MOLECULAR PRESSURE, AND THE TRAJECTORY OF MOLECULES. 
153 
other. If, however, I tilt the bulb so as to bring the indicator half in and half out 
of the molecular shadow cast by the screen, I should expect to see the fly driven 
round to the right or to the left by the molecules striking one side only, thus confirm¬ 
ing the observation on the movement of the thin glass film under the molecular 
impact (541). 
543. The other subject which I had in view was the following. It is well known 
that a movable conductor carrying a current of electricity is deflected under the 
influence of magnetic force, and experiments tried very early in this research, and 
repeated with the apparatus already described, showed that the stream of molecules 
projected from the negative pole obeyed in a very marked manner the power of a 
magnet. It was hoped that the form of apparatus now under experiment would throw 
some light on this action of magnetism on molecules. 
544. The bulb being exhausted to the necessary high degree, the pole b is made 
negative, so as to cast the shadow of the screen e across the tube, where it can be 
Fig. 16. 
traced as a broad band along the lower part of the glass. Fig. 16, plan, shows the 
appearance, the shadow of e projected by the pole b being enclosed within the lines 
f e, g e, shaded diagonally. 
The indicator fly is first brought into position h, where it is entirely screened from 
the molecular stream; no movement takes place. The apparatus is slightly tilted, till 
the fly comes into position i, half in and half out of the shadow ; very rapid rotation 
takes place in the direction of the arrow, showing that impacts occur in the direction 
anticipated. The apparatus being further tilted, so as to bring the indicator quite out¬ 
side the shadow into position j, no movement takes place. When the indicator is 
brought to the other side of the shadow into position k, the rotation is very rapid in 
the direction of the arrow, opposite to what it is when at i. 
MAGNETIC DEFLECTION OF LINES OF MOLECULAR FORCE. 
545. An electro-magnet is placed beneath the bulb, shown at S in fig. 15, elevation, 
and by dotted lines at S in fig. 16, plan. A battery of from 1 to 5 Grove’s cells is 
connected with the magnet. The current is made to pass in such a direction that the 
pole under the bulb (marked S) is the one which would point towards the south were 
the magnet freely suspended. 
MDCCCLXXIX. x 
