which is covered with a layer of phosphoric anhydride, the platinum wires
are introduced, and the air is pumped out to a tension of only a few
millimetres. If the inductive current of a Ruhmkorff coil be sent through
the flask, the whole flask will be filled, as it were, with a violet
light, which otherwise only proceeds from the negative pole, and from
both electrodes a spectrum is obtained composed chiefly of shaded violet
bands.” The comparison of the spectrum of this violet glow with that of
the Aurora gives, according to Ångström, the following results:—
Aurora-lines, wave-lengths 4286 4703 5226
Violet light, wave-lengths 4272 4707 5227
Two weak light bands, found by Dr. Vogel at 4663 and 4629, are also
compared with other lines in the violet light 4654 and 4601; and the
Professor then concludes that it may be in general assumed that the
feeble bands of the Aurora-spectrum belong to the spectrum of the
negative pole, possibly changed more or less by additions from the banded
or the line air-spectrum.
[Sidenote: Bright line is due to fluorescence or phosphorescence.]
4thly. That the only probable explanation of the bright line is, that it
owes its origin to fluorescence or phosphorescence. The Professor remarks
on this point that “an electric discharge may easily be imagined which,
though in itself of feeble light, may be rich in ultra-violet light, and
therefore in a condition to cause a sufficiently strong fluorescence.” He
notes also that oxygen and some of its compounds are fluorescent.
[Sidenote: No need of Dr. Vogel’s theory of variability.]
5thly. That there is no need, in order to account for the spectrum of the
Aurora, to have recourse to the “very great variability of gas-spectra
according to the varying circumstances of pressure and temperature” (Dr.
Vogel’s theory). Professor Ångström does not admit such variability, and
does not admit that the way a gas may be brought to glow or burn can
alter the nature of the spectrum.
[Sidenote: Professor Ångström’s conclusions tested.]
In order to test some of the Professor’s conclusions in an experimental
way, I examined some tube and other spectra not only for line-positions,
but also for general resemblance to an Aurora-spectrum.
These experiments are detailed in the last Chapter, and the results are
comprised in Plates XIV., XV., and XVI., in which the spectra obtained
are represented in black for white.
[Sidenote: Result of examination of the Professor’s propositions.]
The result of the examination of Professor Ångström’s principal
propositions seems to be this:—
1st. Two Auroral spectra. I agree in this, but question whether the
fainter lines may not possibly comprise more than one spectrum.
2nd. I agree also that the bright yellow-green line falls, as Professor
Ångström describes, just behind the second line in the hydrocarbon yellow
group (see Plate V. fig. 7). And I find, in common with the Professor, no
well-marked or prominent line in the air-spectrum with which it accords.
3rd. This may be conveniently divided into two parts, viz.:—
A. The proposition that “moisture in the region of the Aurora must be
regarded as _nil_.”
[Sidenote: Moisture probably not _nil_ in the Aurora region. Reasons for
this given. Aurora in vapour or mist. Frequently near to earth’s surface.]
Here I see reason to differ, since (to quote a letter of Mr. Procter’s)
“the vapour-density of OH₂ is only 9 against 14 for N and 16 for O;” and
again, “electrical or heat-repulsion may carry water-dust up to enormous
heights.” There are, too, I think, circumstances connected with the
Aurora itself which make the assumption of moisture being _nil_ in the
Auroral regions untenable. The first of these is the fact that the white
arc, streamers, and floating patches of light, found in some Auroræ,
have frequently the peculiarly dense and solid look of vapour-clouds—a
circumstance with which I have been frequently struck. Mr. Procter and
others have also remarked that the Aurora is generally formed in a sort
of “mist or imperfect vapour.” The second, that Auroræ, or portions of
them, are frequently near to the earth’s surface. Instances of this are
given in the section on the Height of the Aurora, notably the experiences
of Sir W. Grove and Mr. W. Ladd.
[Sidenote: Coincidence of Auroral lines with telluric solar lines.]
On this point, too, note the peculiarities of the red line, which (and,
as I find, the green line also) are coincident with, or very close
to, telluric bands or groups of lines in the solar spectrum usually
attributed to moisture. (See Plate XIII. fig. 2.)
[Sidenote: Continuous spectrum.]
I think we may also claim the continuous spectrum in the Aurora in
further proof of water-vapour (see Plate XV. spectrum 7). The continuous
spectrum of the Aurora is also, to my observation, more local and dense
in the spectroscope than the glow generally seen between the lines or
bands in gas-spectra.
[Sidenote: Violet-pole spectrum discussed. Most spectra have a general as
well as special character.]
B. The question of the violet-pole spectrum. Here I make the remark that
in comparing other spectra with that of the Aurora, it is, I think, too
much the practice to trust to the coincidence (more or less perfect) of
one or perhaps two lines out of many; whereas we know by experience that
most spectra have so well-marked a general as well as special character
that, when once seen, they are recognized afterwards with the greatest
ease and without measurements. An experience and proof of this is found
in a set of “Photographed Spectra” which the Autotype Company have
reproduced for me.
[Sidenote: Coincidence of one or two lines not sufficient to establish
identity.]
Of course no two given spectra can be considered identical unless their
principal lines coincide; but, on the other hand, the coincidence
of one or two lines out of many, without other features, cannot be
satisfactorily or conclusively held to establish identity.
[Sidenote: Ångström’s compared spectra.]
In Professor Herschel’s letter (Phil. Mag. ser. 4, vol. xlix. p. 71),
Professor Ångström’s representation of the “spectrum of the glow
discharge round the negative pole of air-vacuum tubes” is given, in
comparison with the Aurora-lines and those of olefiant gas. This
illustration is here introduced.
Ångström’s representation of the Spectrum of the glow discharge round the
negative pole of Air-vacuum tubes, and its comparison with the Spectrum
of the Aurora.
[Illustration: Wave-lengths, in hundred-thousandths of a millimetre.]
It is unfortunate that in this illustration and in Professor Herschel’s
paper the wave-lengths of the Aurora-lines are not given in figures, but
must be roughly calculated from the scale. Professor Herschel speaks of
Ångström’s drawing as representing a _normal_ spectrum, and as derived
from authentic sources, such as Vogel, Barker, and others; but beyond
this we are not certain as to its origin.
In illustration of the difficulty of constructing any thing like a
general typical Aurora-spectrum I append a Table of eight Auroral spectra
taken at hazard:—
[Sidenote: Table of compared Aurora.]
Auroral lines and bands.
---------------------+----+---------+------+-----+-----------+-----------
Observers. |Red.| Yellow. |Green.|Blue.| Indigo. | Violet.
---------------------+----+----+----+------+-----+-----+-----+------+----
| | | | | | | |{4694}|
Vogel, April 9, 1871 |6297|5569|5390| 5233 | 5189| 5004| — |{ to }| —
| | | | | | | |{4629}|
| | | | | | | | |
Barker, Nov. 9, 1871 |6230|5620| — | — | 5170| 5020| 4820| — | —
| | | | | | | | |
| | | |{5330}| |{5050| 4930| 4740}|
Barker, Oct. 14, 1873|6300|5550| — |{ to }| — |{ to | to | to }|4310
| | | |{5200}| |{4990| 4850| 4670}|
A. Clarke, junr., | | | | | | | | |
Oct. 24, 1870 | |5690| — | 5320 | — | — | 4850| — |4350
| | | | | | | | |
Backhouse, 1873 |6060|5660| — | — | 5165| 5015| — | 4625 |4305
| | | | | | | | |
Backhouse, Feb. 4, | | | | | | | | |
1874 | * |5570| — | — | 5180| 4980| 4830| 4640 |4320
| | | | | | | | |
H. R. Procter, 1870 | * | * | — | * | — | — | — | * | *
| | | | | | | | |
Lord Lindsay, 1870 | — | * | * | * | * | — | — | — | *
---------------------+----+----+----+------+-----+-----+-----+------+----
* Mr. Procter’s and Lord Lindsay’s lines had no wave-lengths.
[Sidenote: Ångström’s drawing discussed.]
On examining Ångström’s diagram it certainly seems to me that, upon the
showing of the drawing itself, the coincidences are not very exact.
All three of the violet-pole bands appear to be less refrangible than
the Aurora-lines with which they are compared-the middle one (at 47)
considerably so, the one near E (at about 52·30) appreciably so, and the
third (at 43) slightly so.
[Sidenote: Diagram of Vogel’s Aurora and violet-pole spectrum.]
As it seemed desirable to adopt a specific Aurora-spectrum for
comparison, and to show such comparison on a somewhat larger scale than
Ångström’s drawing, I prepared the diagram shown on Plate XI. fig. 1. The
upper spectrum is Vogel’s, already described and figured on Plate XIII.
The lower spectrum is that of “Air, violet pole,” Plate XV. spectrum 2,
graphically shown. I can only find one absolute coincidence in the two
compared spectra in this diagram.
It should, too, I think, be borne in mind that there is a great
difference in the character of the compared spectra, whether as shown in
Ångström’s drawing or mine—the bands of the violet-pole spectrum mostly
degrading towards the violet, while the lines or bands of the Aurora in
no way possess that character[14].
[Sidenote: Dr. Vogel’s violet-pole and Aurora-lines.]
To assist in the foregoing violet-pole comparison I add the following
Table derived from Dr. Vogel’s memoir:—
Violet-pole lines. Aurora-lines.
W.L. W.L.
6100,} broad, moderately
5945,} bright stripe 6297, very bright stripe.
5459,} broad, moderately {5569, brightest line of spectrum.
5289,} bright stripe {5390, extremely faint line.
5224, very bright line 5233, moderately bright.
5147, faint line 5189, moderately bright.
5004, bright line 5004, very bright line.
4912, fainter than last.
4808, very faint line. {6694,}
4704, very intense line. {4663,} band less brilliant in
4646, very faint line. {4629,} the middle.
4569, moderately bright.
4486, moderately bright.
4417, quite faint line.
4346, moderately bright line.
4275, very bright line.
On examination of these figures it will be seen that 5224 and 5233 are
fairly close, and that 5004 is coincident. Beyond these there is little
to identify the spectra.
[Sidenote: Conclusions arrived at adverse to the violet-pole theory.]
As the general result of my observations and a comparison of the
foregoing spectra and tables, I see no reason for giving to the
violet-pole glow any special or distinguished place in a comparison with
the Aurora, and certainly not for assigning to it the nearly absolute
monopoly of the spectrum. It is true that the line γ in the violet-pole
glow (Plate XV. spectrum 2), which, by the way, degrades towards the
red, is in close coincidence with one of the Aurora-lines; but how
are the brighter bands α and β accounted for? These, as I have before
pointed out, alone survive when the tube is placed at a distance from
the slit. It is true they are thus reduced to shaded-off lines in lieu
of bands; but the difficulty still remains, that they are conspicuous
for their absence in the Aurora-spectrum. On the whole, I cannot but
conclude that Professor Ångström’s theory fails. At all events, if the
violet-pole glow-spectrum is to represent the Aurora-spectrum, it must
be under conditions different from those by which it obtains in dry-air
vacuum-tubes or flasks at ordinary temperature.
