J. L. W. (John Louis William) Thudichum.

A treatise on the origin, nature, and varieties of wine; being a complete manual of viticulture and œnology online

. (page 21 of 64)
Online LibraryJ. L. W. (John Louis William) ThudichumA treatise on the origin, nature, and varieties of wine; being a complete manual of viticulture and œnology → online text (page 21 of 64)
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the sugrar and oxyde of copper reduced, holds strictly good.

Any solution which besides the sugars contains other sub-
stances reducing the copper salt, requires previous preparation
so as to remove, if possible, the interfering substance, without
acting on the sugar. Moreover, colourless, or nearly colour-
less, solutions only can be employed with advantage.

Before, therefore, we can proceed to the estimation of sugar
in the wine, some treatment is necessary to remove the colour,
and some substances which also reduce copper ; unfortunately
the latter is only partially attainable. This latter circumstance
is not, however,' of much consequence in wines containing a
moderate amount of sugar, but in wines containing less than
0'2 per cent, it causes frequently a considerable error, and may
even make the process entirely illusory. Light white wines
have merely to be shaken up with charcoal, which not only
decolorizes them, but also removes tannin, and such-like sub-
stances, reducing copper salts. Dark-coloured wines and such
as contain a great quantity of sugar require, however, some
further preparation. The estimation of the alcohol, after the
plan of Tabarie, gives us the specific gravity of the wine
minus its alcohol, from which the total solid contents of
the wine may readily be cafculated, as elsewhere described.
Wines containing less than 2 per cent, of solid matter
generally require no dilution. Wines with between 2 and
4 per cent solid constituents are best diluted with from
two to five times their bulk of water; whilst wines con-
taining upwards of 4 per cent, are diluted so as to yield a

Q 2



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228 SACCHAROMETRY. [CHAP.

mixture containing no more than one-half per cent, of residue.
Thus lOO c. c. of the first class of wines being measured out,
20 c. c. acetate of lead solution are added (the acetate of
lead solution is made by dissolving one part of sugar of lead in
ten parts of distilled water), and the mixture, after being well
stirred, is allowed to stand a short time, and is then filtered.
The clear filtrate is next shaken up with some animal char-
coal and again filtered, this second filtrate being generally
perfectly colourless. Sometimes, though but rarely, a little
more acetate is required (which may be seen by the filtrate
from the first lead precipitate giving a further precipitate, on
the addition of a fresh portion of acetate of lead), or a second
shaking up with a fresh portion of animal charcoal will be
found necessary before a colourless solution is obtained. Of
wines of the second class 50 c. c. are taken, 10 c. c of acetate
of lead solution are added, and this mixture is then diluted
so as to form 100 c. c. to 300 c c. solution : this is then filtered,
shaken up with charcoal, &c., as above. Of the third class of
wine 30 c c. only need be taken, 5 c. c. acetate added, and
water added so as to yield a mixture, giving about 50 c. c.
solution for each per cent, of residue in the wine ; it is filtered,
treated with charcoal, &c The wine is best measured by
means of a pipette, and poured at once into a measure flask
of the required capacity, holding, when filled up to a mark on
the neck, 100 c. c, 200 c. c, 300 c. c, 500 c. c, &c The acetate
of lead is then added, and the flask is filled up to the mark
with distilled water. The clear filtrate thus obtained is now
fit for testing. It is sometimes recommended to render the
wine alkaline with lime water before adding the acetate ; this,
however, should be avoided, inasmuch as in an alkaline solu-
tion grape sugar is precipitated, at least partially, by acetate
of lead, and a loss of sugar is thus occasioned.

10 c. c of our standard copper solution are now measured
into a small porcelain dish, 50 c. c. distilled water added, and
heat applied. The blue solution should remain perfectly clear
when boiled for a few minutes. The heat is then moderated
so as just to keep the solution gently boiling, and the de-
colorized wine is slowly poured in from a burette divided into



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VIII.] CHEMICAL METHOD, 229

tenths of a c. The addition of the wine should be so regulated
that the gentle boiling is not interrupted.

