-------
53. CROOKES’ EXPERIMENT. DARK SPACE AROUND THE CATHODE. _Lect. Brit.
Asso., Shef., Eng._, Aug. 22, ’79.—According to Lenard (_The Electr.,
Lond._, Mar. 23, ’94) Hittorf discovered the cathode rays, and Varley, §
61_a_., and Crookes studied them. The pressure of the residual gas was 1
M. of an atmosphere. Prof. Crookes, F.R.S., maintained the evacuated
space in communication with the air pump and with an absorbent material.
Before his time most experimenters worked with a vacuum not much less
than 30,000 M. The first experiment is illustrated in diagram, at Fig. 6
p. 17, but the vacuum was not the highest in this type. The tube was
cylindrical and was provided with electrodes at the ends. Another
electrode was located at the centre and was made the cathode, while the
two terminal electrodes were made the same pole; namely, the anode. Upon
connecting the tube in circuit with the secondary of a large induction
coil, the luminosity did not extend either continuously or in striae
throughout the length of the tube. Former investigators had likewise
noticed the dark space. The space and glass on each side of the central
cathode were dark. The dark space extended for about one inch on each
side of the negative pole. It is not intended here, any more than in
former cases, to present theories in explanation further than to briefly
allude to any conclusion at which the experimenter himself arrived.
Crookes’ explanation of the phenomena has not been universally accepted,
nor has it been proved otherwise. The knowledge of the existence of
rays, now known as Roentgen rays, will assist in formulating theories
upon the Crookes’ phenomena and may either confirm some of his views or
overthrow them. Crookes considered that the residual atmosphere was in
such a state as to be as different in its properties from gas, as gas is
from liquid and liquid from solid, and therefore he named the attenuated
atmosphere radiant matter, or fourth state of matter. He concluded that
the remaining particles of the gas forming the radiant matter moved in
straight lines over a great distance as compared with that moved through
by molecules at the ordinary pressure. He called this distance the “mean
free path.” If his theory is correct, this dark space is due to the fact
that the molecules in motion at and near the cathode do not bombard each
other and therefore do not produce the effect of light. When the motion
is arrested by particles of gas themselves, within the bulb, then is
light generated. The force propelling the particles from the positive
pole was supposed to be less. In order to let the experiments speak for
themselves, as much as possible, without being too much influenced by
the opinion of the experimenter; the theory is only briefly alluded to
as above, and will not be further applied in the presentation of his
other experiments. In view of the radical discoveries of Lenard and
Roentgen, after the installation of the Crookes phenomena, it has been
the policy of the author to present all the experiments as facts for
evidence in behalf of the general theories, which may be hereafter
formulated independently of old theories. Therefore, the reader should
bear in mind the teachings of the various experiments with the view of
arriving at general principles and hypotheses.
54. RELATION OF VACUUM TO PHOSPHORESCENCE.—He started with such a high
vacuum that he could not obtain any electrical discharge. § 25. There
was, therefore, no phosphorescence in the glass tube, whatever. The
caustic potash, which had been employed to absorb the last trace of
moisture and carbonic acid gas, was slightly heated, and very gradually.
Then it was noticed that a current began to pass and that the glass
became green, and apparently on the inner surface. As the heat
continued, the green passed gradually away and was replaced by striae,
which first appeared to extend across the whole diameter of the glass
tube (§ 40) which was a long cylindrical tube, and then became
concentrated toward the axial line of the tube. Finally, the light
consisted of a pencil of purple. § 10. When the source of heat was
removed so that the moisture and carbonic acid gas could be absorbed
again by the potash, the striae appeared, and then the other effects
just named, only in the reversed order, until the tube acted like an
infinite resistance. Phosphorescence is the correct word, because the
light existed for a few seconds after cutting off the current.