[Sidenote: Phosphorescence or fluorescence of the yellow-green line.]
4th. I feel more in accord with Professor Ångström’s memoir upon the
subject of the phosphorescence or fluorescence of the bright yellow-green
Aurora-line.
[Sidenote: External features of Auroræ confirmatory of this.]
I do not notice that the Professor touches upon the external features of
the Aurora in respect of this question.
October 20, 1870.—I noted the grand Auroral display of that evening,
including “streamers of opaque-white _phosphorescent_ cloud very
different from the more common transparent Auroral diverging streams of
light.”
February 4, 1872.—A fine display. The first signs were (in dull daylight)
“a lurid tinge upon the clouds, which suggested the reflection of a
distant fire, while, scattered among these, torn and broken masses of
white vapour, _having a phosphorescent appearance_, reminded me of a
similar appearance in October 1870.” (Other instances of this effect
will be found in the section Aurora and Phosphorescence.) Day Auroræ,
too, we might suppose could hardly be seen without the presence of some
phosphorescent glow.
[Sidenote: Other confirmatory circumstances. Conclusion in favour of the
theory.]
Having regard to the near proximity of the phosphoretted-hydrogen band to
the bright Aurora-line, to the circumstance of this band brightening by
reduction of temperature (a phenomenon probably connected with ozone),
to the peculiar brightening of one line in the green in the “Aurora” and
“phosphorescent” tubes (the phosphorescent tubes probably containing
O), and to the observed circumstance that the electric discharge has
a phosphorescent or fluorescent after-glow (isolated, I believe, by
Faraday), I feel there is strong evidence in favour of such an origin to
the principal Aurora-line, if not to the red line as well.
[Sidenote: Invariability of gas-spectra questioned.]
5th. Professor Ångström opens a wide door to discussion in his
proposition of the invariability of gas-spectra, and I do not now attempt
to follow in detail this interesting part of the present subject. Suffice
it to say, that if the Professor lays down this proposition in its
strictest sense (I can hardly suppose he so meant it), there is, so far
as I am aware, no one spectrum that can at all claim comparison with the
Aurora-spectrum. Giving greater latitude to the Professor’s words, I
reply, upon competent authority, that lines vary in number and brilliancy
with temperature, and in breadth with pressure. Kirchhoff, too, in
speaking of vapour-films as increasing the intensity of lines, states
“it may happen that the spectrum appears to be totally changed when the
mass of vapour is altered.” We may, too, now add magnetism as capable of
effecting a change in certain spectra, not only as to brilliancy, but
even as to position of lines. (Chautard’s Researches, ‘Philosophical
Magazine,’ 4th series, vol. 1. p. 77, and experiments detailed in Chap.
III. of this work.)
CHAPTER XIII.
THE OXYGEN-SPECTRUM IN RELATION TO THE AURORA (PROCTER AND SCHUSTER).
[Sidenote: Procter’s oxygen-spectrum.]
In a communication to ‘Nature,’ Mr. H. R. Procter has pointed out an
apparent coincidence in position of several of the Auroral lines with
those of a spectrum occasionally obtained from air at low pressure with
a feeble discharge. It is, he says, sometimes exhibited in lumière
(phosphorescent?) tubes, and he believed it, in part at least, to be the
spectrum described by Wüllner (Philosophical Magazine, June 1869) as a
new spectrum of oxygen.
[Sidenote: How obtained.]
He had obtained it very vividly in pure electrolyzed oxygen with a feeble
discharge, but some perplexing observations made him doubtful of its
origin.
Plate XI. fig. 4 gives a representation of this spectrum as shown by Mr.
Procter, except that my drawing is in black for white.
[Sidenote: Compared spectra described.]
The upper spectrum is that above mentioned, the centre one that of the
Aurora, the lower one the lines of Na and H for comparison. The Auroral
yellow-green line, in January 1870, was found by Mr. Procter coincident
with a bright line or band in the tube (with a spectroscope of a 60°
bisulphide prism, and magnifying-power about six). The third and fifth
lines in the Aurora seemed also to correspond with tube-lines. As to
these Mr. Procter says they were not bright enough to be compared with
the same accuracy as the yellow-green line, but that the positions could
not be far wrong.
[Sidenote: Mr. Procter’s subsequent views. Yellow-green line traced to
some form of hydrocarbon.]
Mr. Procter subsequently (‘Edinburgh Encyclopædia,’ art. “Aurora”)
considered he traced the yellow-green tube-line to some form of
hydrocarbon. On examination with instruments of greater dispersion, it
was found that, though more refrangible than the first band of citron
acetylene (candle-flame), it was less so than the Aurora-line. The
tube-band, too, was shaded towards the violet, which was not the case
with the Aurora-line.
The question as between hydrocarbon and oxygen I did not then consider as
disposed of. With the lumière tubes the question might be open, but I did
not see how it could be in the case of the electrolyzed oxygen-spectrum.
From a comparison of the tube-spectra, I have shown that although
the spectra of the carbon and oxygen tubes are proved to be,
photographically, as a whole, distinct, they have, as to position of
some of the principal lines in the central part of the spectrum, a very
close resemblance.
[Sidenote: Probability that O may play a part in the Aurora-spectrum.]
That oxygen may in some form play a part in the Aurora seems highly
probable; how far it is spectroscopically detected seems a different
question.
[Sidenote: Difference between air-spark and tube-spectra.]
Ångström and Herschel suggest its presence in the Aurora in connexion
with phosphorescence or fluorescence. With a spark-discharge in air at
ordinary pressure, a mixed spectrum of bright lines of N and O is found;
while in the case of Geissler vacuum-tubes (representing a glow-discharge
in a much more rarefied atmosphere) the N lines appear mainly to usurp
the spectrum.
[Sidenote: H₂O tube referred to.]
It must, however, be borne in mind that a Geissler tube, as to
temperature at least, in no way represents the conditions of the Aurora;
and when we remember the association of oxygen and ozone, and the way in
which the latter is affected by heat, it may well be that temperature
plays an important part in the matter. In proof of this conduct of
oxygen, it may be cited that, in the case of a H₂O tube, the H lines
come out sharp and brilliant in the spectrum, while what is seen of the
O lines is comparatively weak, misty, and ill-defined. Vogel, it will be
remembered, makes 5189 of the Aurora coincident with an O line.
[Sidenote: Residual phosphorescence in Geissler tubes. Garland tube.]
Professor Herschel has pointed out, and I have confirmed, that the
residual phosphorescence in Geissler tubes, after the spark has passed,
is probably associated with oxygen. He also alludes to the fact that when
one of the globes of a “Garland” tube was heated, it did not shine after
the spark had passed, apparently because of the destruction of the ozone
by heat.
[Some experiments with a tube of this description will be found detailed
in Part III. Oxygen was not, I think, the gas it was filled with.]
[Sidenote: Dr. Schuster’s tubes described.]
Subsequently to my examination and comparison of the O and CO₂ spectra
before detailed, Dr. Arthur Schuster was good enough to send me three
vacuum-tubes of his own preparation, showing an oxygen-spectrum.
One, with large disk-shaped brass electrodes, was unfortunately broken in
transit. Dr. Schuster informed me it showed the carbonic-oxide spectrum
as well as that of oxygen. The other two tubes had aluminium electrodes.
They were similar in shape to ordinary Geissler tubes, but had attached
to each a supplemental bulb containing dry oxide of manganese.
Illuminated by the larger coil, one of these tubes (which had a slight
crack in the manganese bulb) lighted up faintly; the other was fairly
bright, and the glow had a somewhat reddish tint.
Plate XVIII. fig. 15 represents as the upper spectrum Vogel’s Aurora,
with W.L. numbers, as the middle spectrum the capillary part of Dr.
Schuster’s O tube, and as the lower spectrum the negative (violet) pole
of the same tube.
[Sidenote: Spectra described.]
The tube-spectra were mapped out with the aid of the diaphragm micrometer
before described.
[Sidenote: Capillary.]
The capillary spectrum was mainly distinguished by four bright sharp
lines—one in the red, between the red Aurora-line and D, two in
the green, but considerably more refrangible than the yellow-green
Aurora-line, while the fourth was found to be hydrogen F. The other lines
in the spectrum were considerably fainter, and misty and band-like. The
red line, though not brilliant, was fairly bright and sharp.
The place of the less refrangible of the two bright bands in the
violet-pole spectrum was occupied in the capillary spectrum by a faint
glow only.
[Sidenote: Violet-pole.]
The violet-pole spectrum was recognized by two very bright broad
bands of light in the green, each including within its limits one of
the Aurora-lines. The bright red line in the capillary had a faint
representative in the violet-pole spectrum, as also had the two bright
lines in the green. Other fainter lines appeared in the blue, and three
fairly bright ones towards the violet.
[Sidenote: Dr. Schuster’s remarks on the spectra.]
Dr. Schuster remarks that one of these O bright bands is closely
coincident with a band in the CO spectrum, but that the CO band is bright
towards one edge and fades off gradually thence, while the O band is of
pretty uniform strength throughout. Dr. Schuster finds the wave-lengths
of the violet-pole O bands to be as follows:—
5205·0}
5292·5} Brightest part 5255.
5552·8}
5629·6} Brightest part 5586.
[Sidenote: His tubes free from impurity.]
He also gives as weak bands 5840-5900 and 5969-6010. Dr. Schuster comes
to the conclusion that the green line of the Aurora is not due to
oxygen, as, under considerable dispersion and with good definition, the
oxygen-bands can be broken up into a series of lines, when the brightest
part is found to lie at 5586, which is too much towards the red to
compare with the Aurora-line. He notices that the more refrangible of the
O bands corresponds with a line sometimes seen in the Aurora (Vogel’s
5233). The same remark will, however, apply to this last as to the other
coincidence, viz., that a broad band can hardly represent a line—at
least, the line can only be said to coincide in a loose and indefinite
way. It is evident that Dr. Schuster’s tubes were free from what must
now be considered an impurity in those examined by me and by Dr. Vogel,
and that Mr. Procter’s suspicions of carbon impurities in these, and the
ordinary oxygen-tubes, are thereby quite confirmed.
[Sidenote: Experiments with an open Geissler tube.]
In some experiments which we made (after receiving Dr. Schuster’s tubes)
with an open Geissler tube, so arranged as to connect with an air-pump
and gas-receiver, and thus from time to time to wash out the tube and
vary its contents, we found the same impure spectrum as in the case
of the sealed O tubes; and it seems to require a very large amount of
precaution to avoid these impurities.
[Sidenote: Spectra of Dr. Schuster’s O tube examined.]
Dr. Schuster was kind enough to examine the spectra I mapped out, and
which are shown in Plate XVIII. fig. 15, with the following results:—The
lines Oα, Oβ, Oγ are those he has referred to under that designation in
his communications to ‘Nature,’ and undoubtedly belong to oxygen. The
bands A, B, and C are the bands characteristic of the negative pole.
