Scientific American Suppl. No. 299

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Very little practice enables an ordinary workman to judge when
two-thirds of the contents of the big pot are in crystals, and one-third
liquid; and when he sees this to be the case, instead of ladling out the
crystals ladleful by ladleful, as in the old Pattinson process, he taps
out the liquid lead by means of two pipes, controlled by valves, the
crystals being retained in the pot by means of perforated plates.

The liquid lead is run into large cone-shaped moulds on either side of
the pot; and a wrought iron ring being cast into the blocks thus formed,
they are readily lifted, when set, by the crane. To give some idea of
the rapidity of the process, it may be mentioned that from the time the
lead is melted and fit to work in the big pot, to the time that it is
crystallized and ready for tapping, is, in the case of a 36 ton pot,
from thirty-five to forty-five minutes; and the time required for
tapping the liquid lead into the large moulds is about eight minutes.

Before the lead begins to crystallize, the upper pot is charged with
lead of half the richness of that in the lower pot. Thus, when the
liquid lead has been tapped out of the lower pot, it is replaced by a
similar amount of lead of the same richness as the remaining crystals,
by simply tapping the upper or melting pot, and allowing the contents to
run among the crystals.

The same operation is repeated from time to time, until the crystals are
so poor in silver that they are fit to be melted, and run into pigs for
market.

The large blocks of partially worked lead are placed by the crane in
a semicircle round it, and pass successively through the subsequent
operations. The advantages of the steam process, as compared to the old
six-ton Pattinson pots formerly used by the writer's firm, are: (1) a
saving of two-thirds amount of fuel used; (2) the saving of cost of
calcination of the lead to the extent of at least four-fifths of
all that is used; (3) above all, a saving in labor to the extent of
two-thirds. The process has its disadvantages, and these are a larger
original outlay for plant, and a constant expense in renewals and
repairs. This is principally caused by the breakage of pots; but with
increased experience this item has been very much reduced during the
last two or three years.

The "zinc process" of desilverizing, which is largely used by Messrs.
Locke, Blackett & Co., and was patented in the form adopted by them
about fourteen years since. The action of this process is dependent on
the affinity of zinc for silver. The following is a brief description of
it:

A charge of silver lead, usually about fifteen tons, is heated to a
point considerably above that which is used in either the Pattinson or
the steam process. The quantity of zinc added is regulated by the amount
of silver contained in the lead; but for lead containing 50 oz. to the
ton, the quantity of zinc used is in most cases about 11/2 per cent, of
the charge of lead. The lead being melted as described, a portion of
this zinc, usually about half of the total quantity required for the
charge, is added to the melted lead, and thoroughly mixed with it by
continued stirring. The lead is now allowed to cool, when the zinc is
seen gradually to rise to the top, having incorporated with it a large
proportion of the silver. The setting point of zinc being above that
of lead, a zinc crust is gradually formed, and this is broken up and
carefully lifted off into a small pot conveniently placed, care being
taken to let as much lead drain off as possible. The fire is again
applied strongly to the pot, and when the lead is sufficiently heated, a
further quantity of zinc, about one-third of the whole quantity used, is
added, when the same process of cooling and removing the zinc crust is
repeated. This operation is gone through a third time with the remaining
portion--1/4 per cent.--of zinc; and if each of these operations has
been carefully carried out, the lead will be found to be completely
desilverized, and will only show a very small trace of zinc. In some
works this trace of zinc is allowed to remain in the market lead, but
at Messrs. Locke, Blackett & Co.'s works it is invariably removed by
subjecting the lead to a high heat in a calcining furnace. The zinc
crusts, rich in silver, are freed as far as possible from the lead by
allowing this to sweat out in the small pot, after which the crusts are
placed in a covered crucible, where the zinc is distilled off, and a
portion of it recovered. The lead remaining, which is extremely rich in
silver, is then taken to the refinery, and treated in the usual manner.
The writer is given to understand that the quantity of zinc recovered is
as high as from 50 to 60 per cent. of the total quantity used.

Although it was said that the rolling or milling of lead remains
unchanged in its main features since the first mill was established, yet
the writer's firm have introduced many important improvements. When lead
is required for sheet making, instead of running out the market lead
into the usual pigs of about one hundredweight each, it is run into
large blocks of 31/2 tons. These 31/2 ton blocks are taken on a bogie to
the mill-house, where the mill melting pot is charged with them by means
of a double-powered hydraulic crane, lifting, however, with the single
power only.