55. PHOSPHORESCENCE OF OBJECTS WITHIN THE VACUUM TUBE.—The construction
in Fig. 7, p. 17, shows how a diamond was caused to phosphoresce within
a Crookes’ tube, being supported in a convenient manner in the centre of
one of the tubes, while electrodes were located near the ends and were
formed of disks facing the diamond. Upon connecting the disks to the
respective poles of the secondary conductor, and by performing the
experiment in a rather dark room, the diamond became brilliantly
phosphorescent, radiating light in all directions. He experimented with
many substances in this way, but found that the diamond was the
best—almost equal to one candle power. In order to exhibit the
phosphorescence of glass in a striking manner, he charged three small
tubes simultaneously. One was made of uranium glass which radiated a
green light. Another was an English glass which appeared blue, and the
remaining one was German glass which phosphoresced a bright green.
Notice difference with respect to light which does not perceptibly cause
phosphorescence of glass. The uranium glass was the most luminous.
Luminous paint, as prepared by Becquerel, and later by Balmain, which
has the property of storing up light and afterwards radiating it in a
dark room for several hours, became more phosphorescent in the Crookes
tube than when subject to day-light. Phosphorescence of the mineral
phenakite, the chemical name of which is glucinic aluminate, was blue,
the emerald, crimson, and spodumene, which is a double silicate, were
yellow. The ruby phosphoresced red, whatever its tint by day-light. In
one tube he had rubies of all the usual tints by day-light, but they
were all of one shade of red by the action of the disruptive discharge
in the tube.
56. DARKNESS AND LUMINOSITY IN ARMS OF V TUBE. See Fig. 8, p. 17. It
will be noticed that in Fig. 6, p. 17, the tube was straight. Crookes
desired to see what effect would take place in a bent tube. He therefore
employed a V shaped tube, having electrodes in the ends—one in each arm.
Upon causing the electrical discharge to take place through the tube,
one arm was luminous and the other was dark. Whatever the E. M. F. was,
the appearances remained the same. No luminosity would bend from one arm
of the V shaped tube to the other. The cathode arm was always luminous
and the anode dark. With a less degree of vacuum, both arms were
luminous, according to early experimenters who thus brilliantly lighted
tubes of the most fantastic shapes.
57. CATHODE RAYS RECTILINEAR. RADIATE NORMALLY FROM THE SURFACE OF THE
CATHODE. In his lecture he had, side by side, two bulbs, one, in which
the vacuum was of such a degree, that a blue stream of light existed
between the negative pole and positive pole, § 54, at centre. It is
evident that the vacuum in this bulb was not very high. Fig. 9, p. 17,
shows a stream extending from the negative to the positive pole, Fig.
10, p. 17, is the same kind of a tube only the vacuum is about 1 to 2 m.
In other words, the vacuum in the latter was just so high that a
discharge took place, and instead of the luminous effect being like that
with a low vacuum, there was a patch of green light directly opposite
the concave negative pole. The radiations from this pole were
rectilinear, crossing each other at a focus within the bulb and
producing upon the glass a phosphorescent spot. It should be remembered
that the word radiations is used as a mere matter of convenience.
Directly opposite the concave cathode, there was a green patch of light
on the inner surface of the glass. It was shown that it made no
difference where the anode was. This fact becomes useful in carrying on
experiments in connection with Roentgen rays, and it may have a great
deal to do with the solution of the theoretical problems in connection
with electrical discharges in vacuum tubes. In regard to the three
streams shown in Fig. 9, p. 17, it may be stated that only one occurred
at a time in the experiment, for, first one anode was connected in
circuit, and then the next by itself, and then the third one by itself,
while the concave pole was always negative. Each time the anode was
changed, the stream changed, and connected that pole which was in
circuit, § 43, but similar changes made upon the tube with a high
vacuum, did not alter the position of the phosphorescent spot. This and
other experiments show that the radiations took place perpendicularly
from the surface of the cathode.