He finds A divided into two parts by a dark space. The spectrum of the
negative pole, under good exhaustion, stretches into the capillary part;
hence B appears in the capillary as a faint band. A similar thing happens
with nitrogen. I., II., III., and possibly 8 and 9, he thinks, are due
to the spark-spectrum of oxygen, obtained when the jar and a break are
interposed, the brighter lines of the line-spectrum being always present
at the negative pole. These last-mentioned lines I have already referred
to, as having been found by me in a tube showing phosphorescence after
the spark has passed. (Compare Plate XVIII. fig. 15, O violet pole, with
Plate XV. spectrum 5.) Nos. 1 and 2, he thinks, are due to some foreign
matter, as they are not in all his tubes.
Dr. Schuster often finds that a spectrum due to the aluminium electrodes
is seen in tubes under great exhaustion; and this he considers is the
spectrum of aluminium oxide. A drawing of this spectrum is found in
Watts’s ‘Index of Spectra,’ plate iii., “Aluminium first Spectrum.”
To this, he thinks, are also due the bands, or sets of lines in my
aluminium-arc spectrum (‘Photographed Spectra,’ plate ii.), and he
believes lines 3, 4, 5, 6, and 7 in the mapped-out spectra are due to it.
It would thus appear that the lines due to O are few in number, and do
not well compare with the Aurora-spectrum.
PART III.
MAGNETO-ELECTRIC EXPERIMENTS IN CONNEXION WITH THE AURORA.
INTRODUCTION.
[Sidenote: Object of experiments. Description of apparatus employed.
Electro-magnet. Battery.]
The set of experiments detailed in Chapters XIV. to XIX. was mainly
conducted for the purpose of testing, in connexion with the Aurora,
the action of a magnet upon the electric glow _in vacuo_ and on the
spark at ordinary pressure. It also includes some observations on the
glow from the violet pole with and without the magnet, and on the
glow obtained from one wire only. The apparatus employed was a Ladd’s
electro-magnet, with poles 10¼ inches high by 2 inches across, each
pole being surrounded by a movable helix, composed of two sets of stout
copper wire wound together, so that they could be used either in one
length or as independent coils excited at the same time. The latter form
of arrangement was employed by us. In most of the experiments conical
armatures were employed for the purpose of bringing the action of the
poles to bear upon the subjects examined. A contact-maker was added to
the magnet, so that it could be put rapidly in or out of action without
disturbing the wires. The battery used to excite the magnet was of the
form known as that of Dr. Huggins, and consisted of four vulcanite
cells in a frame, each holding seven pints of bichromate solution, and
containing two carbon and one zinc plate, each 13½ by 6 inches.
[Sidenote: Small coils. Larger coil. Magnetic curves obtained.]
A winch and pulley enabled the whole set of plates to be lowered into
the liquid and withdrawn at pleasure, and the large quantity of solution
gave the battery a considerable amount of constancy. We found it could be
used for two evenings’ work, of four hours each, without any material
dropping in power. For obtaining the glow in the Geissler and other small
tubes, a Ruhmkorff coil, giving a ½-inch spark, excited by one plate of a
½-gallon bichromate (bottle form), was used. For the glow in the larger
tubes and the spark in air a larger coil, giving a two-to three-inch
spark, and worked by two ½-gallon double-plate bichromates, was employed.
Notes were taken of the experiments, and drawings of the effects at the
time; and these are reproduced almost literally in the text and Plates
comprised in this Part. To ascertain the direction and extent of the
magnetic curves, we covered large sheets of cardboard, placed over the
poles, with iron filings; excited the magnet so as to obtain the curves,
and then obtained permanent prints from the filings by spraying the
cardboard with tannin solution. The magnetic effects were thus found to
extend to a radius of at least ten inches (see diagram, Plate XVII. fig.
1, showing magnet-poles and curves on a ¼ scale).
[Sidenote: Chautard’s investigations kept in view. Evidence obtained of
change in colour, form, &c. of Aurora. Ångström’s flask-experiment tried.]
In the vacuum-tube experiments we held Mons. J. Chautard’s investigations
(on the action of magnets on rarefied gases in capillary tubes rendered
luminous by the induced current, Phil. Mag. 4th series, vol. 1. p. 77)
in view. We obtained in our experiments plenty of evidence of a change
of colour and form in the discharge under the magnetic influence;
and both simple and compound spectra were found to be much varied by
the exaltation or suppression of some parts of the spectrum, so that
apparently new lines sprang up; but we failed to trace actual change of
position or wave-length in any given line, though we carefully looked for
it. A portion of our researches was directed to the subject of Ångström’s
experiment of filling a dry flask with a violet glow, analogous to
that from the negative pole. We entirely failed in obtaining the same
result while two wires and an uninterrupted circuit were employed. When,
however, we attached a negative wire only (the other wire being left
free) to an exhausted globular receiver, we obtained an effect very
similar to that referred to in Prof. Ångström’s memoir.
[Sidenote: General results of experiments. Bulb effects noticed as a mode
of analysis of gases.]
The general result of the experiments was to prove, assuming the Aurora
to be an electric discharge, the great influence the magnetic forces may
exercise on the colours, form, motions, and probably the spectrum also
of that phenomenon. It is easy to conceive that the variation in number,
and intensity of the lines which has been remarked in Auroral spectra
may have its origin in such a cause. The influence of the magnet on the
capillary stream was mainly in colour and intensity; but in the bulbs the
effects were still more marked and striking, and, in a greater or less
degree, different in the case of each gas which we examined. A careful
and extended study of these effects, conjointly with the changes in the
spectrum, might possibly form a new and valuable mode of analysis of
compound gases. This is well illustrated in the case of the iodine and
sulphur tubes which we examined.
[Illustration: Plate XVII.]
CHAPTER XIV.
EXAMINATION OF GEISSLER TUBES UNDER ACTION OF THE MAGNET.
_Nitrogen-tubes._
[Sidenote: Nitrogen-tube No. 1. Discharge described. Spectrum described.
Capillary stream. Positive bulb. Violet-pole glow.]
(1) A small Geissler tube (No. 1) was lighted up by the small coil.
The capillary part showed a very bright, slightly rosy-tinted stream.
Negative bulb was filled with rosy-purple light, the violet-pole glow
being confined to the extent of the electrode. Positive bulb of the
same rosy-purple colour, but stream slightly contracted in volume. Glow
throughout quiescent, and no stratification in the tube. A compound-prism
spectroscope, taking in the whole of the spectrum, showed in the
capillary stream, from yellow to red, a fairly bright wedge, having a
dark band in the centre, and six bright columns, with dark lines at
intervals, shading off on either side. On the more refrangible side of
the yellow, the spectrum was composed of a set of bright bands and lines
in the green, blue, and purple, one line only (in the green) standing
out very bright. In the yellow and red no bright line stood out alone.
The positive bulb gave a fainter spectrum of the same character, mainly
confined to the centre, the violet, yellow, and red not being well seen.
When the violet-pole glow was examined, the general character of the
spectrum was quite changed: a brilliant broad band in the violet, a
bright narrower one in the blue, and two bright lines in the green, with
intermediate fainter lines throughout, were the main features. The yellow
and red part of the spectrum was also changed. The yellow was fairly
and evenly distinct up to the dark band; then came a somewhat brighter
orange band, and after that the red, but rather obscure and cut off. No
absolutely bright line could be traced in the red.
[Sidenote: Nitrogen-tube No. 2. Glow described. Difference of spectra of
capillary stream and violet-pole glow. Junction of the violet-pole glow
and capillary stream.]
(2) To compare the capillary stream and the violet-glow, a second
nitrogen-tube (No. 2) was used. This tube was larger in bulk and bore
than No. 1. The glow in the bulbs was considerably fainter and more
salmon-coloured; and there was much stratification in both, extending
to the capillary bore. (This stratification was considered due to H,
as the three principal lines of that gas came out very brightly in the
spectrum.) The difference of the spectra of the capillary stream and of
the violet-pole glow was extremely well marked—the former consisting of
a set of bright lines and bands of fairly uniform intensity, while the
latter was split up into a few bright bands with fainter lines between.
The yellow and red of the violet-glow were very weak as compared with
the same region of the capillary spectrum. No bright line appeared in
the red. The tube being properly adjusted for the purpose, the junction
of the violet-pole glow and the capillary red-glow was easily observed.
The bright bands of the violet-pole were seen to run into the capillary
line-spectrum, and then, gradually getting finer and more pointed, to
fade out.
[Sidenote: Tube No. 1 between the poles of the magnet. Change of colour
in capillary stream. “Tailing-over” of capillary stream.]
(3) The capillary part of tube No. 1 was arranged between the poles of
Ladd’s electro-magnet, the conical ends of the armatures almost touching
the tube (Plate XVII. fig. 1). With the magnet not excited, the capillary
stream was bright and of a slightly rosy-yellow tinge. It varied a little
in apparent diameter with the current. As soon as the magnet was excited
the capillary stream, as also (in a less degree) that in the bulbs,
were seen to contract, and to change from a _rosy_ tint to a distinctly
_blue-violet_. The polished armatures, acting as reflectors, showed this
change of tint in a most marked manner each time the magnet was excited.
At the same time the capillary stream was seen to run into the negative
bulb, as if overflowing, and with an effect resembling the “tailing-over”
of a gas-flame. This effect took place each time the magnet was excited,
and was not found at the positive-bulb end.
[Sidenote: Spectrum examined.]
Occasionally, when the magnet was excited, flashes of light were
discharged in the negative bulb from the capillary towards the
violet-pole. The spectrum was then carefully examined. No change was
seen in the actual position of any of the lines or bands when the tube
was influenced by the magnet, but those towards the violet end of the
spectrum were conspicuously brightened.
[Sidenote: Negative bulb between poles of the magnet. Positive bulb
within action of the magnet.]
(4) The extremity of the negative bulb was now placed between the poles
of the magnet. A bright violet-coloured arc, following the magnetic
curve, was at once formed, as in the case of the large Plücker tubes; and
at the same time a straight stream of not very bright light ran along the
bulb. The positive bulb was next placed within the action of the magnet;
and immediately a brilliant spiral of flickering light appeared in the
bulb, lighting it up, and reminding one in shape of the spiral which
water forms on being poured from a lipped jug (see Plate XVII. fig. 9).
[Sidenote: Spiral formed.]
This was repeated each time the magnet was excited. The spiral, though
flickering in character, was permanent in form, and inclined to the side
of the tube which was in contact with the N pole of the magnet.
_Oxygen-tubes._
[Sidenote: O tube No. 1; spectrum described.]
A tube (No. 1) was lighted up and examined with the spectroscope, and
found to give the spectrum shown on Plate XIV. spectrum 3, but with a
strong set of H lines in addition.
[Sidenote: O tube No. 2; spectrum described.]
A second tube (No. 2) was then lighted up. The spectrum was a bright one,
similar to the foregoing, the principal H lines being present, but not
strong.
[Sidenote: Tube-glow described. Effect upon glow when magnet excited.
Bulbs between poles of the magnet. Effect of magnet on spectrum.]
The red region was indistinct, and showed no prominently bright line.
