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ad. ab was 2 mm., ad was 34.8 mm., and the angle Fig. 3. Four-fifths nat. size,
between the planes of ad and be was 55 minutes of arc. The horizontal
width of the wedge was 10 mm. Glass walls surrounded three sides of the
wedge, as the outline indicates. The reason for using the quartz wedge was
to counteract the deviation and dispersion produced by the solution in the
cell. The angle of the liquid wedge could be varied until the deviation
effected by the quartz wedge nullified the average action of the absorbing
solution. At first it was supposed that with liquid wedges of 15 or so
minutes of arc a plane-parallel quartz plate could be used successfully instead



o



10



ATLAS OF ABSORPTION SPECTRA.



of the quartz wedge. This was true for some dyes but for concentrated
solutions of certain other dyes (notably the sodium salt of /-methoxy-
toluene-azo-/J-naphthol-di-sulphonic acid) some compensating system was
absolutely necessary. Finally, the quartz wedge was made with the utmost
care by an expert optician, special pains being taken to have the edge
through d, perpendicular to the plane abed, as sharply denned as possible,
and the surfaces whose traces are denoted by ad and be were accurately
plane.

(4) Figure 4 presents a side view and an end view of the vulcanite frame
into which the quasi-box just described fitted. This frame was shaped

out of a single block of vulcanite, for
experience showed that a cemented
system of several pieces was not dur-
able; also a dielectric was needed to
keep the sparks from jumping to the
screws. P indicates a little depression
which fitted over the point of the



M



l
'




Fig. 5.



1J ijp'

Fig. 4.-Four-fihh natural size.

screw T. P' designates the end of a straight line along which the rounded
extremity of the screw T' slid. P" is the cross-section of a shallow,
V-shaped groove along which the pointed end of the third screw, T", like-
wise slid. The perforations M, M', etc., correspond to each other and to
the associated wooden pegs mentioned above as (5).

Figure 5 is an unconventional

sketch of the cell when completely

assembled.

A cell of the construction just

described is very well suited to the
study of thin layers of solutions in solvents of relatively high boiling-points,
such as water and amyl alcohol, but, unless inclosed in some suitable vessel,
it is not applicable to solvents of lower boiling-points like ethyl alcohol,
ether, chloroform, etc.

CEMENTS.

A few words concerning cements may not be superfluous because a great
many receipts were tried and none was considered entirely satisfactory.
No single cement was found which satisfied the following three necessary
conditions : (a) Of being unaffected by hot or cold water ; (b] of being insolu-
ble in the alcohols, ether, chloroform, carbon bisulphide, etc. ; (c) of drying
or setting in three or four days, at most.

The plan used by Prof. H. N. Morse, in waterproofing cells for the
study of osmotic pressure, gave the best results and hence it was followed



SELECTION OF MATERIAL, APPARATUS, ETC. II

in fastening together the quartz and glass parts of the cell described in the
last section. These parts were first fastened together with Khotinsky
cement in the usual way, that is, by heating them in an air bath, to any
convenient temperature above the melting point of this cement, and by
heating a stick of the adhesive mixture in a Bunsen flame and then applying
it to the surfaces of the hot quartz and glass.

Since this resinous cement is soluble in ethyl and amyl alcohol and other
solvents, and because it is attacked by various liquids, such as an aqueous
solution of potassium permanganate, it was necessary to coat the exposed sur-
faces of the cement with something which was chemically inert towards the
solutions to be studied. Such a substance is a solution of rubber in carbon
bisulphide. This solution was made and used as follows : From an adequate
length of black, soft, rubber connecting-tubing segments about 2 cm. long
were cut and heated in an evaporating dish over a Bunsen flame until the
sections fuse'd, ran together, and formed a very sticky, viscous liquid. (A
single long piece of tubing does not liquefy at all satisfactorily.) The liquid
state persisted after the contents of the evaporating dish had been allowed
to cool down to about room temperature. Carbon bisulphide was next
poured into the dish and the contents of the latter were stirred until a
homogeneous solution resulted.