58. SHADOW CAST WITHIN THE DISCHARGE TUBE. This is illustrated in Fig.
11, p. 17, where there is a negative polar disk at the small end of the
egg shaped tube, and a cross near the large end, the same forming the
positive pole. The cross is made of aluminum. There was a novel action,
however, discovered in addition to the mere casting of a shadow. The
glass which had become phosphorescent except within the shadow, became
after a while, less phosphorescent. Its property to phosphoresce became
less as proved by removing the cross, which was arranged to fall down
upon tipping the bulb. Immediately, the part which was within the shadow
became brighter than the rest of the glass, thereby reversing the
appearances, by making a luminous picture of the cross upon only
partially phosphorescent glass. A remarkable feature is that the glass
never recovered its first exhibited power of phosphorescence, neither
did this power entirely become nothing, however many times the tube was
employed. Was the deposit of metal from the cathode the cause?
58_a_. MECHANICAL MOTION PRODUCED BY RADIATIONS FROM THE NEGATIVE POLE.
It occured to Crookes that the radiations from the cathode might perhaps
cause a wheel to turn around. He therefore had a minute wheel made by
Mr. Gimingham, like an undershot water wheel, and its axle rested on two
rails of glass, so that it might roll along from one end of the tube to
the other. The vanes were exactly opposite to the plane surface of the
cathode. The molecular stream or radiations, or whatever they may be,
possibly vibrations, from the cathode, were so powerful mechanically
that the wheel was caused to run up hill, the tube being inclined very
slightly. On the principle that action and reaction are equal, he built
another device in which the negative electrode was movable, and he
observed that when the current was on, the negative electrode moved
slightly. Upon these principles he built the well-known Crookes
radiometer in which the vanes rotated by reaction of the radiations. The
vanes in this form of radiometer were made of aluminum, and a cup of
hard steel served as the bearing, Fig. 12, p. 17. One side of each disk
was coated with a thin scale of mica. The aluminum disks formed the
cathode, while the anode was located at the top. The operation consisted
in connecting the terminals as stated, so that the vanes were the
negative poles and it was observed that the little wheel rotated. The
vacuum was not as high as that for obtaining phosphorescence. With a low
vacuum, an envelope of violet light existed near the surface of the
aluminum vanes. Effects were carefully studied by maintaining connection
with the pump. At the pressure of .5 mm. there was a dark cylinder
opposite the aluminum extending to the glass, and this was the pressure
at which the vanes began to rotate. The dark spaces opposite each vane
became larger and larger in width, until they appeared to be opposed or
resisted by the inner surface of the glass, and then the rotation became
very rapid. He modified this experiment by having vanes entirely of
mica, and by having the cathode disconnected electrically from the
vanes, Fig. 13, p. 17. A coil of metal near the vanes served as the
cathode. The anode was at a distance in the top of the tube as in Fig.
12, p. 17. During the electrical discharge, the wheels rotated by
radiations from the coil which formed the cathode. He made the discovery
that when this coil was heated red hot conveniently by a current from a
primary battery, the vanes also rotated, showing that there is probably
some relation between the radiations from the cathode and heat rays. The
fact remains however, that both kinds of rays produced rotation,
directly or indirectly.
59. ACTION OF MAGNET UPON CATHODE RAYS.—He had two tubes, one of which
is shown in Fig. 14 and the other in Fig. 15, on page 17. In the former,
the vacuum was so low that a violet stream of light existed between the
electrodes. In the other, the rays were invisible, but were converted
into luminosity by projection at an exceedingly slight angle, upon a
phosphorescent screen arranged along the length of the tube and inside
thereof. Inasmuch as the whole surface of the cathode in the latter case
radiated parallel and invisible rays, he cut off some of them by a mica
screen having a hole in the centre and located near the negative pole,
so that only a pencil of invisible rays could go through the mica screen
and act upon the phosphorescent screen. In both cases, there was visible
a straight pencil of light. Now notice the effect which took place upon
locating a magnet as indicated in the figures. With the low vacuum, the
pencil was bent out of its course but returned again to the line of its
original path. § 28. With the high vacuum, the rays were bent but did
not return to their original direction nor parallel thereto. In the
former case, the magnet acted as upon a very delicate flexible
conductor, while in the latter, it acted, as Crookes said, like the
earth upon projectiles. He modified the latter experiment in order to
determine if the similarity between this phenomenon and gravitation
existed in other respects. He anticipated that if the molecular
resistance to the rays were increased they would be bent more out of
their course like a horizontally projected bullet. He therefore heated
the caustic potash sticks slightly, and in view of the liberation of
molecules of water within the vacuum tube, the rays, he thought, would
be resisted; and such was the case to all appearances, for then the
pencil of light was bent out of its course to a greater extent, although
the magnetic power remained the same as well as the E. M. F. producing
the electric discharge. He therefore established, apparently, the
principle that the magnetic actions upon cathode rays vary somewhat in
their nature according to the degree of vacuum. In either case, it may
be stated incidentally, that when the magnet was moved to and fro, the
pencils of light waved back and forth.