Three such blocks fill the pot, and when melted are tapped on to a large
casting plate, 8 ft. 4 in. by 7 ft. 6 in., and about 7 in. thick. This
block, weighing 101/2 tons, is lifted on to the mill table by the same
crane as fills the pot, but using the double power; and is moved along
to the rolls in the usual manner by means of a rope working on a surging
head. The mill itself, as regards the roll, is much the same as those
of other firms; but instead of an engine with a heavy fly-wheel, always
working in one direction, and connected to the rolls by double clutch
and gearing, the work is done by a pair of horizontal reversing engines,
in connection with which there is a very simple, and at the same time
extremely effectual, system of hydraulic reversing. On the usual method
there is no necessity for full or delicate control of lead mill engines;
but with this system it is essential, and the hydraulic reversing gear
contributes largely to such control. This may be explained as follows:

In all other mills with which the writer is acquainted, when the lead
sheet, or the original block, has passed through the rolls, and before
it can be sent back in the opposite direction, a man on either side of
the mill must work it into the grip of the rolls with crowbars.

In the writer's system this labor is avoided, and the sheet or block is
fed in automatically by means of subsidiary rolls, which are driven by
power. When it is required to cut the block or sheet by the guillotine,
or cross-cutting knife, instead of the block being moved to the desired
point by hand-labor, the subsidiary driven rolls work it up to the
knife; and such perfect control does the engine with its hydraulic
reversing gear possess, that should the sheet overshoot the knife
1/8 in., or even less, the engine would bring it back to this extent
exactly.

Another point, which the writer looks upon as one of the greatest
improvements in this mill, is its being furnished with circular knives,
which can be set to any desired width, and put in or out of gear at
will; and which are used for dressing up the finished sheet in the
longitudinal direction. This is a simple mechanical arrangement, but
one which is found to be of immense benefit, and which, in the writer's
opinion, is far superior to the usual practice of marking off the sheet
with a chalk line, and then dressing off with hand knives. The last
length of the mill table forms a weighbridge, and a hydraulic crane
lifts the sheet from it either on to the warehouse floor or the tramway
communicating with the shipping quay.

* * * * *

APPARATUS USED IN BERLIN FOR THE PREPARATION OF GELATINE PLATES.

I.--MIXING APPARATUS FOR GELATINE EMULSION.

The mixing vessel--a porcelain kettle capable of containing twenty
liters, made at the Royal Porcelain Factory at Berlin, whose products
are unequaled for chemical purposes--is also the boiling vessel, and,
therefore, fits tightly, by means of the tin ring with the wooden
handles, on to a large water bath. The light-tight metal lid, which
can be permanently affixed to the kettle, then supports a stirring
arrangement of fine silver, which dips into the emulsion and has blades
formed like a ship's screw.

The arrangements for injecting the silver vary. The simplest consists of
a large glass vessel containing the silver solution, which is closed
by a glass stopper, and terminates below in a funnel running to a fine
point. This funnel-shaped bottle fits into an opening specially made for
it in the lid of the kettle, and while revolving sends a fine stream
into the gelatine. When it is wished to interrupt it, it is only
necessary to raise the glass stopper in order to see the stream dry up
after a short time.

Another arrangement consists of a contrivance constructed on the
principle of the common India-rubber inhaling apparatus, and sends
the silver solution into the gelatine in the form of the minutest
air-bubbles. After the emulsion is boiled in such a kettle it is allowed
to stand until cool, when the ammonia is added. With such a great
quantity of emulsion and so large a water bath sufficient heat is
retained as to allow the action of the ammonia to take place. As soon
as the time set apart for that reaction has elapsed the water bath is
emptied and filled with pieces of ice and iced water, and the kettle
replaced in it.

If the stirring apparatus be now set in motion, even this large quantity
of emulsion will stiffen in at least an hour and a half. It may be
further remarked that, the outside of the kettle being black, the lid
being light-tight, and all the apertures in it being firmly closed,
nearly the whole process can be conducted by daylight, from the mixing
to the stiffening, so that it is very convenient to be able to keep the
emulsion in the same vessel during all these operations.

II.--DIGESTIVE APPARATUS.