The bulbs were mainly of a slightly blue-grey tint, with a steady glow.
Capillary stream quite pale white, with a very slight tinge of red.
Violet-glow small and confined to the electrode. Upon the magnet being
excited, the capillary stream became intensely brighter, and the glow
in both bulbs contracted into a single bright stream, which curved
towards the sides of the bulbs at right angles to the magnetic poles, and
changed from side to side with the current. This effect was very marked,
and was more apparent in the positive than the negative pole. A faint
stratification was seen in both bulbs. Upon either bulb being placed
between the armatures, the glow left the electrode point and condensed
into one bright stream, running along the side of the tube and curving at
each end (Plate XVII. fig. 10). No trace whatever of tendency to form a
spiral was seen. The spectrum with the magnet on was very conspicuously
brightened up throughout. A set of fluted bands with a bright line
among them appeared in the red, and several lines or bands appeared in
the violet which could not be seen before. The bright red line, upon
measurement, proved to be the hydrogen-line C. It thus seemed brighter
in proportion than the F line, although, with the magnet off, the latter
was well seen, while the C was not. No actual change in position of the
spectrum-lines could be detected.
[It is to be noticed that the O tubes employed were those used by me
in former experiments, and had the bright lines now attributed to
hydrocarbon impurity. Their bulb-effects differed, however, entirely from
those of the CO₂ tube. (Compare figs. 10 and 11, Plate XVII.)]
_Hydrogen-tubes._
[Sidenote: H tube, No. 1; glow described.]
A small H Geissler tube (No. 1) was selected, and lighted up by the small
coil. The capillary was a bright white-pink stream, with a tendency to
redden at times. The bulbs were both of a faint blue-grey tint, with
coarse lenticular stratification. The violet-pole glow was pale and
white as compared with that of N.
[Sidenote: When the magnet was excited, whole character of tube changed.
Unexcited spectrum described. Effect when magnet was excited.]
When the magnet was excited, the whole character of the tube changed.
The capillary stream diminished in brightness and in apparent volume,
and changed to a deep amber-yellow. The bulbs lost some of their light,
and their coarse stratification; being, in lieu, filled with a vertical
condensed stream of moderate light, in which a fine stratification only
was seen. The stream in the positive bulb had a tendency to the spiral
form. The capillary, each time the magnet was excited, “tailed over”
into the negative bulb, as in the case of N, looking as if it were
squeezed out of the capillary bore. The unexcited spectrum was found to
consist of the usual principal lines of H on a continuous glow, with the
intermediate bands and finer lines, which are usually suspected to be due
to impurity. The sodium-line was also seen. When the magnet was excited,
the spectrum grew much fainter—the continuous glow in the red and blue,
and the red and blue lines, nearly disappearing, and the line in the
green alone shining out conspicuously. No change of place in the lines
could be noticed.
[Sidenote: No. 2 H tube; effects described.]
A longer H tube (No. 2) was then tried, with similar effects, except that
the diminution in brightness was not so conspicuous. When the negative
bulb of the tube No. 1 was placed between the poles of the magnet, a
stream of light was formed, and the stratification became finer. The same
effect took place with the positive bulb, with a tendency to the spiral
form.
_Water-Gas (H₂O) tube._
[Sidenote: Water-gas (H₂O) tube; effects produced described.]
A faint purple glow was seen in each bulb, the tube not lighting-up
brightly. The capillary showed a slightly rosy-tinted, grey stream of
brighter light. With the magnet on, the glow in the bulbs was condensed
into a single bright stream. The capillary brightened up, and assumed a
yellow tint—this effect being principally confined to that portion which
was between the conical ends of the armatures, and gradually diminishing
as the distance increased from these. Without the magnet, the principal
H lines showed brightly in the spectrum, the O lines being misty and
indistinct. With the magnet on, the O lines and spectrum generally
brightened up.
_Ammonia-tube._
[Sidenote: Ammonia-tube; lighting-up described. Spectrum described.]
This tube was difficult to light up. Hardly any light was seen in
the bulbs, except a very faint purple glow at the electrodes. In the
capillary part a fairly bright stream of purple-white light appeared. The
spectrum was a faintly shown one of N and H. The effect of the magnet
was to reduce the brightness of the glow in the capillary, but with
little marked action on the bulbs, except to condense the faint glow into
a slightly bright stream running along the side of the tube.
On a subsequent examination the tube and spectrum both brightened up
under the influence of the magnet. The N lines, which were faint without
the magnet, shone out under its influence distinctly—the red and yellow
parts of the spectrum specially showing this effect. The H lines also
brightened up, but hardly so much in proportion as the N.
_Carbonic-Acid tube._
[Sidenote: Tube marked C A; lighting-up described.]
A Geissler tube marked C A was examined. Capillary stream a brilliant
bluish white; bulbs grey-blue, with a slight tint of green; slight
stratification in positive bulb; stream diffuse, not quite filling
the bulbs, and changing in volume as the coil-break was touched; glow
round the violet-pole considerable, but markedly white in tint, rather
than violet; stratification strong in capillary. With magnet excited,
the capillary stream diminished in volume, but greatly increased in
brightness. It “tailed over” into the negative bulb, and the stream
through both bulbs curved towards the sides. A slight pattering noise
was heard in the tube. In the positive bulb bright, imperfectly formed,
saddle-shaped rings of light, with a tendency to spiral formation, were
seen, somewhat similar to the effects in the Plücker tube after described
(see Plate XVII. fig. 11).
[Sidenote: Effects when magnet was excited.]
The whole spectrum, under influence of the magnet, became much brightened
up. Faint bands in the red came out bright, as also did some in the
violet. The violet-glow was examined (without the magnet), and the light
was found condensed into four prominent shaded bands, one red, one
yellow-green, one green, and one blue, with fainter bands seen between.
_Chlorine-tubes._
[Sidenote: Chlorine-tube No. 1 lighted-up. Action of magnet upon the tube
and spectrum.]
A chlorine-tube (No. 1) was lighted-up with the small-coil. Capillary
stream of a pale green tint. Bulbs with very little glow in them;
spectrum pale, and not very distinct. Under action of the magnet this
tube brightened up throughout, and the glow became more condensed, and
ran to the sides of the tube. The spectrum also brightened, the faint
lines becoming stronger, but the general character was preserved.
[Sidenote: Chlorine-tube No. 2 lighted-up. Effect on glow when magnet was
put on.]
A second chlorine-tube (No. 2) was then tried. Both bulbs were
completely filled with a dense white (very slightly rosy-tinted) opaque
light, and capillary the same, but brighter. A very slight violet tinge
was seen at the negative pole. When the magnet was put on, both bulbs
were at once filled with flickering bright streams of light, running
towards the side of the tube, according to the direction of the current.
The capillary stream at the same time changed from white to an intense
bright green. The spectrum without the magnet consisted of sets of
lines, with two well-marked absorption-spaces between, all seen somewhat
faintly, as if through a mist.
[Sidenote: Changes in spectrum when magnet was excited.]
When the magnet was put on, the marked character of the absorption spaces
was lost. The sets of lines in the yellow-green and green started up
intensely bright, while those in the blue only slightly brightened.
The misty appearance was altogether lost, and the bright lines all shone
up upon a perfectly dark background, with a strikingly metallic look; we
could not, however, trace change of position or actually new lines. It
seemed as if lines which had been faint in the yellow-green and green
region suddenly increased in intensity, the other parts of the spectrum
not being similarly influenced. They quite flashed up when sudden contact
was made with the magnet commutator.
_Iodine-tubes._
[Sidenote: Iodine-tube No. 1.]
This tube (No. 1) had been used for photographic purposes, and the bulbs
were partly obscured by a white deposit.
[Sidenote: Lighting-up described. Effect of touching one wire with the
finger.]
On lighting it up, both bulbs were filled with a violet-grey diffused
light, with much coarse well-marked lenticular stratification. This
stratification was mainly lost on changing the direction of the current,
but made its reappearance when one conducting-wire was touched with a
finger. This effect was still more marked when one finger of each hand
was applied to the wire. The capillary stream was of a pale lemon-yellow.
On putting on the magnet the light in the whole tube was nearly
extinguished, a faint thin stream of condensed light running through the
centre of the tube alone remaining.
[Sidenote: Effect of the magnet.]
On placing the bulbs between the magnet-poles, effects were produced
similar to those in the case of the tube, after described (p. 144, and
marked Si Fl₆), but in a less marked degree.
[Sidenote: Tube again tested.]
The iodine-tube was subsequently again tested, and it lighted-up better
than on the last occasion, showing nearly the same effects in bulbs and
capillary, the former having somewhat of a rosy tint and the latter an
amber.
[Sidenote: Magnet effects. The spectrum described. Change when magnet was
excited.]
On exciting the magnet, the capillary part of the tube changed from
amber to a decided light green. The spectrum, without the magnet,
gave one very bright line, and several less bright ones near, in
the blue-green. The rest of the spectrum, with the exception of the
absorption-spaces, was misty and continuous, with lines showing faintly
through. The red and yellow portions of the spectrum were quite bright.
When the magnet was excited, the spectrum entirely changed. The red
and yellow portions of the spectrum, and the misty continuous light,
all quite disappeared; while a set of sharp lines on the yellow-green
and green flashed up bright and clear, and stood out alone upon a dark
background, in which the absorption-spaces were lost. The effect was very
strongly marked, and gave a totally different character to the appearance
of the spectrum. The change seemed to arise from the suppression of one
part of the spectrum, and the increase in intensity of the lines in the
other part.
[Sidenote: No change in line-position.]
The principal lines could not be traced to change in actual position.
This tube differing somewhat from a second one we examined (No. 2) in
tint of glow and spectrum, it suggested itself to us that there might be
a partial mixture of N or H (or both) with the iodine vapour, giving rise
to some of the brighter parts of the spectrum which were extinguished
under the action of the magnet.
[Sidenote: Comparison of iodine-tubes No. 1 and No. 2. Comparison of the
spectra.]
We therefore compared these two tubes, viz. the old one (No. 1) and the
new one (No. 2), and also their spectra, by means of a comparison-prism
on the slit of the spectroscope. To the eye, the tubes differed much
in appearance. No. 1 had a distinct transparent rosy tint throughout,
with considerable coarse flickering stratification; and this contrasted
strongly with the dense whitish light of tube No. 2, which showed neither
movement nor stratification. The spectra were also found different in
general look. That of tube No. 1 was strongly tinged in the red and
yellow, and showed a bright continuous spectrum, crossed by many sharp
lines, with little trace of absorption-spaces. The spectrum of No. 2
was much whiter in tint, showed very little of the red and yellow, and
the absorption-spaces were very dark. A few bright lines, mainly in the
yellow-green and green, were faintly seen.
[Sidenote: The two tubes examined in detail. No. 2.]
The two tubes were then examined separately in detail. No. 2, excited
by the magnet, showed curious effects. The glow was rendered weak and
intermittent, and the rosy tint almost disappeared. The capillary changed
to a decided green colour, and the positive electrode was surrounded
by a yellow glow. The changes in the spectrum were no less decided.