A yellowish precipitate is produced, if sugar be present,
generally becoming speedily red and pulverulent, so as readily
to settle to the bottom. Sometimes, however, it remains of
a dirty greenish colour, flocculent, or very finely divided, so
that it scarcely settles down at all. This is chiefly observed
in wines poor in sugar, and is most probably due to other
substances than sugar, rendering the estimation of sugar in
these wines by means of this test extremely unsatisfactory.
The addition of wine from the burette is continued until the
blue colour of the copper solution is entirely destroyed. For
the purpose of observing this, the heat is moderated so as to
stop ebullition and allow the precipitate to subside, when on
slightly tilting the dish the colour of the supernatant liquid
can be observed. The solution should not, however, be
allowed to stand too long without boiling, as in the cold
the suboxyde of copper thrown down is redissolved, again
forming a blue solution. The point of disappearance of the
blue colour may be observed with tolerable accuracy if the
sugar solution employed contained not much less than 05 per
cent, and is free from other admixtures. In the case of most
wines, however, it is not easy to observe the point distinctly,
because the supernatant liquid generally acquires a yellowish
tint, which makes it very difficult to observe whether or no
the last trace of blue has disappeared. In such cases it is
advisable, as soon as the blue becomes difficult to observe, to
filter a small quantity of the boiling liquid (about \ c c.) into
a small test tube or watch-glass, and add a drop of ferro-
cyanide of potassium and a few drops of diluted acetic acid ; a
brown precipitate or coloration indicates that there is still some
copper in solution ; some more wine must be added, and the
boiling continued until a portion thus tested gives no longer
any indication of the presence of copper. The intensity of
the coloration produced by the ferrocyanide will, after a
little practice, be a good indication of the amount of wine
still to be added, so that generally no more than two or three
filtrations are necessary. It is essential that the small frac-



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230 SACCHAROMETRY, [CHAP.

tion filtered should be perfectly clear, as, if it contains even
a trace of suboxyde of copper in suspension, this will dissolve
in the acetic acid, and thus copper will be found in solution,
though perhaps the whole of it had been precipitated. The
precipitated suboxyde is often dense and coarse enough to be
readily removed by filtration through ordinary filtering-paper ;
sometimes, however, it is so fine that it passes even through
Swedish paper, and great care is therefore required in using
this test. The filtration has also to be conducted with the
boiling hot solution, otherwise there is danger of redissolving
some of the precipitate as previously stated. If by one of
these means the point has been fixed when all the copper
is reduced, the amount of decolorized wine used is read oflT
from the burette ; it gives the quantity of solution which con-
tains cos grm. of sugar, the quantity required to reduce the
copper in the lo c. c. of standard solution employed. From
this the percentage of sugar in the diluted wine is calculated,
which, being multiplied by the number of times the wine
had been diluted, gives the percentage of sugar present in
the wine. Thus let us suppose 50 c. c. wine had been diluted
to 200 c. c, and that 15 c. c. of this are required to reduce the
copper of the 10 c. c. standard solution employed. If 15 c, a
contain 0*05 grms., lOO c. c will contain 0*333 grm., or

15 : 005 = 100 :x '\'X = o*333 per cent. ;
and as the wine had been diluted four times, the wine itself
will contain 1*332 per cent, of sugar.

If less than 10 c. c of the decolorized wine suffice to pre-
cipitate the copper from the 10 c. c. copper test, the dilution
had not been enough, and a second experiment must be made
with more diluted wine.

The process, as here described, estimates only the grape,
fruit, or invert sugar, all of which, as previously stated, reduce
oxyde of copper from its solution to the suboxyde, one equi-
valent of one of these sugars reducing ten equivalents of
oxyde of copper. Cane sugar does not, however, reduce
oxyde of copper, and cannot, therefore, be thus estimated. *
Cane sugar is, however, readily converted into invert sugar
by diluted acids, and may therefore be estimated as such.



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VIH,] OPTICAL METHODS, 231

One atom of cane sugar becomes two atoms of invert sugar,
or Cm Hj, Ou becomes 2 x Ce H^ O9. or 342 parts of cane
sugar become 360 parts of invert sugar. The amount of
invert sugar found has, therefore, to be reduced in the pro-
portion of 360 to 342, to give the amount of cane sugar from
which it was produced. For the purpose of conversion
100 c c of the solution containing the cane sugar are mixed
with 10 cc of strong hydrochloric acid and heated in a
water-bath for fifteen minutes to a temperature of 70° C,
after which the sugar is estimated as above. In doing so
it is sometimes necessary to adopt the following precau-
tion. The reduction of oxyde of copper to suboxyde, and
its consequent removal from the solution, takes place only
in alkaline solutions ; if then the wine is very acid, and much
of it has to be added to the copper test, it may sometimes
neutralize the alkali to a sufficient extent to prevent the
due action of the test. In such a case it is advisable to
examine whether the liquid in the dish is still strongly alka-
line, and, if necessary, to add a small piece of hydrate of soda.