In the modified form of construction over that shown in Fig. 15, p. 17,
he caused a wheel to rotate that was located in the high vacuum. The
vanes of the wheel were so located that the faces of the same were
perpendicular to the direction of the pencil of the rays radiating from
the cathode. When the magnet deflected the rays, the wheel ceased
rotation.
60. MUTUAL REPULSION OF CATHODE RAYS.—If the little mica screen, as
shown in Fig. 16, p. 17 has two holes, and if there are two cathodes
instead of one, there will also be two pencils of light. He performed an
experiment involving the latter modification, and the result was
something that could not have been predicted. The two pencils, as
displayed by the long fluorescent screen, repelled each other like
molecules similarly electrified. The white pencils, it will be noticed,
were repelled from each other and showed their condition when both of
the negative poles were in circuit. The black pencils show the location
of both of the pencils when only one pole is in circuit at a time, the
direction being perpendicular to the plane of the cathode disc (§ 57) at
end.
61. HEATING AND LIGHTING POWER OF CATHODE RAYS. HEAT OF PHOSPHORESCENT
SPOT.—By making the cathode concave as in Fig. 10, p. 17, and so
locating it that the focus of the cathode rays falls upon some
substance, the latter becomes very hot. In this way Crookes melted wax
on the outside of the bulb at the phosphorescent spot. Further than
this, the heat was so great that it cracked the glass without at first
injuring the vacuum; next the glass at this point softened, and the air,
by its pressure, rushed into the bulb, forcing a hole through the soft
part. He performed an experiment also which illustrated the intensity of
the heat when the rays were brought to a focus. He used an unusually
large electrode like a concave mirror, and in the focus, which was near
the centre of the bulb, he supported a small piece of iridio-platinum.
At first, with a moderately low E. M. F., the metal was made white hot.
When a magnet was caused to approach, the rays were drawn to one side, §
59, and the little piece of metal cooled. He then put in all the coils
of an inductorium, and allowed the metal not only to become white hot,
but to become so heated that it melted. How little did Prof. Crookes
know about the most important phenomena associated with his experiment.
Although he was so exceedingly enthusiastic and ingenious in planning
his experiments, and in reasoning, yet it seems almost mysterious that
he should have been subjected to what have become known as X-rays, which
passed into his body, and would have photographed portions of his
skeleton, and which would have performed outside of the tube many of the
acts that were noticed within. Seventeen years elapsed between the time
of Crookes on the one hand, and Lenard and Roentgen’s discoveries on the
other. Dr. Lodge, F.R.S., (_The Elect._, Lon., Jan. 31, ’96, p. 438,)
and Lenard, in his first paper, attributed to Hittorf the discovery of
the mere existence of cathode rays, but credited to Crookes the full
establishment of their properties, deduction of their principles and
formulation of an ingenious theory.
61_a_ As an appropriate conclusion to Crookes’ work, I cannot do better
than to let Lord Kelvin repeat what he said in his Pres. Addr., _Ro.