The relative proportions of the carbon bisulphide and rubber used were
immaterial and were determined by convenience only The solution can be
retained indefinitely in a tightly stoppered bottle and used whenever needed.
A thin layer of the solution was painted over the Khotinsky cement, after
which the quartz-glass system was heated in an air bath at about 100 C.
until the layer became dry and hard, and was no longer sticky. (Of course,
during the first part of the process the transparent elements of the cell had
to fit over a suitable wooden "form," because Khotinsky cement softens too
much at 100 C. to maintain objects in their proper relative positions.) After
this another thin coat of the rubber solution was applied and the heating
continued. This succession of operations was repeated until a thick, hard,
dark-brown covering for the joints was obtained. It then made little differ-
ence whether the original cement were present or not, as the hard rubber
held the quartz and glass together very satisfactorily.

A cement which dissolves readily in water and in acetic acid but which
is not affected by ethyl alcohol, amyl alcohol, carbon bisulphide, glycerin,
chloroform, ether, benzol, nitrobenzol, aniline oil B, benzaldehyde, toluol,
etc., is made by dissolving 2 pounds of pure gelatin in one quart of water and
adding to the resulting solution 7 ounces of nitric acid (sp. gr. 1.35 to 1.42).
The final solution is colorless and when applied in thin layers dries in a day
or so. It is called Dumoulin's liquid glue. This glue does not keep well,



12 ATLAS OF ABSORPTION SPECTRA.

even in a tightly stoppered bottle, and is best made up fresh just before
being applied as an adhesive.

Since the completion of the experimental work on the aniline dyes a
cell, in the construction of which no cement at all was employed, has been
designed and successfully used by one of us.* This cell could retain any
liquids which would not attack glass and quartz and, although it was
designed to confine the solutions in plane-parallel layers, nevertheless, the
principles involved in its construction were such as to admit of extension to
the production of a cell which would be wedge-shaped in the interior and
would, at the same time, hold organic solvents, prevent evaporation, etc.

THE SPECTROGRAPH.

The essential parts of a vertical section of the spectrograph are outlined
in figure 6. They may be tersely described, with the aid of symbols,
as follows: In the first place, the elements of the system were adjustable
in every respect. Light from the Nernst filament, N, was focused by the
concave speculum mirror, R, on the slit, S, whence it continued to the
grating, G, from which a portion of it was dispersed in the direction of the
sensitized film, F. The distances from the middle of the slit to the centers
of the mirror and grating were respectively about 89.5 cm. and 97.1 cm.
The electrodes, E, were usually at the distance of 4. 2 cm. above the slit and
they did not interfere with the passage of the light from the reflector to the
slit. No lenses or other reflectors were used. The micrometer head at M
indicated the separation of the slit- jaws. Q and Q' denote a screen system
such that when Q was vertical the passage of light from the grating to the
camera was not interfered with, whereas when Q was horizontal only ultra-
violet light of shorter wave-length than 0.4/1 could reach the photographic
film. PP is a horizontal platform with a scale along its front edge. By
sliding projecting, horizontal, opaque screens of various widths along this
platform it was possible to cut out completely any region or regions of wave-
lengths desired.

In making certain tests, the platform and sliding screens were very
convenient. L is the section of a thin, black, metal shutter capable of
motion in a horizontal direction and hence at right angles to the length of
the photographic films ; in other words, parallel to the slit and to the rulings
of the grating. A number of long, rectangular slots or openings, suitably
spaced and proportioned, were present in this screen so that strips of differ-
ent widths of the films or plates could be exposed to the light from the
grating without causing any displacement of the sensitized surfaces with refer-

*The description of the details of the cell is given on pages 241 to 243 of Publication No. 60 of the
Carnegie Institution of Washington, entitled : Hydrates in Aqueous Solution. By Harry C. Jones.



THE SPECTROGRAPH.



ence to the grating and slit. This was necessary for impressing comparison
spectra, etc. H and H' suggest the rack-and-pinion system by the aid of
which the films could have unexposed portions brought successively opposite
to some selected opening in the slide-screen L. D and D' denote two of

the four doors which gave access to
the interior of the spectrograph, and
which made it possible to close up
the camera light-tight, while making
various adjustments with the rest of the
system. The camera was made so
that, when it contained neither a film
nor a plate, it was possible for the
experimenter to look directly at the
grating and to make observations with
the assistance of an eye-piece.