It is very desirable that those who do not prepare their emulsion by
boiling, but by prolonged digestion, should possess a regulator which
will keep the temperature at a given point. Such an apparatus would also
be very useful for warming the emulsion for the preparation of plates,
as then one would have no further occasion to pay attention to the
thermometer and gas stove. In the accompanying diagram a simple
contrivance is shown. The gas which feeds the stove passes through
a narrow glass tube, a b, into the wider tube, c d e, which is made
air-tight at e. This latter tube has an exit tube at f, by which the gas
is supplied to the gas stove. At e it is hermetically closed, and at its
deepest part it contains mercury, upon which a little sulphuric ether
floats in the hermetically-closed limb, e.g. Lastly, there is a minute
opening in the narrowest tube at i. The whole apparatus, or, at least,
the under part of it, is dipped into the water bath warmed by the gas
boiler. It acts thus: As the temperature rises the ethereal vapor in the
shorter limb expands and drives the mercury up the longer tube until
it closes the opening of the narrow tube, a b, and thereby impedes the
power of the stream of gas. Still, the Bunsen burner does not go out,
being always fed by the small opening, i, with sufficient gas to support
a small flame until the water bath has so far cooled as to leave the
opening at b free, when the burner again burns with a strong flame. By
removing the cork, c, from the tube the temperature of the water bath is
raised, while by pushing it in it is lowered. The apparatus never goes
wrong, and is very cheap. It was first made by Herr C. Braun, of Berlin.

[Illustration: FIG. 1.]

III.--TRITURATING APPARATUS.

The apparatus hereafter described is in general use, and was invented
by Herr Paul Grundner, of Berlin. It is particularly adapted for finely
dividing large quantities of emulsion. It consists essentially of a
wooden lid, a b, fitting upon a large stone pot, to the under side of
which two strong trapezoid pieces of wood, e d and e f, are fixed, in
the under part of which semicircular incisions are cut and held together
by two leather straps, supporting a strong, easily-removable iron
transverse bar, g h. Through the center of the lid, and turned by the
crank, m, passes the axle i, which ends under the lid in the long ring,
n.

The stiffened emulsion is then placed in the bag, o p q r, made of
fine but strong canvas, with meshes about 0.5 mm. (such as is used for
working upon with Berlin wool). The iron rod, g h, is then slipped
through the four loops at the bottom of the bag, the open end is slung
upon the ring, n, and bound tightly to it by the ribbons, r1. The loops
upon the iron bar are then pushed as close together in the middle as
possible, and the stone vessel is filled with water until o p q r is
completely covered. The crank is then turned, by which the bag is wrung,
and the emulsion squeezed through the meshes immediately into the water.
When this process is continued until the purse between n and g h feels
like a metal rod, the best part of the emulsion has been squeezed
through, and if one now take out the bag and dissolve its contents, it
will be found that the loss of emulsion is almost _nil_.

[Illustration: FIG. 2.]

It may be remarked that the whole apparatus, with the exception of the
crank, must be coated with asphalt varnish; also that the corners, r and
q, must be separated off from the purse, as shown by the dotted line, s
s s s, otherwise the emulsion would lodge there without being squeezed
through. Instead of g h a strong glass rod may be used for small
apparatus; but for large apparatus it is indispensable, as the power
that requires to be exerted would be far too great for glass.

IV.--WASHING APPARATUS.

The fundamental idea of the apparatus shown in Fig. 3 first occurred
to Herr Jos. Junk, of Berlin. In the present form all the subsequent
improvements made by Herren Carl Such, Paul Grundner, and others are
incorporated. It may be described as follows:

A tin vessel, the bottom of which sinks at e into the shape of a funnel,
rests upon strong iron feet, f f, and is covered with a lid, having a
double edge closing it light-tight. Through the center of the lid passes
the tube, g h, by which the water enters. In the interior of the vessel
upon iron hooks stands a wooden vessel saturated with paraffine, open at
the ends, and over one end of which the finest hair cloth is stretched
at o p. The water which enters the vessel runs off through the siphon.
The proceedings are as follows: Turn the granulated gelatine and the
water in which it is contained into the horsehair sieve, m n o p. Place
the lid upon the apparatus and turn on the water. The whole apparatus
fills with water until the siphon begins to act. If the diameter of
the siphon be properly measured--one inch should be sufficient for the
largest apparatus--and the cock by which the water is turned on properly
adjusted, more water will run out by the siphon than runs in through the
supply pipe, and the apparatus becomes completely empty.