Without the magnet, the spectrum was found to be a bright continuous one
of H (with a full set of principal and intermediate lines) and N—the N
spectrum being rather faint and misty, with very slight, if any, traces
of the iodine-spectrum. On the magnet being excited, the spectrum changed
as if by magic; the H and N spectra disappeared (except hydrogen F, which
still faintly remained), and the iodine lines, mostly in the yellow-green
and green, shone out wonderfully sharp and bright on quite a dark ground.
No. 1, upon examination, showed between the magnet-poles only the same
changes as on last occasion. The spectrum seemed to be one of iodine,
with the addition of slight traces of the H spectrum.
[Sidenote: Effects discussed.]
On excitation of the magnet, the misty continuous part of the spectrum
nearly disappeared, and the bright lines shone up sharply upon the dark
background as before. The effects in the case of both tubes were strongly
marked. The impression as to tube No. 2 was that, without the magnet,
the slight iodine-spectrum was overpowered and masked by the N and H
spectra; while under the influence of the magnet the N and H spectra
were almost altogether suppressed, the iodine-spectrum being at the same
time intensified. The disappearance of the continuous spectrum under the
action of the magnet in No. 1 (with the supposition it was mainly H)
would be accounted for in the same way.
_Bromine-tubes._
[Sidenote: Bromine-tube No. 1. Lighting-up described. Effect of the
magnet. Bromine-tube No. 2. Effect of magnet.]
This tube (No. 1) had been previously worked for photographic purposes.
Excited by the small coil, the whole tube was filled with a faint
flickering light. The positive bulb contained a faint purple glow, with
a yellow-green tinge at the electrode, a curious flickering stream of
light flashing from the electrode to the side of the tube. The negative
pole showed pretty much the same effect as the positive. The capillary
stream expanded at the opening into the positive bulb, but ran in a
condensed stream into the negative bulb. In colour it was of a rather
bright lilac. Upon putting the magnet on, the light-glow in the tube was
at once and permanently extinguished, the coil still working as if the
current passed. The same effect happened repeatedly; but now and then the
tube lighted-up for a second, showing spiral arrangement in the bulb.
We tried another bromine-tube (No. 2): it lighted-up easily; both bulbs
were filled with a purple stream of light; capillary stream bright grey.
The glass of the tube was strongly fluorescent and of a yellow tinge.
When the magnet was excited the stream of light was somewhat condensed in
the bulbs, and flew to the side of the tube; while the capillary stream
at the same time brightened. The spectrum without the magnet was fairly
bright; it increased in brightness under the influence of the magnet,
and additional lines appeared; but we considered them to be only faint
existing ones brightened up. No change in the position of the principal
lines was traced.
_Silicic-Fluoride tubes._
[Sidenote: Si Fl₆ tube. Lighting-up described. Effect of magnet.]
(1) A tube marked Si Fl₆ had been worked for photographic purposes; it
lighted-up easily. Both bulbs were filled with a brown-pink diffused
light, inclined to condense into a stream in the positive bulb. The
violet glow was very bright, and nearly filled the space round the
electrode. The capillary stream was of a bright violet tint. The effect
of the magnet was to decrease the intensity of the light throughout the
whole tube.
In the positive bulb the stream broke up into a number of vibrating
streamlets, with little bright threads of light intermixed, which flew
towards the side of the tube at right angles to the magnetic poles. There
was an inclination to spiral arrangement in the streamlets. This stream
changed from side to side of the tube coincidently with change in the
magnetic poles. At the negative pole the violet glow formed an arc in the
direction of the magnetic curves, while a spiral of fainter (positive?)
light was formed in the upper part of the bulb. A slight ringing sound
was heard in the tube.
[Sidenote: Comparison of Si F₄ and Si Fl₆ tubes.]
(2) We compared two tubes (Si F₄ and the one marked Si Fl₆). The Si Fl₆
tube in general effect, and in its spectrum, when lighted-up, resembled
Si F₄. We compared the one tube under the influence of the magnet with
the other not so, by means of a comparison-prism on the slit. As the
spectroscope and second tube were necessarily removed some distance from
the magnet, the spectrum of the tube between the poles was not bright.
We could not trace a change of position in any of the principal lines.
The tube between the poles was brightened up when the magnet was in
action[15].
_Sulphuric-Acid (SO₃) tubes._
[Sidenote: SO₃ tube No. 1; lighting-up described. Effect of the magnet.
Changes in the spectrum.]
(1) Excited by the small coil, both bulbs of this tube (No. 1) lighted-up
brightly, with a misty light-blue tinted stream of opaque light, a
yellow glow appearing at the negative pole. The capillary stream
partook of the same blue tint, but was whiter and brighter. Under the
magnet’s influence, the glow in the bulbs flew to the side of the tube
in flickering streams of light, the capillary at the same time changing
to a distinctly green tint. The spectrum without the magnet consisted
of four fairly bright bands of light in the yellow, green, blue, and
violet, connected by a faint misty continuous spectrum (O or possibly the
hydrocarbon spectrum found in O tubes by way of impurity).
When the magnet was excited, this spectrum entirely disappeared;
and a set of bright metallic-looking lines upon a dark background
(line-spectrum of S) took its place. This effect was produced whenever
the magnet was excited, and we tried it several times, to make sure of
the complete change. After a time, when the magnet, battery, and the
coil-power were all weaker, with the magnet on, we obtained a compound of
both spectra, the bright lines being seen upon the continuous spectrum in
which the bands appeared. When the magnet was taken off, the bright lines
disappeared, and the O spectrum alone remained.
[Sidenote: SO₃ tube No. 2 examined.]
(2) We also tried another SO₃ tube (No. 2) which had been worked for
photographic purposes, and was suspected of a carbon impurity. Without
the magnet, the spectrum was very like that of the first tube; but when
the magnet was excited, the spectrum only brightened, and no bright
metallic-looking lines appeared.
_Sulphur-tube._
[Sidenote: Sulphur-tube. Lighting-up (without heating) described.]
(1) A small bent vacuum-tube containing some solid sulphur, excited by
the smaller coil, and without being heated, gave a narrow stream of
bright blue-green light running straightly through it. With the magnet
on, this stream was deflected in the bulbs, and the capillary changed
from a blue-green to a distinct rosy tint.
[Sidenote: Effect of magnet. Changes in the spectrum.]
Without the magnet, the spectrum consisted of four bright bands, with a
continuous spectrum between, resembling that of SO₃ tube No. 1. With the
magnet on, the spectrum brightened, especially in the yellow and red,
which were dull before; and a set of lines appeared upon it (a line or
band in the yellow especially showing) which were not seen before. The
lines were distinct, but not very bright. The action on the capillary was
noticed to be strongest just between the conical points of the armatures;
and, in accordance with this, the central part of the spectrum-band in
the red and yellow showed an increased brightness.
[Sidenote: Effects when one of the bulbs of the tube was heated. Changes
in the spectrum under influence of magnet.]
(2) One of the bulbs of the tube was then gradually heated with a
small gas-flame. The single stream in the heated bulb became somewhat
deflected and broken up into a number of smaller streams; and these,
when placed under the magnetic influence, had small spark-like threads
of light running among them. The capillary, as the tube was heated,
and the sulphur rose in it, changed somewhat in tint, and, under the
magnetic influence, became of yellow-rose hue. As the heat was applied
to the bulb the bands of sulphur gradually appeared in the field of
the spectroscope, until at last the band-spectrum of sulphur entirely
took the place of the spectrum seen in the cool tube. The magnet being
excited, the spectrum changed at once, a set of bright sharp lines
(line-spectrum of S) appearing upon a faint and dull image of the
band-spectrum.
This effect was constantly repeated upon the magnet being excited. The
magnet being taken off, the band-spectrum alone was to be seen.
CHAPTER XV.
EFFECT OF MAGNET ON A CAPILLARY GLASS TUBE.
[Sidenote: Capillary portion of a Geissler tube tested in three ways.]
The capillary portion of a Geissler tube was cut away from the bulbs,
cleaned, and connected by a small vulcanite tube with the gas-pipe in
the room conveying coal-gas at ordinary pressure. The flame was small
and oval in shape, 8 millims. high, by 4 millims. wide, and burnt quite
steadily. (Plate XVII. fig. 13.)
[Sidenote: No effect on flame.]
(1) The capillary tube was placed between the poles of the excited
magnet, almost, but not quite, touching them; no effect at all was
produced on the flame.
(2) The tube was placed so that the conical ends of the armatures were
allowed to compress the centre of it between them; still no effect was
produced on the flame.
(3) The tube was placed so that the straight sides of the armatures
compressed it between them; still no effect took place on the flame.
[Sidenote: Flame between poles of magnet.]
(4) The flame itself was placed between the poles of the magnet. It was
slightly drawn towards one pole with an inclination to form the magnetic
curve.
[Sidenote: Quill glass tubing tested. No effect on the flame.]
(5) A piece of quill glass tubing was selected, 5 millims. in diameter
and 1 millim. thick, and drawn out to a point, the end of which was
snapped off and the tubing connected as before. The flame was 20 millims.
high, and 5 millims. across, and somewhat lambent. On being placed (1)
between the conical ends and (2) between the flat ends of the armatures,
no effect could be seen on the flame.
[Sidenote: Effect on taper and spirit-lamp flames.]
(6) A small taper-flame was placed between the poles of the magnet: no
effect was produced, except that the flame gave a slight “jump” each time
the magnet was excited. A spirit-lamp flame was tried with a similar
result.
_Action of Magnet on a bar of heavy glass._
[Sidenote: Heavy glass bar and mounting described.]
A piece of heavy yellow-tinted glass was selected, being a bar 10
centimetres in length, and 8 millimetres square. This was mounted in a
frame with a Nicol prism at one end, and a double-image prism (next the
eye) at the other.
[Sidenote: Placed along poles of magnet. Effect of magnet on
candle-images.]
(1) The glass bar and mounting were placed upon and along the poles
of the magnet (in the direction of the magnetic curves), and the
double-image prism and Nicol were so adjusted that two images of a candle
were seen—the one below bright and normal, the one above, by rotation of
the prism, as nearly as possible extinguished (Plate XVII. fig. 4). On
exciting the magnet the faint image at once conspicuously brightened,
at the same time assuming a slightly green tinge. To get full effect of
brightening, it seemed necessary to have good pressure-contact between
the battery-wires and the binding-screws.
[Sidenote: Effect on using a tourmaline as analyzer.]
(2) Using a tourmaline as analyzer in lieu of the double-image prism,
the candle-flame was seen alternately brightened and darkened, as the
tourmaline was rotated; and when the image was obscured by rotation,
excitation of the magnet caused it to brighten strongly. This effect was
accompanied by the apparent removal of a dusky red patch or spot, which
occupied the centre of the field when the flame was obscured.
[Sidenote: Bar placed at right angles to the poles: no effect produced.]
(3) The bar of glass and double-image prism being placed between the
conical ends of the armatures, but at right angles to, instead of along,
the poles, upon excitation of the magnet no effect at all was produced.