Cane sugar, as before stated, is rapidly changed into invert
sugar in the presence of acid, and is, practically, never found
in wine.

There are sometimes found in wine substances other than
cane sugar, which, when boiled with dilute sulphuric acid,
become converted into grape sugar. They may be approxi-
mately estimated by the amount of sugar they yield. After
estimating the amount of sugar in the wine, the operator has.
only to heat lOO cc. of it with i cc of strong sulphuric acid, and
again to estimate the sugar. Any increase in the sugar found
is due to the conversion into sugar of such glycogenetic matters.

Optical Methods for the Estimation of Sugar,
Solutions of the different kinds of sugar have the property
of rotating the plane of polarized light. The degree of such
rotation depends on the amount of sugar present in a certain
volume of solution, the length of the column of such solution
through which the light passes, the temperature of the solution,
and the colour of the light. Upon this property of sugar an
easy and accurate method for its estimation has been based.



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232



THEORY OF



[chap.




An ordinary ray of light falling upon a polished surface is
reflected, whatever may be the direction in which the ray-
falls upon the surface. If, however, such a ray has once
been reflected from a polished surface (which must not, how-
ever, be metallic) under a certain angle, 35** in the case of
glass, it has acquired the remarkable property of not being
reflected, under all circumstances, from a second polished
surface upon which it may fall.

Let m m be a plate of glass, on
which a ray of light, A T, falls in a
direction making an angle of 35**
with the plane of the plate ; part of
the light will pass through, but part
of it will be reflected in the direc-
tion of t'. Now if this ray at T' is
made to fall upon a second mirror,
m' m\ it will be reflected like an
ordinary beam, if the planes of
incidence (a plane containing both
the incident beam and the per-
pendicular erected at the point of
incidence of the beam with the
mirror) of the two mirrors coin-
Now let the mirror m' m be turned
round T t', as an axis ; it will be found that the intensity of
the beam T' E, gradually diminishes, and, as soon as the
planes of incidence of the two mirrors make an angle
of 90° with each other, the beam T t' will not be re-
flected at all from the upper mirror. Continuing the rota-
tion, light will again be reflected, the intensity of the
reflected ray gradually increasing until the upper mirror has
been turned through an angle of 180**, when the beam T' E
will again be at a maximum, the planes of incidence of the
two mirrors coinciding once more. If the mirror be still
further rotated, the ray t' E will diminish again ; and after
a further rotation of 90°, or 270° from the first position,
will once more vanish, to be again at a maximum when
brought back to the first position.




Fig. 36.— Polarization of Li^ht
by reflection.



cide, as in the figure.



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VIII.] POLARIZATION. 233

During one rotation of the mirror there are, therefore, two
points 180*^ apart, where the reflected beam t' E is at a maxi-
mum, and two intermediate points in which it is at a minimum.
The first takes place when the planes of incidence in the two
mirrors coincide ; the second, when they are at right angles
to each other.

A ray of light which possesses the property of T t' (being
reflected or not reflected according to circumstances) is called a
polarized ray. It is assumed that in such a ray all vibrations
of the ether are performed in one plane, called the plane of
vibration ; it is at right angles to the plane of incidence in the
above case : this latter is called the plane of polarization.



Fig. 37.>^Showing the direction of the vibration-s in a Ray of Light, &c., polarized by reflection.
The plate e dj g is the reflecting mirror, a b the incidental ray of common light, b c the re-
flected ray of polariied light, the polarization consisting in the peculixuity that the light vibrates
in the plane indicated by A / ^ /, as sketclied by the undulating line.