Certain black-on-white scales and
ruby-glass windows (Z, for example)
enabled the experimenter to know the
precise relative positions of the various
accessories on the interior of the spec-
trograph, when the entire system was
shut up and exposures were being made.
Numerous dull black diaphragms and
screens (A 1( A 2 , A 3 , A 4 , A 6 , etc.) pro-
tected the photographic film from the
unusable light which came from the cen-
tral image, I, and from all the spectra
except the one desired. Ui and d
give the extreme rays of so much of the
first order spectrum as was studied,
that is, Ui and Oi correspond respec-
tively to about o. 2O/i and 0.625^. Ob-
viously, the spectrograph was dull black,
both inside and out, and contained
plaited black velvet in appropriate
places. A general idea of the size of
the apparatus may be derived from
the following dimensions: From R to
the plane of BC 198.5 cm. ; BC
34. 5 cm. ; the bottom edge perpen-
dicular to BC=-27.5 cm.; BJ 116
cm. ;'_ and JK = 29 cm.




Fig. 6. One-tenth natural size.



14 ATLAS OF ABSORPTION SPECTRA.

MANNER OF EXPERIMENTING.

SOLUTIONS.

A small, known mass of a selected dye was carefully weighed on a
chemical balance, and put at the bottom of a medium-sized test-tube.
Then distilled water was run from a burette into the test-tube, and the
latter shaken up from time to time, until the resulting solution appeared to
have the proper concentration. As would be expected, practice produced
skill in judging absorption of visible light, but to get the right concentration
with respect to ultra-violet light was not always so easy. The greatest error
in measuring the solvents was about o. 2 per cent. Since the concentrations
are only intended to serve as general guides to an understanding of the
spectrograms, a higher degree of accuracy would have been superfluous.
Neither was there any reason, in general, for noting the volume of solution
which contained a known number of grams of pure solvend; in other words,
changes in volume due to the processes of solution were not regarded.

ADJUSTMENT OF THE CELL.

Especial care was taken to remove all coloring matter from the cell
before introducing another solution into it. Dust caused more trouble than
anything else. After cleaning the quartz and glass elements of the cell the
various parts of the latter were assembled and, when a prism of liquid was
to be studied, the micrometer screws regulated in the following manner :
All the screws were turned down so as not to touch the vulcanite framework,
and- thus to cause the quartz wedge to rest on the quartz plate. Then the
screw T had its point elevated again and again until it just touched the
deepest part of the depression P. (See figures 2, 3, 4, and 5.) This
condition was attained by gently rocking the system around the edge d of
the quartz wedge, somewhat after the fashion of experimenting with certain
types of spherometer. Thus the zero position of the cell was determined,
before each experiment, of course. Next, guided by the circular and plane
scales, the observer turned up the screw T until the desired angle, between
the wedge and plate, was known to obtain. After this, the screw corre-
sponding to T' was turned up until its tip projected far enough into the groove
P" to prevent the quartz wedge and its accessories from sliding over the
quartz plate around the point T as pivot, but yet not far enough to raise the
vulcanite frame the least bit. Finally, a small amount of the solution was
poured into the cell and the latter was then placed on the very thin brass
sheet which rested upon and protected the jaws of the slit.

As soon as the cell was placed over the slit and the glower had been
lighted the cell was moved forward and backward, parallel to the slit, while
one edge of the field of view was examined with an eye-piece, until a position
of the cell was obtained for which the light passing through the quartz wedge



MANNER OF EXPERIMENTING. 15

at its refracting edge (d of figure 3) illuminated the very limit of the
field of view as seen through the chosen slot of the shutter (L of figure 6).
The position of the mark on the handle of the cell (F of figure 2), with
respect to the horizontal scale in the plane of the slit-jaws, was then read off.
If the cell were then moved, ever so little, in one direction the width of the
brightly illuminated field could be seen to be less than the opening in the
shutter ; whereas, if the cell were translated in the opposite sense no increase
in the width of the illuminated field occurred. At this opportunity, eye-
observations of the absorption between 0.400^ and 0.625^ were always
made and the facts recorded.