(4) The bar and prism being placed in the same position between the flat
ends of the armatures, no effect at all was produced.
[Sidenote: Slight effect on second experiment.]
(4_a_) Experiment No. 4 was repeated. It was thought that on excitation
of the magnet the secondary image slightly brightened; but there was a
doubt about it, and the effect (if any) was slight.
[Sidenote: Effects produced when a biquartz was introduced.]
(5) The apparatus was now changed for one of the following
arrangement:—1, a rotating Nicol prism next the eye; 2, the glass bar;
3, a biquartz with the halves horizontal; 4, another Nicol prism. The
neutral-passage tint of the biquartz was found to be rather green (from
mixture with the yellow of the glass).
[Sidenote: Change in colour of the halves.]
(i.) Placed _along_ the poles of the magnet and the magnet excited, a
change of tint was seen in both halves of the biquartz, the slightly
purple-reddish tint of the upper half passing into a full purple. Effect
not so marked as with the double-image prism.
(ii.) Placed _across_ flat ends of the armatures (as in experiment No. 4)
no effect was seen.
CHAPTER XVI.
EFFECT OF MAGNET ON WIDE AIR (AURORA) TUBE.
[Sidenote: Wide air-tube described.]
A large, wide air-tube was tried; it was 14½ inches long by 1 inch
in diameter, of the same bore throughout, and with straight platinum
electrodes.
[Sidenote: Magnet effect when tube placed vertically between conical
armatures.]
(1) To excite it the larger coil was used. The tube was filled with
bright, steady, rosy light, and beautiful stratification, which, as
it flickered, seemed to incline to a continuous spiral (Plate X. fig.
8). This stratification was very close and fine, and extended nearly
throughout the tube. On excitation of the magnet (the tube having
been placed _vertically_ between the conical armatures), the glow was
condensed into a bright solid line or stream of light at the point
which lay directly between the poles. This line or stream expanded into
an elongated funnel-shape as it retreated from this centre towards
the extremities of the tube, the stratification showing itself more
distinctly as the glow of light became less dense (Plate XVIII. fig. 3).
The stream of light was driven away at right angles to the poles, and
changed from side to side of the tube with the direction of the current.
[With the small coil this tube showed only a flickering stream of light,
with very slight indications of stratification.]
[Sidenote: Effect when tube placed horizontally between the armatures.]
(2) The tube was placed _horizontally_ between the conical ends of
the armatures. The condensed stratified stream of light flew upwards
and downwards (according to direction of current) instead of to the
respective sides of the tube.
[Sidenote: Tube placed along the poles of the magnet.]
(3) The tube was placed along the poles of the magnet. In the interval
between these the stream was driven upwards, but at either end sideways,
right or left according to whether the pole was N. or S. (Plate XVIII.
fig. 4). The result gave a complete spiral of stratified condensed light
within the tube.
_Note on Stratification._
[Sidenote: Stratification in small tubes arranged in series.]
The current from the large coil was sent through a set of five small
French vacuum-tubes, of equal calibre, containing salts of strontium and
calcium, and showing phosphorescent effects. These tubes were arranged in
single series; and, from the colour of the glow-discharge, were presumed
to contain rarefied air in contact with the salts.
A strong coarse stratification was seen in the central (No. 3) tube.
Tubes Nos. 2 and 4 also showed stratification, but in a less degree;
while the outside tubes, Nos. 1 and 5, showed no stratification at all.
The current was steady, and these effects did not fluctuate.
_Effect of Magnet on Plücker (Air-) Tube._
[Sidenote: Plücker air-tube. Lighting-up described. Effect of magnet on
the positive-pole stream.]
(1) A Plücker air-tube was selected of the form shown on Plate V. fig.
1, and was excited by the small coil. The ring was used for the positive
pole, the straight electrode for the negative. When lighted up, the
tube glowed with a perfectly steady and quiescent light. The negative
electrode was surrounded by the usual bright violet glow, extending
itself and being gradually lost at a short distance from the wire, while
the ring let fall a faint, tubular, salmon-coloured, diffused stream of
light, which met the violet glow as it approached the negative pole. The
tube was then placed vertically between the poles of the electro-magnet,
the armatures being almost in contact with the sides of the tube around
the negative pole. On excitation of the magnet, an instantaneous change
took place. The stream of light from the positive pole contracted itself,
so that it became of a long funnel-shape (the ring forming the mouth of
the funnel), while it tapered almost to a point where it met the violet
glow.
[Sidenote: Effect on negative violet glow.]
The stream also became very brilliant (the sides of the tube being left
proportionately free from light), and crossing it were a set of bands,
or striæ, having a waving or vibratory motion. The whole of the negative
violet glow was simultaneously gathered into a brilliant narrow arc,
which stretched across between the poles of the magnet. These effects are
shown on Plate V. fig. 1. The edges of the arc were remarkably sharp and
well defined, and with no surrounding aura or shading off.
[Sidenote: Arc of light followed the magnetic curves.]
By moving the tube between the armatures it was seen that the arc of
light followed the magnetic curves. If the tube was moved upwards, the
arc curved towards the zenith, if downwards, contrariwise; and a middle
position could be selected, in which the edges of the arc were nearly
parallel. Moving the tube a short distance from the pole had the effect
of rendering the arc more diffuse, but not of otherwise altering its
character.
[Sidenote: Direction of the current changed. Effects on glow described.]
(2) The direction of the current in the tube was then changed; and,
without the magnet, the ring electrode was surrounded by a diffused
violet glow; while the straight wire gave forth a faint salmon-coloured
stream of light, spreading up to the ring.
[Sidenote: Magnet effects described. On negative pole. Rings from
positive pole described. Effects on rings of making and breaking contact
with magnet. Shape of rings described.]
On excitation by the magnet (the positive pole being now placed between
the armatures), the violet glow of the negative pole contracted into a
compact mass round the ring electrode. At the same time from the positive
pole sprang a set of bright saddle-shaped rings, which increased in size
as they advanced; and spreading upwards with a rapid but smooth motion
towards the negative pole, closely approached to, but never actually
came in contact with, the violet glow. The positive end of the tube
was otherwise but slightly lighted, and the sudden appearance of this
brilliant stream of rings of light was very striking. A single bright ray
was also seen running from the positive wire, in a somewhat transverse
course, along one side of the tube. When wire-contact with the magnet
ceased, so that it was not excited, the rings ran back in succession to
the positive pole and disappeared, and by making and breaking contact
they were caused to advance and retire at will. They were accompanied by
a waving or vibratory motion, and were evidently of the same character
as the smaller striæ or bands mentioned as seen when the ring formed
the positive pole. The general appearance was that of a hollow cone of
light (the base towards the negative pole), composed of brilliant rings
with dark spaces between, which appeared and expanded under the magnetic
influence, and contracted and disappeared on its removal. The rings did
not appear to be flat disks, but were somewhat curved or saddle-shaped.
They reminded one much of the diatom _Campylodiscus spiralis_; that is
to say, they were apparently flat if looked at from above, but like a
figure of 8 when viewed sideways, the peak of the saddle forming a kind
of brilliant point or apex.
All this is difficult to describe; but an illustration from a sketch made
of the tube is given on Plate XVII. fig. 2.
[Sidenote: Negative pole placed vertically on the magnet.]
(3) The negative pole (straight electrode) was then placed vertically on
one of the poles of the electro-magnet. On excitation, the violet glow
was contracted into a small upright brush or column of bright light, with
a slight inclination to curvature.
[Sidenote: Tube laid horizontally across poles of magnet.]
(4) The same Plücker tube was laid horizontally across the poles of the
electro-magnet (without armatures), the respective electrodes being above
each pole.
[Sidenote: Effects produced.]
From the negative (straight electrode) pole sprang a dense and compact
arc of violet light, in the direction of the magnetic curves, which
terminated at the upper circumference of the tube, but which, if
prolonged, would have followed the curves to the opposite pole. The
stream from the positive pole was very considerably brightened, as in the
other experiments, but did not appear in the form of rings or waves. It
assumed that of a bright steady continuous glow, which formed round the
tube a not perfectly continuous, but distinct and well-marked, spiral.
This form of discharge seems connected with the peculiar contour of
the rings mentioned in experiment 2. One might, indeed, conjecture the
spiral-shaped glow to be a ring of light extended or drawn out towards
the negative pole.
[Sidenote: Effects like those obtained by Gassiot.]
Experiment No. 2 seems in result very like that of Gassiot’s with his
grand battery and the Royal Institution magnet, the effects (though of
course upon a smaller scale) being similar to those obtained by him.
_Effect of Magnet on Plücker Tube (Tin Chloride)._
[Sidenote: Plücker tube (tin chloride). Lighting-up described.]
A large Plücker tube was examined, which had a bulb attached at each end,
communicating with the central portion by a narrow neck or constriction.
On connexion with the small coil, a narrow stream of pale diffused
cobalt-blue light ran along the whole tube, from point to point of the
electrodes, the positive wire at the same time glowing with an aura of
amber-yellow light. (See Plate XVII. fig. 3, where the narrow stream of
light is shown by dotted lines.) At the two necks or constrictions the
stream of light was perceptibly brightened.
[Sidenote: Effects of magnet upon the stream.]
When the magnet was connected, the stream in the positive bulb was not
much changed, but only slightly bent. In the central partition of the
tube and in the negative bulb, the stream of light was broken and split
into a number of smaller streams, and at the same time bent or forced
against the sides of the tube. (See Plate XVII. fig. 3.)
[Sidenote: Peculiar noise within the tube.]
In the central partition, the blue streamlets were accompanied by a
number of spark-like threads of golden light, which shone out among them
as the whole vibrated against the side of the tube; at the same time a
peculiar pattering, as of a miniature hail-storm within the tube, made it
ring with a slightly metallic tinkle.
The direction of the bending or deflection of the stream was at right
angles to the axis of the poles of the magnet, and changed from side to
side of the tube as the direction of the current from the coil was varied.
[Sidenote: Spectrum described. Without magnet. With the magnet excited.]
In the positive bulb the stream, instead of joining the point of the
electrode, left this and ran along one side of the whole length of the
wire. (See effect, Plate XVII. fig. 3.) The spectroscope was applied to
the neck of one of the bulbs where the stream was bright. Without the
magnet a faint continuous spectrum, mainly of the blue and green, with
very slight traces of the yellow and red, was seen. Upon this, five or
six faint but sharp and metallic-looking lines were seen. On the magnet
being excited, the continuous spectrum was not changed; but the sharp
lines shone out brighter and clearer, one in the blue being especially
conspicuous. These lines were measured with a micrometer; and their
places being compared with Lecoq de Boisbaudran’s “Spectres lumineux,”
they were easily recognized to be those of tin. On each excitation of the
magnet the same brightening of the lines took place.
_Effect of Magnet on Tin-Chloride Geissler Tube._
[Sidenote: Geissler tube, Sn Cl₄, examined. Glow described. Effect of the
magnet.]