A similar alteration in a ray of light is produced by its
passage, in certain directions, through various transparent
media, e.g. all crystals, except those belonging to the regular
system. Crystallized rhombohedric calcic carbonate, called
Iceland spar or double spar, shows this property of polarizing
a beam of light passing through it, in great perfection. A ray
of light passing through such a crystal, in the direction of the
principal axis, undergoes but a single refraction; but when
passing in any other direction, it is split into two rays, which
emerge from the crystal in different directions. Both rays
are polarized, the plane of polarization of the one being
at right angles to that of the other. One of these two rays
follows the ordinary law of refraction: the proportion existing
between the sines of the angle of incidence and angle of re-
fraction is constant ; it is called the ordinary ray. The second
does not follow that law ; it is called the extraordinary ray.

For most purposes for which polarized light is employed, the



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234



NI COL'S PRISM.



[chap.



existence of two rays polarized at right angles to each other
is not only needless but injurious, one polarized beam being
all that is required. The superfluous ray is therefore mostly
excluded by mechanical contrivances, such as Nicols prism.
Let A B E G (Fig. 38) be a natural elongated rhombohedron,
the axis of which passes through the corners C and E ; a sec-
tion through A C E G will, therefore, be a principal section.

The face, A B C D, will be at right
angles to this principal section, and
make an angle of 71° with the edge
A E. This face, A B c D, and
the opposite one parallel with it,
are first replaced (by grinding) by
two other faces also at right angles
to the above principal section, and
making an angle of only 68° with the
edge A E. The crystal is next cut
in two, along a plane at right angles
to the principal section, and to the
newly cut faces ; the cuts are polished
and the pieces cemented together
again in their original position by
means of Canada balsam. A prism
thus prepared has the following pro-
perty. A ray of light falling on the
face A C (Fig. 39, on opposite page)
parallel with the edges A E and C G, is
split into two rays, — ia, the ordinary,
and i d, the extraordinary one The
first strikes the layer of Canada balsam within the limiting
angle, and is totally reflected in the direction a r ; the second,
i d, passes through and emerges as a single ray from the face
E G, parallel to the original direction and perfectly polar-
ized in a plane at right angles to the plane A C E G, the
principal section (in the figure the plane of the paper), or
parallel to the longer diagonal of the rhombic face A C.
The plane of vibration of the ray is, therefore, parallel to
the shorter diagonal of that face



Fig. 38. — Elevation and section
of an elongated prism of Ice-
land spar, snowing the direction
in which it is cut when made
into a Nicol's prism.



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VIII.J



POLARIZATION AND ANALYSATION.



235



If two Nicol's prisms are placed one behind the other,
light which has passed through the first ,

prism will pass without hindrance through '

the second if both prisms are in similar
po5ition,^-that is to say, if the principal
sections of both are parallel ; the light
will, however, be entirely stopped by the
second prism if the principal sections
are at right angles to each other ; in any
other intermediate position, some light
will pass, Some will be stopped. The
intensity of the ray, transmitted by the
second prism, is greatest with parallel
Nicols, «// with crossed Nicols, intermediate
in all other positions.

Supposing now two Nicols, placed one
behind the other, some distance apart,
and a ray of light to enter the first prism,
which we will call the polarizing prism ;
it will issue from it polarized in a certain

direction, as before described. If, then,

the second Nicol, called the analysing

prism, is placed with its principal section

at right angles to that of the first, the

field of view will be dark, at least the

central part will be black, the sides

showing some light The interposition

of a piece of glass, or a tube filled

with water, between the two prisms will

produce no effect ; the field remains

dark. But if, instead of these, a plate

of rock crystal (cut perpendicularly to

the principal axis) be interposed, the

field of view will become illuminated,

and the analysing Niqol will have to

be turned round a certain amount to

render the field dark again ; a similar

effect is produced by certain liquids, — oil of turpentine, or



Fig. 39. — Longitudinal
section of Nicol's prism
through the edges a e
and c o of Fig. 38 ;
namely, through the
shorter diagonal of the
rhombic cross section.



^../X




Fig. 4o.~Sections of two Ni-
col's prisms superimposed.
In the upper figure the prin-
cipal sections of the two
prisms being parallel, light
IS transmitted througn both
prisms ; in the lower, the
principal sections being at



right angles, all the light
which has passed through
the first prism is stopped by



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236 CIRCULAR POLARIZATION. [chap.

solutions of sugar, for example. This experiment shows
that the plane of polarization of the ray is turned a certain
amount whilst passing through these solids or liquids, and
the analysing prism has to be turned to the same amount to
bring its principal section again into parallelism with the plane
of polarization of the beam as it issues from the substance.