When the concentration of the liquid in the cell was much too great or
far too small this instrument had to be cleansed and filled with a solution of
more suitable absorbing power, obviously, but when the concentration was
not too remote from the best value the effective depth of the cell was varied
until the desired result was obtained.

All three screws were raised and regulated in an obvious manner when
prisms of liquid having nowhere infinitesimal thickness were wanted. When
layers of liquid of uniform depth were studied a system much like that
shown in figure 3, but which had for bottom a plane-parallel plate of
quartz 2 mm. thick, was substituted for the quartz-wedge system.

CALIBRATION OF THE CELL.

The diedral angles formed by the cell were calculated from the dimen-
sions of the instrument, and also from measurements made with a spec-
trometer.

EXPOSURES AND SPECTROGRAMS.

The majority of the spectrograms consist of three distinct photographs
taken side by side and as close together as possible. (See the plates.) The
width of each photograph was practically the same as the width of the
opening in the shutter L. Numerous trials showed that this field of view
was completely filled with light, with no overlapping on the grating-side of
the opaque portions of the shutter, when the length of the slit was dia-
phragmed down to 10.5 mm. Consequently, the slit was limited to a length
of a very little more than this number and the cell was moved along exactly
10.5 mm. between the taking of two adjacent photographic strips on the
same film. By this means, the thickness of absorbing liquid through which
the light passed to the very edge of one photographic strip was equal to the
thickness subsequently traversed by the light which recorded itself at the
contiguous edge of the adjacent strip. Of course, the best appearing records
were obtained when the film holder, actuated by the rack-and-pinion system,
was moved, by an amount exactly equal to the width of the opening in the
shutter. A casual inspection of the positives reproduced in the appended



1 6 ATLAS OF ABSORPTION SPECTRA.

plates shows that mechanical shifts, in wave-lengths, of the strips on one
complete spectrogram, with reference to one another, exist. This may mar
the appearance of the photographs somewhat, but the ultra-violet spark lines
show the magnitude of the displacements so that corrections can be made,
and hence the ultimate scientific value of the results is not decreased.

The order of events in taking a complete negative of three strips was
invariably as follows : The thickest layer of absorbing liquid was over the
opening of the slit first, then the intermediate layer, and last of all, the
thinnest layer, which usually tapered to infinitesimal depth. This sequence
enabled the comparison spectrum to be taken by moving the shutter, L,
without jarring the film-holder, so as to minimize the shift of this spectrum
relative to the adjacent photographic strip. For negatives of more than three
strips precisely the reverse succession was adopted because it was easier to
commence with the cell in adjustment and then to raise the quartz wedge
parallel to itself than to lower all three micrometer screws by the same
number of turns until the quartz wedge just barely came into contact with
the quartz bottom of the cell. With the screen Q horizontal the first expo-
sure with the spark was taken. The screen was lowered and the second
exposure was made, this time with the Nernst glower. These two exposures
produced the first of the three photographic strips. Next the film-holder
and cell were moved the proper distances, as explained above. The glower
and spark exposures followed in the order named. After again moving the
film-holder and cell, the fifth and sixth exposures were produced by
the spark and glower respectively. Finally the cell and diaphragm were
removed from the slit, another opening in the shutter was adjusted before
the film, and the comparison spectrum impressed. In general, the glower
exposures lasted 60 seconds, the ultra-violet exposures 75 seconds, and the
comparison exposures 35 seconds. The width of the slit was always 0.008
cm. In any one complete spectrogram the exposures to the Nernst light
were all equal to each other and those for the ultra-violet were related to
one another in the same manner. Experience showed that the intervals 60
and 75 seconds were best suited to cause the overlapping ends of the photo-
graphic impressions to blend as if they had been produced simultaneously
by light from a single source. With the longest exposures used, the light
from the glower did not affect the films and plates for wave-lengths as short
as o. 3 1 5/i and, since the field photographed did not comprise wave-lengths
longer than 0.63/4, there was no trouble produced by the ultra-violet of the
second order. The screen Q took care of this matter so far as the spark
exposures were concerned. Figures 14 and 15, plate 4, indicate how the
processes just explained can be extended to negatives as wide as may be
cjesirable and hence to as deep layers of absorbing liquid as may be wished. *

*Of course, a cell deeper than 6 mm. would be necessary if the matter were pushed very far.



