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THE PHOTOGRAPHS 37

to occur with slowly moving particles more readily than with
fast ones ; a particle moving faster than a certain speed would
not combine with a negative corpuscle, so that there would be
a superior limit to the speed of the particles In secondary rays
of this kind. There does not, however, seem to be any reason
why there should be an Inferior limit to the velocity, provided
the slow particles have managed to retain their charges up to
the beginning of the magnetic field, and as a matter of fact
this type of secondary often shows itself, as In Fig. 25, Plate II.,
as the limit of a patch of fogging on the photographic plate
rather than as a sharply defined line. There are, however,
cases notable with the mercury lines, when this type of
secondary Is more sharply defined than we should expect,
since there are among the particles which produce the primary
parabolas some with a smaller velocity than can be detected
In the secondaries of this type.

The question arises whether the corpuscles which produce
the secondaries by neutralizing a positively charged particle
or Ionizing a neutral one are free, or are those bound up in
the molecules of the gas through which the positive rays are
travelling. There are several reasons for thinking that the
latter hypothesis is the more probable one.

For If the corpuscles which neutralize the positive particles
are free they should be removed by a strong electric field
which ought therefore to dimmish the brightness of the
secondaries. I have, however, never been able to detect an
effect of this kind.

Again if free corpuscles were those which neutralized the
positively charged particles, the distance such a particle would
travel before it got neutralized would depend only upon the
density of the free corpuscles. Now this density depends
upon the amount of ionization produced by the positive rays
after they have passed through the cathode; this will vary



	THE PHOTOGRAPHS 37 to occur with slowly moving particles more readily than with fast ones ; a particle moving faster than a certain speed would not combine with a negative corpuscle, so that there would be a superior limit to the speed of the particles In secondary rays of this kind. There does not, however, seem to be any reason why there should be an Inferior limit to the velocity, provided the slow particles have managed to retain their charges up to the beginning of the magnetic field, and as a matter of fact this type of secondary often shows itself, as In Fig. 25, Plate II., as the limit of a patch of fogging on the photographic plate rather than as a sharply defined line. There are, however, cases notable with the mercury lines, when this type of secondary Is more sharply defined than we should expect, since there are among the particles which produce the primary parabolas some with a smaller velocity than can be detected In the secondaries of this type. The question arises whether the corpuscles which produce the secondaries by neutralizing a positively charged particle or Ionizing a neutral one are free, or are those bound up in the molecules of the gas through which the positive rays are travelling. There are several reasons for thinking that the latter hypothesis is the more probable one. For If the corpuscles which neutralize the positive particles are free they should be removed by a strong electric field which ought therefore to dimmish the brightness of the secondaries. I have, however, never been able to detect an effect of this kind. Again if free corpuscles were those which neutralized the positively charged particles, the distance such a particle would travel before it got neutralized would depend only upon the density of the free corpuscles. Now this density depends upon the amount of ionization produced by the positive rays after they have passed through the cathode; this will vary