The effect here described is called circular polarization
or rotation. The amount of such rotation depends, firstly,
upon the nature of the substance ; secondly, upon the thick-
ness of the layer through which the ray passes ; thirdly,
upon the colour of the light, and lastly, the temperature.
In one and the same substance the amount of turning is
proportional to the thickness of the layer, red light being
at the same time turned least, violet light most A plate
of quartz, for example, i mm. thick, turns the plane of
red light 19°, green light 28**, and violet light 41°; a plate
of 2 mm. thick giving 38°, 56°, and 82° respectively.



Fig. 41.— Cylinder of a substance possessing the power of circular polarization,
showing how the plane of polarization a b of a ray of light, passing in the direction

' of the arrow, is turned (a b^ a' b' t^' If') in exact proportion to the length of sub-
stance passed through.

Let A B C D represent a column of a substance possessing
this power of circular polarization. Let a ray of light enter
the face A B in the direction c Wy and having its plane of
polarization vertical, as A B. This plane of polarization will
gradually be turned the more the deeper the ray enters the
substance, assuming, successively, the positions a by a' b\ and
will finally emerge from the face C D, polarized in the direc-
tion ci' V'y making an .angle c w a" with its original position.
It is this angle through which the analysing prism would
have to be turned so as again to extinguish the light This
angle, however, as before stated, varies with the nature of
the light, being least for red, greatest for violet. It follows
that if the ray entering at A B be compound light, — white



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VIIL] POLARIZING SACCHAROMETERS. 237

light, for example, — it will on emerging from C D be separated
into its components, the red portion being turned least, the
violet most If, under these circumstances, the analysing
prism is turned, instead of arriving at a point of darkness,
as with homogeneous light, the field of view will never be-
come quite dark, but will be illuminated by coloured light,
the colour varying with the position of the Nicol. If, in
order to extinguish red, green, and violet light successively,
the prism has to be turned to the right, the substance shows
right-handed rotation ; if, in order to extinguish the colours
in the above order, we must turn the prism to the left, the
substance possesses left-handed circular polarization. What-
ever be the colour of the light, the amount to which this
colour is rotated is generally proportional to the length and
strength of active substance interposed between the Nicols.
If, then, one of the different tints produced by turning the
analysing Nicol, when white light is used, could be selected,
which we could always readily fix upon, it would be unne-
cessary to employ monochromatic light Such a tint is found
at the point where the blue or violet change into red ;
here a very little turning of the Nicol in one direction
will render the field distinctly blue ; a little turning in the
opposite direction will render it as distinctly red. The change
of one colour into the other takes place tolerably abruptly ;
the analysing prism may be placed in such a manner that one
side of the field is red, the other side blue, the centre a kind
of violet The position of the Nicol required to produce this
central tint, can indeed be fixed with much greater accuracy
than that corresponding to the point of maximum darkness ;
it is the point selected by Mitscherlich to measure the power of
circular polarization possessed by various substances.

POLARIZING SACCHAROMETERS OF MITSCHERLICH, SOLEIL,
AND JELLETT.

As will be seen from the preceding, it is necessary, in order
to estimate the degree of circular polarization shown by any
substance, that we should be able accurately to fix the position
of the plane of polarization of a ray of polarized light A



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238 MITSCHERLICWS SACCHAROMETER, [chap.

great number of instruments have been devised to accomplish
this, of which, however, a few only can here be described.

M it seller licKs Polaroscope. — This was one of the earliest
proposed, and is one of the most simple. It consists
essentially of a polarizing Nicol's prism ; a lens to render
the rays of light parallel ; a tube, closed at both ends by
plate glass, to hold the solutions to be examined ; and an
analysing Nicol's prism. This latter is mounted in such a
manner that it can be readily turned round its longest axis,
whilst the degrees of the circle round which it is turned
can be read off a brass plate. The instrument is used as
follows : — The tube filled with pure water being placed between
the Nicols, the index of the analysing prism is placed ac-
curately on zero, and the observer, looking through the



Online LibraryJ. L. W. (John Louis William) ThudichumA treatise on the origin, nature, and varieties of wine; being a complete manual of viticulture and œnology → online text (page 21 of 64)