RESULTS. ' 17

RESULTS.

INTERPRETATION OF THE CURVES.

If the distances from the edge of a positive which is adjacent to the
comparison spectrum (which edge therefore corresponds to zero depth of
liquid in the cell) to arbitrary points on the boundary of a sharply-defined
absorption curve be called ordinates, and if wave-lengths be considered as
abscissae, we may say that the absorption constants* associated with any
two chosen wave-lengths are inversely proportional to the ordinates belong-
ing to these wave-lengths. This statement involves certain assumptions,
about emission curves and sensibility curves, a discussion of which will not
be given here.

If the edge of an absorption band is a straight line at right angles to the
length of the picture it means that the position of this side of the band
will not appreciably change with wide variations in the concentration of the
solution; in other words, the limit of absorption will remain at the same
wave-length regardless of the concentration. This is roughly the case in
figs. 4 and 15 of plates i and 4 at the respective wave-lengths o. 29/4 and
0.515/4, and for most of the narrow bands of figs. 96, 100, and 101. If this
condition holds for all the bands of a given substance, which are within or
near the confines of the visible spectrum, the color of the light transmitted
by the solution will be the same no matter how much the concentration be
varied. This is well illustrated by solutions of the salts of neodymium and
praseodymium.

When the boundary of an absorption band is a straight line inclined to
the axis of wave-lengths it may be inferred that the limit of the band will be
displaced in proportion to the change of concentration, and that the factor of
proportionality depends upon the angle which the line makes with the axis of
abscissae. This is exemplified in fig. 45, plate 12, by the portions of the
band, at wave-length 0.47^ corresponding to the thicker layers of liquid.

In like manner, the general relation between the displacements of the
limits of absorption and the associated changes in concentration may be
easily inferred when the confines of the absorption bands are curved either
convex or concave.

EXPLANATION OF THE TABLES.

Two plans suggest themselves for the sequence of the experimental data,
viz : (a) To classify the material on the basis of the characteristics of the
absorption spectra, i. e., the succession, intensity, etc., of the bands and
regions of absorption ; (b) to arrange the results according to the chemical



1 8 ATLAS OF ABSORPTION SPECTRA.

nature of the absorbing media. Because the first method conforms more
closely to the professed object of the present research than the second, every
scheme consistent with it was tried which suggested itself. The great num-
ber of combinations on the negatives of the effects of weak, general absorp-
tion with definite, intense bands, combined with more or less uncertainty as
to the interpretation of the negatives in the region for which the source of
the discontinuous spectrum had to be used, made it impossible to find a
satisfactory permutation of the photographic records. Consequently the
second plan suggested above was followed as far as the text is concerned.
The spectrograms, on the contrary, are arranged, as far as possible, so as
not to have widely different absorption spectra succeed one another on the
same plate. * The organic coloring matters succeed one another in the same
order as is given to them in the English translation by A. G. Green of a
book by G. Schultz and P. Julius entitled "A Systematic Survey of the
Organic Colouring Matters" (Macmillan & Co., London, 1904). This con-
nection between the contents of the volume just named and the material
recorded below has the advantage of making it easy to find out many things
about the dyes which can not be appropriately given here, such as the names
of their discoverers, their literature, patents, methods of preparation, their
behavior with various reagents, chemical constitution, etc.

The descriptive tables following this explanatory section present the
experimental results in the following order:

(1) The absorption of a small number of interesting intermediate prod-
ucts, so-called, arranged according to the alphabetical order of their names.

(2) The absorption of such dyes as were studied and were capable of
identification with the dyes discussed in the book by Schultz & Julius.

(3) The absorption of such dyes as were not unquestionably the same
as any given in the reference volume. The accounts of these dyes follow
the alphabetical order of their commercial names.

(4) The absorption of certain miscellaneous objects of more or less
interest, in alphabetical order.

Whenever a number without qualification is given to a substance it
refers to the present account, but when a number is quoted from the volume
by Schultz & Julius attention is called to the fact by the abbreviation S. & J.

In the brief account of any one dye the details are presented in the


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Online LibraryHorace Scudder UhlerAtlas of absorption spectra → online text (page 2 of 9)