So._, Nov. ’93, see also _The Elect._, Lon. Feb. 14, ’96, p. 522,
showing that a small portion of the credit is due not only to Hittorf, §
53, but to Varley. “His short paper of 1871, which, strange to say has
lain almost or quite unperceived in the _Proceedings_ during the 22
years since its publication, contains an important first instalment of
discovery in a new field, the molecular torrent § 53, at centre, from
the ‘negative pole,’ the control of its course by a magnet, § 59, its
pressure against either end of a pivoted vane of mica, § 59, at end, and
the shadow produced by its interception by a mica screen, § 58. Quite
independently of Varley, and not knowing what he had done, Crookes
(_Roy. Inst. Proc._, April 4, ’79, vol. LX, p. 138. _Ro. So. Trans._,
’74, “On attractions and repulsions resulting from radiation” Part II,
’76, parts III and IV, ’76, part V, ’78, part VI, ’79) was led to the
same primary discovery, not by accident and not merely by experimental
skill and acuteness of observation.” * * * * “He brought all his work
more and more into touch with the kinetic theory of gases; so much so,
that when he discovered the molecular torrent he immediately gave it its
true explanation—molecules of residual air, or gas or vapor projected at
great velocities (probably, I believe not greater in any case than 2 or
3 kilometers per second, § 61_b_), by electric repulsion from the
negative electrode. This explanation has been repeatedly and strenuously
attacked by many other able investigators, but Crookes has defended
(Presidential address to the _Inst. Elect. Eng._, 1891.) it, and
thoroughly established it by what I believe is irrefragable evidence of
experiment. Skillful investigations perseveringly continued brought out
more and more wonderful and valuable results; the non-importance of the
position of the positive electrode, § 57, near end, the projection of
the torrent perpendicularly from the surface of the negative electrode,
§ 57, at end; its convergence into a focus and divergence thenceforward
when the surface is slightly concave, § 47, near beginning; the slight
but perceptible repulsion, § 60, between two parallel torrents due,
according to Crookes, to negative electrifications of their constituent
molecules; the change of the direction of the molecular torrent by a
neighboring magnet, § 59. the tremendous heating effect of the torrent
from a concave electrode when glass, metal or any ponderable substance
is placed in the focus, § 61. the phosphorescence procured on a plate
coated with sensitive paint by a molecular torrent skirting along it,
Fig. 15, p. 17; the brilliant colors—turquoise blue, emerald, orange,
ruby-red—with which grey, colorless objects, and clear, colorless
crystals glow on their struck faces when lying separately or piled up in
a heap in the course of a molecular torrent, § 55. “electrical
evaporation” of negatively electrified liquids and solids, § 59. (_Ro.
So. Proc._, June 11, ’91.) the seemingly red-hot glow, but with no heat
conducted inwards from the surface, of cool solid silver kept negatively
electrified in a vacuum 1/1,000,000 of an atmosphere, and thereby caused
to rapidly evaporate, § 40 and 139_a_. This last named result is almost
more surprising than the phosphorescent glow excited by molecular
impacts on bodies not rendered perceptibly phosphorescent by light, §
55, at centre. Both phenomena will usually be found very telling in
respect to the molecular constitution of matter and origination of
thermal radiation, whether visible as light or not. In the whole train
of Crookes investigations on the radiometer, the viscosity of gases at
high exhaustion, and the electro-phenomena of high vacuums, ether seems
to have nothing to do except the humble function of showing to our eye
something of what the molecules and atoms are doing. The same confession
of ignorance must be made with reference to the subject dealt with in
the important researches of Schuster and J. J. Thomson on the passage of
electricity through gases. Even in Thomson’s beautiful experiments,
showing currents produced by circuital electro-magnetic induction in
complete poleless circuits, the presence of molecules of residual gas or
vapor seems to be _the essential_. It seems certainly true that without
the molecules, electricity has no meaning. But in obedience to logic, I
must withdraw one expression I have used. We must not imagine the
“presence of molecules is _the_ essential.” It is certainly _an_
essential. Ether is certainly also _an_ essential, and certainly has
more to do than merely to telegraph to our eyes to tell us what the
molecules and atoms are about. If the first step towards understanding
the relations between ether and ponderable matter is to be made it seems
to me that the most hopeful foundation for it is knowledge derived from
experiment on electricity in high vacuum; and if, as I believe is true
there is good reason for hoping to see this step made, we owe a debt of
gratitude to the able and persevering workers of the last 40 years who
have given us the knowledge we have; and we may hope for more and more
from some of themselves and from others encouraged by the fruitfulness
of their labors to persevere in the work.”