The siphon has then performed its function, the apparatus fills again,
and the play begins anew. The tube, g h, which reaches right down nearly
to the bottom of the sieve, takes the water so deep into the vessel
that, as long as the water in the apparatus stands high enough above o
p, the gelatine nodules are in continuous motion. In order to prevent
the finest particles of the emulsion from stopping up the pores of the
sieve too much, and thereby incurring the danger of the water in the
sieve overflowing its upper edge, thus occasioning loss of emulsion,
the tube, g h, is now sometimes omitted and replaced by a supply pipe,
represented in the diagram by the dotted lines, x y. In this way
every possibility of loss is excluded, and yet a very careful washing
provided. Then when, after being emptied by the siphon, the apparatus
fills again, every particle of the emulsion which might have formerly
been pressed down into the interstices of the sieve would now be driven
up again by the upward pressure of the water entering from below, and
thus the sieve would always be kept clear and open.

[Illustration: FIG. 3.]

The great advantages of this apparatus are as follows: 1. From the
moment the lid is closed one can work by daylight. 2. The method of
washing in moving water is combined with that of complete change of
water. 3. The emulsion never comes in contact with metal. 4. Whoever
wishes to prepare dry gelatine only requires, when the washing is over
and the vessel perfectly emptied, to leave the emulsion to drip for a
time, and then to lift out the sieve and its contents and place it in
a suitable vessel with absolute alcohol. The latter should be changed
once, and when sufficient water has been extracted the sieve should be
withdrawn from the vessel and the emulsion allowed to dry spontaneously.
In this way all trouble occasioned by changing from vessel to vessel is
avoided, and there is no loss of material.

This apparatus is principally valuable in dealing with large quantities,
since it saves a great deal of labor, and affords perfect certainty of
the emulsion being well washed. It may not be unnecessary to maintain
that the difficulties of perfect washing--particularly if one do not
wash with running water--increase at least in quadruple proportion to
the quantity of emulsion manipulated.--_Franz Stoke, Ph.D., in Br. Jour,
of Photography_.

* * * * *

HOW TO MAKE EMULSION IN HOT WEATHER.

By A. L. HENDERSON.

Numerous complaints have reached me within the last few weeks of the
difficulty experienced in preparing emulsion and coating plates; one is
very likely to blame everything but the right, but doubtless the weather
is the culprit.

I have always held that to boil gelatine is to spoil it, and, even when
emulsification is made with a few grains to the ounce and cooled down
before adding the bulk, the damage is done to the smaller quantity,
so that when mixed it contaminates the whole mass; moreover, it is
impossible to set and wash the gelatine without the aid of ice.

I have lately made several batches (with the thermometer at 92 deg. in the
shade, and the washing water at 78 deg.) as follows:

Hard gelatine...............,...... 1/2 ounce.
Water.............................. 2 ounces.
Alcohol............................ 2 "
Bromide ammonia....................150 grains.
Liquor ammonia, 880................ 60 drops.

When all is thoroughly dissolved and of about 120 deg. temperature, add,
stirring all the time,

Nitrate silver..................... 60 grains,
Water.............................. 3/4 ounce.
Alcohol............................ 3/4 "

Then again add,

Nitrate silver.....................140 grains.
Water.............................. 1 ounce.
Alcohol............................ 1 "

Both solutions being warmed to about 120 deg..

My object is adding the silver in two quantities will be obvious to
many--viz., when the first portion of silver is mixed, nitrate of
ammonia is liberated (which is a powerful restrainer), and the bulk of
the solution being increased, the remainder of the silver may be added
in a much more concentrated state.

The alcohol, both in the gelatine and silver solutions, plays a most
important part: (1) It prevents decomposition of the gelatine. (2) It
allows the gelatine to be precipitated with a much smaller quantity of
alcohol (say about 10 ounces).

After letting the emulsion stand for a few minutes to ripen, I pour in
slowly about eight ounces of alcohol, stirring all the time, and keeping
the emulsion warm; the emulsion will adhere to the stirring-rod and the
bottom of the vessel in a soft mass, and all that is now required is to
pour away the alcohol, allow the emulsion to cool, tear it into small
pieces, wash in several changes of cold water, make up the quantity to
ten ounces, and strain; it is then ready for coating.

