought into 
position q, where it is entirely immersed in the shadow whilst deflected by the magnet. 
No movement occurs until battery contact is broken with the electro-magnet, when 
the shadow instantly returns to its normal position, and the further half of the fly 
being thereby left uncovered it rotates rapidly in the same direction as it did at Jc. 
549. The pole b being negative, no difference in any of the phenomena here recorded 
could be detected, whether the positive wire was connected with the pole a or the 
pole c, or even when the screen itself, e, was made the positive pole. (519, 526, 527.) 
550. The position of the magnet was now moved from under the positive pole a, to 
under the negative pole b, its axis still remaining parallel to that of the axis of the 
bulb. The shadow was now deflected when the magnet was excited with two cells, to 
a greater extent than it was with five cells in the former position. 
THE TRAJECTORY OF MOLECULES. 
551. The indicator in the apparatus just described (541) is supported by a thin fibre 
of German glass, which becomes highly fluorescent in the molecular stream. By moving 
it about in different parts of the bulb the shape of the shadow can be determined as it 
passes through space. Fig. 17 shows the trajectory of the molecules forming the shadow 
Fig. 17. 
when under the influence of the electro-magnet actuated by 3 cells. The normal 
position being represented by a a, b b, the deflected shadow is shown by c c, d d. On 
tracing c d back past the screen to the negative pole the trajectory of the molecules 
forming it is found to be a curved line. The lines b b and a a are straight, but the 
corresponding lines, c c and d d, are curved. Moreover, the line c c starts from the 
negative pole in a contrary direction to that which it ultimately takes. This shows 
that the line of molecules whose straight path forms the boundary line of shadow b b is 
x 2 