61_b_. THOMSON’S EXPERIMENT. VELOCITY OF CATHODE RAYS. _The Elect._,
Lon., Oct. 5, ’94, p. 762; _Phil. Mag._, ’94.—The object of the
experiment of J. J. Thomson was to determine whether the velocity
approached that of light or that of molecules. The apparatus he employed
involved the rotating mirror, which was fully described in _Proc. Royal
So._, ’90, slightly modified. The rays were caused to produce
phosphorescence, while the mirror was so adjusted that when at rest, the
two images on the phosphorescent strips appeared in the same rectilinear
line. Many other elements comprised the apparatus. All the steps were
performed carefully and according to the best methods, but the results
are those which in this experiment are of particular interest, for by
knowing the velocity of the rays, their nature is better appreciated and
that of the X-rays can be better deduced. The velocity bore a close
relation to that of the mean square of the molecules of gases at
temperatures zero ° C. or in the case of hydrogen, 1.8 × 10^5 cm. per
second. As compared with such a velocity, that of the cathode rays was
found to be in the neighborhood of 100 times as great, and this agrees
very nearly with the velocity of a negatively electrified atom of
hydrogen acquired under the influence of the potential fall, which
occurred at the cathode. In further evidence of the verity of this
statement, he made a rough calculation upon the curve or displacement
produced by a magnet upon the rays. § 59. He stated: “The action of a
magnetic force in deflecting the rays shows, assuming that the
deflection is due to the action of a magnet on a moving electrified
body, that the velocity of the atom must be at least of the order we
have found.”
[Illustration:
FIG. 1.
]
61_b_. PERRIN’S EXPERIMENT. CATHODE RAYS CHARGED WITH NEGATIVE
ELECTRICITY. CORRESPONDING POSITIVE CHARGES PROPAGATED IN THE REVERSE
DIRECTION AND PRECIPITATED UPON THE CATHODE. _Comptes Rendus_, CXXI.,
No. 20, p. 1130; _The Elect._, Lon., Feb. 14, ’96, p. 523.—Jean Perrin’s
object was to discover whether or not internal “Cathode rays were
charged with negative electricity.” That they were had often been
assumed by others, namely, Prof. J. J. Thomson, who considered cathode
rays as due to negatively charged matter moving at high speed. § 61_b_.
Again, Prof. Crookes, principally, and others, showed that they were
possessed of mechanical properties and that they were deflected by a
magnet. § 59. Perrin called attention to the above investigations and
also alluded to the theoretical considerations of Goldstein, Hertz and
Lenard, who favored the analogy of cathode rays to light—whose phenomena
are well answered by the accepted theory concerning assumed etherial
vibrations, which, in both cases, have rectilinear propagation, § 57,
excite phosphorescence, § 54 and 55, and produce chemical action upon
photographic plates. Great ingenuity was displayed, as might be
expected, in the manner in which Jean Perrin proved the proposition
named in the title of this section, at the Laboratory of the École
Normale and also in M. Pallet’s Laboratory. First, therefore, let the
elements of the discharge tube be thoroughly understood. As usual, the
disk N is the cathode, referring to accompanying Fig. 1. A, B, C, D, is
a metal cylinder having a small opening at the right hand end toward the
cathode. Concentrically, is a similar cylinder, acting as an electrical
screen and having a like opening similarly located as indicated. It
corresponds to and plays the part of the Faraday cylinder, being
connected to earth. The principle involved in this apparatus was based
upon the laws of influence, which permitted him to ascertain the
introduction of electric charges within a conducting envelope, and to
measure such charges. During the discharge, the cathode rays were
propagated from the cathode to and within the cylinder A, B, C, D, which