By this formula I have no difficulties whatever; my plates set in about
five minutes, and their quality is such that, "unless a better method is
devised," I intend to adopt it in all weathers.

One word more as to the alcohol. It will prevent the decomposition of
gelatine when boiling goes on, or when in the presence of foreign salts;
no flocculent deposit is noticed in the alcohol after the emulsion has
been precipitated.--_Photographic News_.

* * * * *

THE DISTILLATION AND RECTIFICATION OF ALCOHOLS BY THE RATIONAL USE OF
LOW TEMPERATURES.

By RAOUL PICTET.

The industrial problem of the rectification of alcohols is based
entirely upon the properties of volatile liquids, upon the laws of the
maximum tensions of the vapors of these liquids, and upon the influence
of temperature upon those different elements which find themselves in
presence of each other in an alembic.

If we desire to follow, in their least details, all the phenomena which
succeed one another in a rectifying column, and which are connected with
one another by a continuous chain of reciprocal influences, the problem
becomes exceedingly complex.

[Illustration: PICTET'S APPARATUS FOR THE RECTIFICATION OF ALCOHOL BY
COLD.]

In order that the new applications of the mechanical theory of heat may
be readily understood, we shall divide this problem into a series of
propositions, which we shall examine separately, and which collectively
constitutes in its general features the methodical rectification of
liquids.

I. Knowing the maximum tensions of pure water and pure alcohol, can we
calculate directly the tensions of the vapors of any mixture whatever of
alcohol and water?

Yes, we can calculate this tension by a general formula, provided we
take into account the affinity of water for alcohol, which increases the
value of the total latent heat of evaporation of the liquid. The results
of the calculation are fully confirmed by experience. We thus establish
the following laws:

a. For any temperature whatever, the maximum tension of the vapors of a
mixture of water and alcohol is always comprised between that of pure
water and that of pure alcohol.

b. The tension of the vapors of a mixture of water and alcohol
approaches the tension of alcohol so much the nearer in proportion as
the proof is higher; and, reciprocally, if water is in excess, the
tension of the vapors approaches the tension of the vapors of water.

c. The curves of the maximum tensions of vapors formed by all mixtures
of alcohol and water are represented by the same general formula, one
factor only of which is a function of the richness of the alcoholic
solution.

It results, then, from these laws that we may determine with the
greatest exactness the richness of a solution containing alcohol and
water, if we know the tension of the vapors that it gives off at a
certain temperature. Such indications are confirmed by the centigrade
alcoholmeter.

We see likewise that, for these solutions of alcohol and water, the
laws of Dalton are completely at fault, since the total pressure of the
vapors is never equal to the sum of the tensions of the two liquids,
water and alcohol.

II. Being given a solution of water and alcohol, mixed in equal volumes,
what will be the quality of the vapors emitted from it?

In other terms, do the vapors which escape from a definite mixture of
water and alcohol also contain volumes of vapor of water and alcohol in
the same proportion as the liquids?

We have discovered the following laws:

d. The quality of the vapors emitted by a mixture of water and alcohol
varies according to the alcoholic richness of the solution, but is not
in simple proportion thereto.

e. The quality of the vapors emitted by a definite mixture of water and
alcohol varies according to the temperature.

f. In a same solution of water and alcohol, it is at low temperatures
that the vapors emitted by the mixture contain the largest proportion of
alcohol.

g. The more the temperature rises the more the tensions of the two
liquids tend to become equalized.

We have been able to verify these different laws experimentally, and
to find an interesting confirmation of our general formula of maximum
tensions, in the following way:

Let us take a test tube containing a 50 per cent. solution of alcohol
and water, plunge it into water of 20 deg.C., and put its interior in
hermetic communication with the receiver of a mercurial air-pump.

We vaporize at 20 deg. a certain quantity of the liquid, and the vapors
fill the known capacity of the pump. The pressure of the gases in the
interior is ascertained by a pressure gauge, and this pressure should be
constant if care is taken to act upon a sufficient mass of liquid and
with moderate speed. When the receiver of the air-pump is full of
vapors, communication between it and the test-tube is shut off, and
communication is effected with a second test-tube, like the first,
plunged into the same water at 20 deg.. Care must be taken beforehand to
create a perfect vacuum in this test-tube.

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