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° in the
shade, and the washing water at 78°) as follows:
Hard gelatine...............,...... ½ ounce.
Water.............................. 2 ounces.
Alcohol............................ 2 "
Bromide ammonia....................150 grains.
Liquor ammonia, 880................ 60 drops.
When all is thoroughly dissolved and of about 120° temperature, add,
stirring all the time,
Nitrate silver..................... 60 grains,
Water.............................. ¾ ounce.
Alcohol............................ ¾ "
Then again add,
Nitrate silver.....................140 grains.
Water.............................. 1 ounce.
Alcohol............................ 1 "
Both solutions being warmed to about 120°.
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°C., and put its interior in
hermetic communication with the receiver of a mercurial air-pump.
We vaporize at 20° 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°. Care must be taken beforehand to
create a perfect vacuum in this test-tube.
On causing the mercury to rise into the space that it previously
occupied, the vapors are made to condense in the second test-tube at the
same temperature as that at which they were formed.
We immediately ascertain that the pressure-gauge shows an elevation
of pressure; moreover, the proof of the condensed alcohol has very
perceptibly risen.
If, instead of causing these vapors to condense in the second test-tube,
we leave the first communication open, the vapors recondense in the
first test-tube without any elevation of pressure; and we do not see the
least trace of liquid forming in the second test tube.
This difference of pressure in the two foregoing experiments must be
attributed, then, to the specific action of the water on the vapors of
alcohol. Now we can calculate the difference of the work of the pump,
and put at 1 kilogramme of condensed liquid the difference of mechanical
work represented in kilogrammeters. What is remarkable is that this
difference is absolutely the equivalent of the heat disengaged when the
condensed liquid and the old liquid are remixed; there is a complete
identity. Thus the affinity of the water for the alcohol modifies the
tension of the vapors which form or condense upon the free surface of
the mixture. The two phenomena are closely connected by the law of
equivalence.
It results from all the laws that we have cited that by properly
regulating the tensions of the vapors of a mixture of alcohol and water,
and the temperature of the liquid, we shall be able to obtain a liquid
of a desired richness by the condensation of these vapors.
III. It was likewise indispensable to make sure of one important fact:
When the temperature of a liquid like alcohol is considerably lowered,
can the distillation of a given weight of this substance be effected
with sufficient rapidity for industrial requirements? Repeated
experiments with a host of volatile liquids have demonstrated the
following laws:
If we introduce a volatile liquid into two spherical receivers connected
by a wide tube, and if these be kept at different temperatures after
driving out all the air from the apparatus, the liquid distills from the
warmer into the cooler receiver, and we ascertain that:
h. The weight of the liquid which distills in the unit of time increases
with the deviation of temperature between the two receivers.
i. The weight of the liquid which distills in the unit of time is
constant for a same deviation of temperature between the receivers,
whatever be, moreover, the absolute temperature of the receivers.
k. The weight of the liquid distilled in the unit of time is
proportional to the active surfaces of the receivers; that is to say,
to the surfaces which are the seat of passage of heat through their
thickness.
l. The least trace of a foreign gas in the vapors left in the apparatus
throws the preceding laws into confusion, and checks distillation to a
considerable degree, especially at low temperatures.
Thus, water distilling between 100° and 60° will pass over as quickly
as that which is distilling between 40° and 0°. Absolute temperature is
without influence, provided every trace of air or foreign gas be got rid
of.
The distillatory apparatus should be provided with an excellent
air-pump, capable of preventing all those entrances of air which are
inevitable in practice.
The following is the industrial application that we have endeavored to
make of these theoretical views: The rectification of alcohols is one
of the most complex of operations; it looks toward several results
simultaneously. Alcohol derived from the fermentation of grain, sugar,
and of all starchy matters in general, contains an innumerable host of
different products, which may be grouped under four principal heads:
1. Empyreumatic essential oils, characteristic of the source of the
alcohol, and having a powerful odor which infects the total mass of
the crude spirits. 2. A considerable quantity of water. 3. A certain
quantity of pure alcohol. 4. A variable proportion of volatile
substances, composed in great part of ethers, different alcohols, and
bodies as yet not well defined. These latter affect the quality of
the alcohol by an odor which is entirely different from that of the
essential oils.
The object of rectification is to bring out No. 3 all alone; that is
to say, to extract the alcohol in a pure state by ridding it of oils,
water, ether, and foreign alcohols.
The alcohol industry never realizes this operation in an absolutely
complete manner. All the rectifying apparatus in operation at the
present day are based on the use of high temperatures varying between
78.5° and 100°. The successive condensation and vaporization of the
vapors issuing from the spirits effect in the rectifying columns a
partial separation of these liquids, and there are received successively
as products of rectification:
1. Bad tasting alcohols, containing the majority of the ethers and
impure alcohols.