immediately and invariably became charged with negative electricity. To
prove that the charge was due to the cathode rays, he deflected them
away from the opening in the protecting cylinder E, F, G, H. The
cylinder was not under these circumstances charged, the rays being
outside. He went further and made some quantitative analysis in a rough
way to begin with. He related: “I may give an idea of the amount of the
charges obtained when I state that with one of my tubes, at a pressure
of .001 m. of mercury, and for a single interruption of the primary
coil, the cylinder A, B, C, D, received sufficient electricity to bring
a capacity of 600 C. G. S. units to a potential of 300 volts.” Upon the
principle of the conservation of energy, he was induced, he said, to
search for corresponding positive charges. “I believe I have found them
in the very region where the cathode rays are generated, and that they
travel in the reverse direction and precipitate themselves on to the
cathode.” He verified this corollary by means of a modified feature of
the apparatus shown in Fig. 2. The construction was the same except that
there was a diaphragm having a perforation β´ within the protecting
cylinder and opposite the smaller cylinder exactly as indicated, so that
the positive electricity which had entered through β could only act on
the cylinder A, B, C, D, by traversing also the hole β´. “When N was the
cathode, the rays emitted traversed the two apertures at β and β´
without any difficulty, and caused the gold leaves of the electroscope
to diverge widely. But when the protecting cylinder was the cathode, the
positive flux, which, as was shown by a previous experiment, enters by
the aperture β, did not succeed in separating the gold leaves, except at
very low pressures. If we substitute an electrometer for the
electroscope we shall see that the action of the positive flux is real,
but that it is very small and increases as the pressure decreases.”
[Illustration:
FIG. 2.
]
He inferred that: “These results, taken as a whole, do not appear to be
easily reconcilable with the theory that the cathode rays are
ultra-violet light. On the contrary, they support the theory that
attributes these rays to radiant matter, § 54, near centre, a theory,
which may at present, it seems to me, be enunciated as follows: In the
vicinity of the cathode the electric field is sufficiently strong to
tear asunder into _ions_ some of the molecules of the residual gas. The
negative ions start off toward the region where the potential increases,
acquire a considerable velocity, and form cathode rays; their electric
charge, and consequently their mass (at the rate of one gramme
equivalent per 100,000 coulombs) is easily measured. The positive ions
move in the reverse direction; they form a diffused tuft, susceptible to
magnetism, but are not a regular radiation.”
61_c_. ZEUGEN. _Comptes Rendus_, Jan. 27, 1896.—In a note regarding the
experiments of Roentgen, called attention to his own communications to
the Academie des Sciences in February and August 1886, describing his
photographs of Mt. Blanc taken in the night by the invisible
ultra-violet rays. This note is entered as many newspapers reported the
photograph to be due to cathode rays, imagine the intense
phosphorescence upon a screen at the top of the mountain, if such were
the case.
62. GOLDSTEIN’S EXPERIMENT. PHOSPHORESCENCE OF PARTICULAR CHEMICALS BY
CATHODE RAYS. _Nature_, Lon. Feb. 21, ’95, p. 406. _Weid. Ann._, No. II,
’95.—Lithium chloride when acted upon by cathode rays, phosphoresced to
a dark violet color or heliotrope, which it retained for some time in a
sealed tube. Chlorides generally and other haloid salts of potassium and
sodium showed similar effects. The colors were superficial and could be
driven away rapidly either by heating or the action of moisture.