We then examined a Geissler tube, marked Sn Cl₄. When first excited
by the small coil, the spark passed freely. The glow in the bulbs was
of a diffused, light purple tint; the positive electrode had a bright
yellow glow around it. The capillary stream was of a sharp green-yellow,
at times brightening up to a metallic-looking green. When the magnet
was first employed, the tube distinctly and permanently brightened up
throughout.
[Sidenote: Spiral formed in positive bulb. Glow in tube extinguished.]
The negative bulb was not much changed in appearance; but in the positive
bulb a curious permanent and steady cloud-like spiral, of a purple
colour, made its appearance, and lasted while the tube was under the
magnetic influence. (See Plate XVII. fig. 12.) After a short time the
tube seemed to lose a great deal of its conducting-power, and to light
up in a feeble and intermittent manner, brightening only when the coil
was made to work its best. While in this condition, the magnet (which had
been previously disconnected) was excited, and at once what moderate glow
was still shining in the tube was totally extinguished. At first it was
thought some accident might have happened to the conducting-coil wires;
but repeated trials satisfied us that the effect was due to the magnetic
influence alone. Efforts were made, by looking to the coil and battery,
to brighten up the tube as at first, but they quite failed; and it was
evident some change had taken place in its conducting qualities. This
tube was accidentally broken, so that we had no opportunity to renew the
experiments.
[Sidenote: Another tin-chloride tube tried. Glow described.]
We subsequently tried another tin-chloride tube, purchased of Mr.
Browning. This lighted up like the former tube, but brighter. There was
an amber glow at the junction of the negative bulb which adjoined the
capillary part. This was lost on putting on the magnet. At the same
time a perceptible pattering ringing noise was heard in the tube, and
metallic-looking threads of light ran through the bulbs.
[Sidenote: Spectrum described.]
Without the magnet, the spectrum was a continuous faint misty one,
with bright lines of tin occasionally flashing up. With the magnet,
the tin lines at once shone out bright, strong, and clear upon a black
background, the change in effect being very marked.
CHAPTER XVII.
EFFECT OF MAGNET ON BULBED PHOSPHORESCENT TUBE.
[Sidenote: Large phosphorescent bulbed tube. Lighting-up described.
Spectrum described. Glow when discharge stopped, described.]
Mr. John Browning kindly lent me a large phosphorescent tube with five
bulbs, said to be filled with anhydrous sulphurous-acid gas (SO₂). (See
Plate XVIII. fig. 1.) This tube lighted up beautifully with the large
coil. The connecting tubular parts of it were filled with a bright,
beaded, transparent, rosy light; while the bulbs glowed with a more
opaque blue-tinted effect. The spectrum of the tubular part was found to
agree exactly with the principal bright band seen in a SO₃ Geissler tube.
The spectrum of the bulb-glow was a faint green-blue continuous one, with
bright bands or lines faintly flashing up at times. When the discharge
was stopped, the tube still glowed with a moderately bright, opaque,
grey-green light. This glow gradually faded out, always commencing with
the bulb forming the negative or violet pole, and so dying out, bulb by
bulb, towards the positive pole. The negative-pole bulb at times was, on
suddenly stopping the current, hardly lighted at all, the other bulbs
being luminous.
[Sidenote: Comparison with SO₃ Geissler tube.]
(1) We compared the large tube with a SO₃ Geissler tube, by means of
a comparison-prism on the slit, with the result before detailed. The
Geissler tube, however, showed no after-glow.
[Sidenote: Effects in bulbs on lighting-up the tube described. Effects of
reversal of the current. After-glow restored by passing of current.]
(2) We lighted up the Browning tube with the large coil. The negative
bulb was always the least filled with the blue opaque vapour, and the
other bulbs increased in vapour-density in the order they approached
towards the positive bulb. When the current was reversed, so that the
negative and positive glow changed places, the negative bulb still
remained transparent, although the positive opaque glow had (presumably)
been thrown into it. When the after-glow had quite disappeared in the
bulbs, it was again strongly restored, by the passing of the current for
a few seconds only through the tube.
[Sidenote: Effect of reversal of current on positive-pole glow.]
(3) The tube was well excited, and the four bulbs (other than the
negative one), upon stopping the current, glowed strongly. The current
was then sent through reversed, so as to throw the negative glow for a
few seconds into the positive bulb. The after-glow in the positive bulb
was at once extinguished. On once more reversing the current, it was only
restored after a certain amount of continuance of the positive stream.
[Illustration: Plate XVIII.]
[Sidenote: Effect of change of current on the three central bulbs.]
The time during which the negative glow was thrown into the positive bulb
did not appear sufficient to have heated it. After rapidly changing the
direction of the current several times and then stopping it, the three
central bulbs alone had an after-glow, the two extreme ones having none,
being both equally transparent.
[Sidenote: Effect of heat on the bulbs. Effect of cooling by ether-spray.]
(4) A moderate heat from a spirit-lamp was applied to the centre bulb
(_a_) while the current was on; and also (_b_) when this was stopped,
and the bulb glowed. In the first case the bulb was found to get more
transparent; and in the second case the after-glow disappeared in a
proportionately shorter time in the heated bulb than in the others. To
test the result of cooling the bulbs, the negative-pole bulb and also the
central one were each subjected to the action of ether-spray, and also
of ether and water-spray mixed. This was done, (_a_) when the current
was passing, and (_b_) when it was stopped and the glow only was in the
bulb. The bulbs were cooled until a marked cold effect to the touch was
produced. We did not notice any difference in the behaviour of the bulbs
so treated as compared with the others, either when the current was
passing or in the case of the after-glow.
[Sidenote: Negative-pole bulb between the armatures of magnet. Effects on
negative and positive glow.]
(5) We placed the negative-pole bulb between the conical points of the
armatures, and excited the magnet. The negative glow contracted itself
into a condensed violet-tinted crescent, in accord with the magnetic
curves. The positive glow of the same bulb lost its beaded (stratified)
character, and was condensed into a bright stream of light, which latter
protruded from the small inner tube and formed a spreading spiral set of
cloud-rings within the bulb (see Plate XVIII. fig. 2). The action of the
magnet seemed to be exercised in subduing the stratification, condensing
the glow into a bright stream of light, and forcing the latter to “tail
over” at each extremity of the tubular joints into the bulbs—this effect
extending even so far as the second bulb.
When the positive bulb was placed between the poles of the magnet, the
glow was simply condensed into a bright stratified stream, which flew to
either side of the bulb.
[Sidenote: Effect of magnet on glow in bulb No. 4.]
(6) _a._ Bulb No. 4 (see Plate XVIII. fig. 1) was placed between the
poles of the excited magnet, and the current was passed and then stopped.
The glow in that bulb faded away out of its order, and earlier than in
ordinary cases (nearly as soon as No. 2).
[Sidenote: Other bulbs tested in similar manner.]
_b._ The same and other bulbs were tested in a similar manner. In all
cases the bulb influenced by the magnet, when the current was stopped,
was found perceptibly fainter in after-glow.
[Sidenote: Effect of magnet upon the after-glow itself.]
_c._ The tube was arranged with one of the bulbs between the poles
of the unexcited magnet; the current was passed and stopped, and
the after-glow obtained. The magnet being then quickly excited, the
after-glow in the bulb, under its influence, faded out; and the bulb
became transparent, perceptibly sooner than under ordinary circumstances.
We tried this several times, with the same result in each case.
[Sidenote: Mr. Thompson’s experiments on action of magnets upon liquid
rings.]
_Note._—In relation to these experiments, it may be mentioned that Mr.
S. P. Thompson, of Bristol, is reported to have studied the action of
magnetism upon rings of coloured liquid projected through water, and
to have observed their retardation and partial destruction in passing
through a powerful magnetic field.
[Sidenote: Mr. Ladd’s explanation of some of the phenomena observed.]
Mr. Ladd has suggested to me that some of the phenomena produced
indicate a driving of the gas in the direction from the negative to the
positive pole—a theory which is supported by the action of the magnet
on the bulbs, if this be considered a repulsive one as regards the gas
influenced.
_Effect of Magnet on small Phosphorescent (powder) Tubes._
[Sidenote: Tubes containing phosphorescent powders described.]
We examined six vacuum-tubes containing phosphorescent powders, which,
upon exposure to sunlight and removal to the dark, or after passing of
the electric current over them, continued to glow in the tubes after the
exciting cause had ceased. They were of thin glass, and of equal calibre
throughout.
One was 6½ inches long and ⅝ inch in diameter, and had no label; the
other five were 7½ inches long and ½ inch in diameter, and were labelled
respectively:—
Strontium vert,
” jaune,
Calcium violet,
” orange,
” vert-bleuâtre.
[Sidenote: Lighting-up of the tubes described. Effect of magnet on
⅝-diameter tube. Spectrum without magnet.]
The powders in tubes of this description are said to contain either
sulphide of strontium, or calcium, or sulphate of quinine. The
first-mentioned tube shone with a white and bright light, and probably
contained the latter substance. The general effect of the current on the
tubes was similar in all cases. Under a sufficiently strong current,
they lighted up with a brilliant, slightly green-white glow; in which,
however, by looking sideways, it was possible to detect a delicate rosy
tint. Any colours beyond these in the tubes seemed to depend on the
powders enclosed in them. When the current was stopped, the powders
alone glowed in accordance with the colours mentioned on the labels, the
rarefied gas or air in the tubes not giving any after-glow, as in the
case of the sulphurous-acid tube. When the ⅝-diameter tube was excited
by the small coil, the effect of the magnet was to entirely suppress and
extinguish the glow. When this and the other tubes were worked with the
larger coil, the spectrum, without the magnet, was bright and continuous,
either showing no lines or else very faint traces of them, and, extending
through the whole range of colours was brightest in and about the green.
[Sidenote: Magnet effect on glow. Same on spectrum.]
With the magnet excited, a bright line of pink light was condensed
against the upper side of the tube; while the glow in the tube generally
became very decidedly fainter, except at the electrodes, which still
preserved a certain amount of brilliancy. The spectrum also was much
changed. The bright continuous glow became much fainter, and many sharp
and fairly bright lines were seen upon it. These lines were, as to
character, not easy to recognize. Hydrogen (F) was, however, plainly
distinguished; and other lines, which we considered to be N, were common
to all the tubes. Some lines were also remarked as being, without the
magnet, not so constant.
[Sidenote: Tubes examined and compared for spectra.]
Calcium orange and calcium violet, compared for spectra, were identical;
the two strontium tubes hardly so, but with strontium vert a bright
continuous spectrum mainly hid the lines.
Strontium jaune and calcium orange were not alike; strontium vert and
calcium violet differed. Calcium orange and calcium vert-bleuâtre were
considered alike; but the comparison was not easy, as the calcium vert
was bright, and the lines were only seen faintly upon the continuous
spectrum.
In order not to shift the powders, the tubes were laid horizontally, and
two spectra simultaneously examined across the tubes.
_Lighting-up Tubes with One Wire only (Marquis of Salisbury’s
Observations)._
[Sidenote: One wire only connected with an electrode.]
