rable with
that of an atom, and so very much greater than that of a corpuscle. At
very low pressures part of the phosphorescence disappears, while the
upper portion breaks up into two patches (fig. 27). For one of these the
maximum value of e/m is 10^4 and for the other 5 X 10^3. At low pressures
the appearance of the patches and the values of e/m are the same whether
the tube is filled originally with air, hydrogen or helium. In some of
the experiments the tube was exhausted until the pressure was too low to
allow the discharge to pass. A very small quantity of the gas under
investigation was then admitted into the tube, just sufficient to allow
the discharge to pass, and the deflection of the phosphorescent patch
measured. The following gases were admitted into the tube, air, carbonic
oxide, oxygen, hydrogen, helium, argon and neon, but whatever the gas
the appearance of the phosphorescence was the same; in every case there
were two patches, for one of which e/m = 10^4 and for the other e/m =
5 X 10^3. In helium at higher pressures another patch was observed, for
which e/m = 2.5 X 10^8. The continuous band into which the
phosphorescent spot is drawn out when the pressure is not exceedingly
low, which involves the existence of particles for which the mean value
of e/m varies from zero to 10^4, can be explained as follows. The rays
on their way to the phosphorescent screen have to pass through gas which
is ionized by the passage through it of the positive rays; this gas will
therefore contain free corpuscles. The particles which constitute the
rays start with a charge of positive electricity. Some of these
particles in their journey through the gas attract a corpuscle whose
negative charge neutralizes the positive charge on the particle. The
particles when in this neutral state may be ionized by collision and
reacquire a positive charge, or by attracting another particle may
become negatively charged, and this process may be repeated several
times on their journey to the phosphorescent screen. Thus some of the
particles, instead of being positively charged for the whole of the time
they are exposed to the electric and magnetic forces, may be for a part
of that time without a charge or even have a negative charge. The
deflection of a particle is proportional to the average value of its
charge whilst under the influence of the deflecting forces. Thus if a
particle is without a charge for a part of the time, its deflection w