63. KIRN’S EXPERIMENT. SPECTRUM OF POST PHOSPHORESCENCE OF DISCHARGE
TUBES. _Wied. Ann._, May, ’94. _Nature_, Lon. June 7, ’94, p. 131.—Carl
Kirn compared the spectra of the phosphorescence of a vacuum bulb,
during and immediately after the discharge. The details are as follows:
The spectrum of the after-glow, § 54, at end and 22, was found to be
continuous. In this connection, see a plate showing different kinds of
spectra, for example, _Ganot’s Physics_, frontispiece. The spectrum
shortened from both directions to a band between the wave lengths of 555
and 495µµ. The spectrum then continued to grow shorter and shorter until
it disappeared at the line E, which is the position of the greatest
luminosity of the solar spectrum. For experiments on spectrum, see
Fraunhofer in _Gilbert’s Ann._, LVI. During the discharge, the
spectroscope showed a line spectrum corresponding very closely to those
of carbonic acid gas and nitrogen. Some authorities had suggested that
perhaps the after phosphorescence and the beginning of the incandescence
of a solid body, were the same kind of light, but this experiment shows
that such is not the case, unless some relation exists on the ground
that the two phenomena are exactly opposite to each other, and it
confirms similar results obtained by Morrin and Riess. The result
indicates that the nature of the phenomenon is not identical in all
respects with light produced at a high temperature.
63_a_. DE METZ’S EXPERIMENT. CHEMICAL ACTION IN THE INTERIOR OF THE
DISCHARGE TUBE. INTERNAL CATHODE RAYS. _L’Ind. Eler._, May 10, ’96, and
_Comptes Rendus_, about April, ’96. Translated by Louis M. Pignolet. He
used a cylindrical discharge tube divided into two halves which fitted
together by an air-tight ground joint. In one-half were the anode and
the cathode; in the other half was the holder containing the sensitive
paper or films. The holder was exposed to the direct action of the
cathode rays and was closed by a cover of cardboard or sheet aluminum.
The objects to be photographed were placed between the cover and the
sensitive film or paper. The tube was connected to a Sprengel pump which
maintained its vacuum during the experiments. In this way, twelve
photographs were taken from which it appeared that cathode rays, like
X-rays, penetrate cardboard and aluminum, but are stopped by copper
(1.26 mm.) and platinum (0.32 mm.). Poincaré, in a note in the same
publications as the foregoing, criticised the results of the experiments
of De Metz, claiming they did not prove irrefutably that cathode rays
possessed the essential properties of X-rays, for the cathode rays in
impinging on the cover of the holder would generate X-rays, § 91, which
would give the results obtained. Poincaré did not deny the fact.
63_b_. HERTZ’S EXPERIMENT. THE PASSAGE OF CATHODE RAYS THROUGH THIN
METAL PLATES WITHIN THE DISCHARGE TUBE. DIFFUSION. _Wied. Ann._, N. F.
45; 28, 1892. Contributed by request, by Mr. N. D. C. Hodges of the
_Hodges Scientific News Agency_, N.Y. Found in records at Astor
Library.—A piece of uranium glass was covered partly on one side (which
he calls the front side) with gold-leaf, and on the gold leaf were
attached several pieces of mica. This front side was then exposed to
cathode rays. So long as the exhaustion had not proceeded far, and the
cathode rays filled the whole tube with a blue cone of light, only the
portion of the uranium glass outside the gold-leaf screen showed any
phosphorescence. But as soon as the exhaustion had progressed far
enough, and the light began to disappear, the genuine cathode rays
struck the covered glass, and the phosphorescence manifested itself
behind the gold-leaf. When the cathode rays were fully developed, the
gold-leaf hardly had any effect, while the mica cast deep black shadows.
The same experiment was tried with silver-leaf, aluminum and alloys of
tin, zinc and copper. Aluminum showed the best results; sheets which
allowed no light to pass, allowing the cathode rays free passage. The
rays after their passage through the metal screens did not continue
their straight course, but seemed to be diffused much as light is
diffused by passing through a cloudy medium. In this connection
reference is made to the work of Goldstein, who had noticed also the
reflection of “electric” rays. _Wied. Ann._, N. F. 15; 246, 1882. In
1893, Goldstein published further accounts concerning actions in
discharge tube. _Wied. Ann._, vol. 48, p. 785.
[Illustration:
DIAGRAM OF LENARD’S APPARATUS. pp. 53 to 69.
]
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