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18 OF POSITIVE ELECTRICITY

iBolectiles. When the tube contains helium there are three
to be seen as in Fig. 8, The values of elm at the tips of
bands are respectively io⁴, 5 x io³,2 . 5 x io³, indicating

we have here again bands due to the atom and molecule of
hydrogen, and in addition a new one due to atoms of helium,
for fas the atomic weight of helium Is 4) e\m for the helium atom
is one quarter of that for the hydrogen atom. It is remarkable
that the slope of these bands, and therefore, by page 12, the
velocity of the particles, varies little if at all with the potential
difference between the anode and cathode of the discharge tube.
This potential difference may be Increased three or four times
without producing any appreciable effect upon the slope of the
bands of phosphorescence. When air is in the tube, the ap-
pearances of the bands Is much the same as when the tube con-
tains hydrogen, though the phosphorescence is not so bright.
The most conspicuous things on the screen In this case are the
two bands corresponding to the atom and molecule of hydrogen
respectively.

In addition to the two bands deflected in the direction in-
dicating a positive charge on the particles, there is another
fainter band deflected in the opposite direction which must
therefore be due to particles with a negative charge. The value
of elm for the tip of this band Is io⁴, thus these negative
particles are not cathode rays for which ejm Is 17 x io⁷, but
have a mass equal to that of an atom of hydrogen. The
existence of particles deflected in the opposite direction to
that of the majority of the particles had also been observed
by Wien.

EFFECT AT VERY LOW PRESSURES.

When the pressure Is reduced to as low a value as is
possible the appearance of the luminosity on the screen



	18 OF POSITIVE ELECTRICITY iBolectiles. When the tube contains helium there are three to be seen as in Fig. 8, The values of elm at the tips of bands are respectively io⁴, 5 x io³,2 . 5 x io³, indicating we have here again bands due to the atom and molecule of hydrogen, and in addition a new one due to atoms of helium, for fas the atomic weight of helium Is 4) e\m for the helium atom is one quarter of that for the hydrogen atom. It is remarkable that the slope of these bands, and therefore, by page 12, the velocity of the particles, varies little if at all with the potential difference between the anode and cathode of the discharge tube. This potential difference may be Increased three or four times without producing any appreciable effect upon the slope of the bands of phosphorescence. When air is in the tube, the ap- pearances of the bands Is much the same as when the tube con- tains hydrogen, though the phosphorescence is not so bright. The most conspicuous things on the screen In this case are the two bands corresponding to the atom and molecule of hydrogen respectively. In addition to the two bands deflected in the direction in- dicating a positive charge on the particles, there is another fainter band deflected in the opposite direction which must therefore be due to particles with a negative charge. The value of elm for the tip of this band Is io⁴, thus these negative particles are not cathode rays for which ejm Is 17 x io⁷, but have a mass equal to that of an atom of hydrogen. The existence of particles deflected in the opposite direction to that of the majority of the particles had also been observed by Wien. EFFECT AT VERY LOW PRESSURES. When the pressure Is reduced to as low a value as is possible the appearance of the luminosity on the screen