The vacuum-tubes employed were examined in the usual way, but one wire
only was connected with an electrode. The other wire was attached to the
end of a glass rod, and circuit was from time to time completed while the
tube was before the spectroscope.
The large coil was used. In all cases, with the one wire, the glow was
very faint as compared with that of the closed circuit.
[Sidenote: Ether vapour.]
(1) _Ether Vapour._—With both wires, in company with the usual bright
bands of the carbon spectrum, shading-off towards the violet, the H lines
were very sharp and brilliant. With the one wire only, the carbon bands
were left faintly shining, with both sides nebulous alike, and with no
shading-off towards the violet. (We were not quite sure whether this
was not the effect of the reduction of the light.) The H lines, though
originally stronger than the carbon bands, quite disappeared from the
spectrum.
[Sidenote: Coal-gas.]
(2) _Coal-gas._—The same effects were produced; but we thought we could
detect very faint traces of the H lines.
[Sidenote: Nitrogen.]
(3) _Nitrogen._—The N lines, as well as those of H (also seen in the
tube), were much fainter with one wire, but the H lines more so in
proportion.
[Sidenote: Hydrogen.]
(4) _Hydrogen._—Only a marked reduction in brilliancy of the whole
spectrum.
[Sidenote: Oxygen, N and H.]
(5) _Oxygen._—An impure tube, showing O (some of the lines hydrocarbon?),
N, and H spectra simultaneously. With one wire the O lines still remained
fairly bright, the N and H being only faintly seen.
[Sidenote: Water-gas.]
(6) _Water-gas._—Same effect.
[Sidenote: Turpentine vapour.]
(7) _Turpentine Vapour._—Same effect as ether, but the H lines could be
faintly seen.
CHAPTER XVIII.
ACTION OF THE MAGNET ON THE ELECTRIC SPARK.
[Sidenote: Apparatus employed.]
The magnet was excited with two plates of the large battery, and the
larger coil with the other two plates, the action in both cases being
strong.
1. A spark from the coil was passed between two platinum wire electrodes,
about three centimetres apart.
[Sidenote: Spark and aura described.]
It consisted centrally of a thin stream of bluish-white light, vividly
bright, around which was seen a narrow, uniform, diffuse, yellow-tinted
aura, which accompanied the spark in all its movements. The spark always
struck across from the extreme points of the electrodes (see Plate XVII.
fig. 5).
[Sidenote: Effect of magnet upon the aura.]
2. On being placed between the conical poles of the excited magnet
the bright thread of the spark did not change; but instead of the
inconsiderable yellow-tinted aura which accompanied the unmagnetized
spark, there now struck out, at right angles to the magnet-poles, a
thin rosy-tinted half-disk of aura-like flame. This extended aura ran
considerably along each electrode, though the spark proper still struck
from the points.
[Sidenote: Extended aura described.]
The aura was somewhat larger in extent upon one electrode than on the
other. In the first case, it sprang from a considerable number of minute
illuminated points; on the other electrode, these illuminated points
were fewer in number, and the flame was more purple in tint. Reversing
the current these effects were reversed. The aura was uniformly thin and
disk-like, and the curved edge remarkably true in shape (see Plate XVII.
fig. 6).
The lateral direction of the aura was changed when the current was
reversed.
[Sidenote: Aura not proportionate to length of spark.]
3. The aura was found not proportionate to the length of the spark. When
the electrodes were approached, so as to very much shorten the spark, the
aura still sprang out to a distance and extent quite out of proportion
to the length of the spark. Even when the electrodes were approached so
close that the spark was very short indeed, still, under the magnetic
influence, a very considerable aura made its appearance.
[Sidenote: Effect of working coil-break upon the aura.]
4. Upon working the coil-break, it was found that in proportion as the
contact screw was drawn apart from the break, so the aura gradually
diminished in extent, until at last, by continuing to increase the
distance between the screw and the break, a point was reached when thin
bright sparks, without any aura, passed. Upon the screw being worked up
closer, thicker sparks passed, and the aura again made its appearance. As
the aura diminished in size it gradually changed in tint from yellowish
rose-pink to purple.
[Sidenote: Spark taken in glass bulb.]
5. The spark was taken in a glass bulb, the tube in which it was blown
being open at both ends, with the same effect as in the open air.
6. A plate of glass was laid on the poles of the magnet, and the spark
was passed _along_ the poles (in the same direction as the heavy glass
was laid in the Faraday experiment). No aura was formed. The points were
then moved round, so as to carry the spark at right angles to the poles,
and the aura was formed as before.
[Sidenote: Aura could be blown away from the spark.]
7. The aura, it was found, could be blown away at right angles to the
spark. When strongly urged, it assumed the shape of a flickering tongued
curtain of flame, flying away in the contrary direction to that from
which the current of air proceeded, and again returning to its original
shape as the impulse was removed. The spark proper was not influenced
(see Plate XVII. fig. 8).
[Sidenote: Effect of withdrawing spark from central position between the
poles.]
8. As the spark was withdrawn from its central position between the poles
of the magnet, the convex edge of the aura became gradually less perfect,
and assumed a ragged and broken-up appearance, the inequality at times
amounting almost to jets or flickering sprays of light. The spark was
also slightly curved away from the electrodes (see Plate XVII. fig. 7).
[Sidenote: Magnet had no effect upon condensed spark.]
9. A condenser of four coated plates was introduced into the circuit,
causing a sharp brilliant blue-white spark, apparently divided into
streams and with no aura. The magnet had no effect whatever upon this
form of spark.
CHAPTER XIX.
THE DISCHARGE IN VACUO IN LARGER VESSELS, AND MAGNETIC EFFECTS THEREON.
A Tate’s air-pump was used, and the spark from the larger coil. The
exhaustion could not be carried very far.
[Sidenote: Globular receiver described. Discharge described.]
(1) A globular receiver was used, having brass caps for exhaustion,
and platinum wires passing through the opposite sides for electrodes
(see Plate XVIII. fig. 6). With partial exhaustion, from the positive
electrode proceeded long, sharp, bright, rosy sparks, striking in zigzags
across the receiver. From the negative terminal sprang a larger number of
bluer and more diffuse streams of light, like spiders’ webs; and these
were enveloped, for a short distance from the terminal, in a slight misty
aura. Both sets played round the sides of the glass as well as across.
[Sidenote: Bell-shaped receiver described. Discharge described.]
(2) A bell-shaped receiver, with terminals inserted at the sides and one
also at the top, was next used (see Plate XVIII. fig. 9). When the side
terminals were employed, the effect was much the same as in the last
case. When the top terminal was used for one wire (the other wire being
connected with the pump-plate) a single stream of bright rosy light ran
from the upper terminal to the plate. First striking the central part of
the plate, the stream then glided towards one of the lateral terminals,
and so to the edge of the receiver. After partly discharging itself by
contact with the terminal, the stream as rapidly retreated to the centre
of the plate again—this effect being from time to time repeated while the
current was passing. The current being reversed, a number of bright, but
weaker and more diffused, streams of light had the appearance of shooting
from the upper electrode, and of striking upon the plate below; with a
tendency to fly off from where they struck, in a similar manner to the
single stream before described. Where each stream touched the plate a
brilliant point of light appeared, and a strong pattering noise was heard
in the receiver.
[Sidenote: Bell-shaped receiver without electrodes. Induction discharge
described.]
(3) Another bell-shaped receiver of similar shape was used. This had no
electrodes forming a direct communication with the interior; but, in
lieu of these, two wafers of thin sheet brass were cemented, one inside
and one outside the glass, opposite to one another. On connexion being
made with the outside wafer, the effects produced by induction were
similar to, and very nearly as strong as, those in the cases where direct
communication with the interior of the receiver was made.
[Sidenote: Long large tube exhausted and illuminated. Spiral form of
discharge.]
(4) A large tube, 24 inches long and 2 inches in diameter, with ball and
point electrodes respectively, was exhausted, and the current passed
through it. The effects were similar in most respects to those produced
in the globular and bell receivers, but the streams of light assumed a
distinctly spiral form in their passage (see Plate XVIII. fig. 5). This
tube when placed between the poles of the magnet showed no effect, except
a slight condensation of the streams of light towards the sides of the
glass.
[Sidenote: Globular receiver again used.]
(5) The globular receiver first described was again used (the Tate pump
having been cleaned and working easier).
[Sidenote: Phosphorescent after-glow succeeding the spark.]
(_a_) When exhaustion was as good as it could be got, the spark struck
across in a single, slightly expanded, stream of rosy light, having a
tendency to curve upwards (see Plate XVIII. fig. 6). The electrodes had
but little glow round them, only just enough to distinguish the poles
apart. When the flow of the stream was interrupted by breaking contact
with one terminal, so that sparks passed in succession, we thought we
detected a faint blue phosphorescent after-glow succeeding each spark.
[Sidenote: Positive wire only attached.]
(_b_) The positive wire only was attached to one electrode, the negative
being unconnected. A set of faint whity-blue cobweb-looking streams of
light spread from the electrode all over the receiver, having a vibratory
motion. The spaces between these were dark, and there was no aura—the
effect being similar, but not quite so bright and pronounced, as when
both wires were attached (see Plate XVIII. fig. 7).
[Sidenote: Negative wire only attached.]
(_c_) The negative wire only was attached. The cobweb streams were
absent, or only shot out very occasionally. The main effect was a
straight nebulous stream of violet light, which commenced at the
electrode and spread out in a fan-shape towards the lower brass cap of
the receiver; while, at the same time, an aura or glow of similar light,
but fainter in quality, spread from the electrode over at least one half
of the receiver. This aura would no doubt have filled a small flask (see
Plate XVIII. fig. 8).
[Sidenote: Effect of gradual exhaustion on the discharge.]
(_d_) When exhaustion was first commenced, both electrodes of the
receiver (both wires being connected) threw out spider-web-like streams,
as in Experiment 1, pale blue from the one pole and somewhat rosy from
the other.
As the exhaustion progressed the pale-blue streams disappeared, while the
rosy flickering ones diminished in quantity and extent until ultimately
a single rosy stream of light crossed the receiver as in Experiment
5_a_. Upon admitting the air, these effects took place in an inverse
order—the single stream being gradually broken up, and the spider-webs
taking its place.
[Sidenote: Globular receiver placed on poles of the magnet. Magnet
effect.]
(_e_) The exhausted globular receiver was placed upon the poles of the
excited magnet, with the stream at right angles to them. Looking across
the S. pole of the magnet, the negative electrode was on the left hand,
and the positive on the right. The effect of the magnet on the stream
was apparently to split it up into several; but this appearance must
have been due to vibration only, as a revolving mirror showed the stream
as single. When the current was reversed, the stream which, without the
magnet, was somewhat flickering and vibrating, slightly straightened at
the positive pole, and the whole stream became steadier.
[Sidenote: Single wires attached.]
(_f_) Single wires were successively attached to the negative and
positive poles, and the cobweb streamers and glow before described
obtained. The magnet was found to have no decided effect on either of
these.
[Sidenote: Plücker tube placed between poles of magnet with negative wire
only attached.]