Scientific American Supplement, No. 458, October 11, 1884 by Various
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Various >> Scientific American Supplement, No. 458, October 11, 1884
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The foregoing particulars, it will be found, furnish all necessary data
for determining with absolute precision the _diffusion_ of rays acting on
the central vessel of the solar pyrometer. But the determination of
temperature which uninterrupted solar radiation is capable of transmitting
to the polygonal reflector calls for a correct knowledge of atmospheric
absorption. Besides, an accurate estimate of the loss of radiant heat
attending the reflection of the rays by the mirrors is indispensable. Let
us consider these points separately.
[Illustration: _Fig._ 2.]
_Atmospheric Absorption._--The principal object of conducting the
investigation during the summer solstice has been the facilities afforded
for determining atmospheric absorption, the sun's zenith distance at noon
being only 17 deg. 12' at New York. The retardation of the sun's rays in
passing through a clear atmosphere obviously depends on the depth
penetrated; hence--neglecting the curvature of the atmospheric limit--the
retardation will be as the secants of the zenith distances. Accordingly,
an observation of the temperature produced by solar radiation at a zenith
distance whose secant is _twice_ that of the secant of 17 deg. 12', viz., 61 deg.
28', determines the minimum atmospheric absorption at New York. The result
of observations conducted during a series of years shows that the maximum
solar intensity at 17 deg. 12' reaches 66.2 deg. F., while at a zenith distance of
61 deg. 28' it is 52.5 deg. F.; hence, minimum atmospheric absorption at New York,
during the summer solstice,
13.7
is 66.2 deg.-52.5 deg. = 13.7 deg. F., or ------ = 0.207 of the sun's
66.2
radiant energy where the rays enter the terrestrial atmosphere.
[Illustration: CAPTAIN ERICSSON'S SOLAR PYROMETER, ERECTED AT NEW YORK,
1884.]
In order to determine the loss of energy attending the reflection of the
rays by the diagonal mirrors, I have constructed a special apparatus,
which, by means of a parallactic mechanism, faces the sun at right angles
during observations. It consists principally of two small mirrors,
manufactured of the same materials as the reflector, placed diagonally at
right angles to each other; a thermometer being applied between the two,
whose stem points toward the sun. The direct solar rays entering through
perforations of an appropriate shade, and reflected by the inclined
mirrors, act simultaneously on opposite sides of the bulb. The mean result
of repeated trials, all differing but slightly, show that the energy of
the direct solar rays acting on the polygonal reflector is reduced 0.235
before reaching the heater.
In accordance with the previous article, the investigation has been based
on the assumption that _the temperatures produced by radiant heat at given
distances from its source are inversely as the diffusion of the rays at
those distances. In other words, the temperature produced by solar
radiation is as the density of the rays._
It will be remembered that Sir Isaac Newton, in estimating the temperature
to which the comet of 1680 was subjected when nearest to the sun, based
his calculations on the result of his practical observations that the
maximum temperature produced by solar radiation was one-third of that of
boiling water. Modern research shows that the observer of 1680 underrated
solar intensity only 5 deg. for the latitude of London. The distance of the
comet from the center of the sun being to the distance of the earth from
the same as 6 to 1,000, the author of the "Principia" asserted that the
density of the rays was as 1,000 squared to 6 squared = 28,000 to 1; hence the comet was
subjected to a temperature of 28,000 x 180 deg./3 = 1,680,000 deg., an intensity
exactly "2,000 times greater than that of red-hot iron" at a temperature
of 840 deg.. The distance of the comet from the solar surface being equal to
one-third of the sun's radius, it will be seen that, in accordance with
the Newtonian doctrine, the temperature to which it was subjected
indicated a solar intensity of
4 squared x 1,680,000
-------------- = 2,986,000 deg. F.
3
The writer has established the correctness of the assumption that "the
temperature is as the density of the rays," by showing practically that
the _diminution_ of solar temperature (for corresponding zenith distances)
when the earth is in aphelion corresponds with the increased diffusion of
the rays consequent on increased distance from the sun. This practical
demonstration, however, has been questioned on the insufficient ground
that "the eccentricity of the earth's orbit is too small and the
temperature produced by solar radiation too low" to furnish a safe basis
for computations of solar temperature.
In order to meet the objection that the diffusion of the rays in aphelion
do not differ sufficiently, the solar pyrometer has been so arranged that
the density, _i. e._, the diffusion of the reflected rays, can be changed
from a ratio of 1 in 5,040 to that of 1 in 10,241. This has been effected
by employing heaters respectively 10 inches and 20 inches in diameter.
With reference to the "low" solar temperature pointed out, it will be
perceived that the adopted expedient of increasing the density of the rays
without raising the temperature by _converging_ radiation, removes the
objection urged.
Agreeably to the dimensions already specified, the area of the 10-inch
heater acted upon by the reflected solar rays is 331.65 square inches, the
area of the 20-inch heater being 673.9 square inches. The section of the
annular sunbeam whose direct rays act upon the polygonal reflector is
3,130 square inches, as before stated.
Regarding the diffusion of the solar rays during the investigation, the
following demonstration will be readily understood. The area of a sphere
whose radius is equal to the earth's distance from the sun in aphelion
being to the sun's area as 218.1 squared to 1, while the reflecter of the solar
pyrometer intercepts a sunbeam of 3,130 square inches section, it follows
that the reflector will receive the radiant heat developed by 3,130 /
218.1 squared = 0.0658 square inch of the solar surface. Hence, as the 10-inch
heater presents an area of 331.65 square inches, we establish the fact
that the reflected solar rays, acting on the same, are _diffused_ in the
ratio of 331.65 to 0.0658, or 331.65 / 0.0658 = 5,040 to 1; the diffusion
of the rays acting on the 20-inch heater being as 673.9 to 0.0658, or
673.9 / 0.0658 = 10,241 to 1.
The atmospheric conditions having proved unfavorable during the
investigation, maximum solar temperature was not recorded. Accordingly,
the heaters of the solar pyrometer did not reach maximum temperature, the
highest indication by the thermometer of the small heater being 336.5 deg.,
that of the large one being 200.5 deg. above the surrounding air. No
compensation will, however, be introduced on account of deficient solar
heat, the intention being to base the computation of solar temperature
solely on the result of observations conducted at New York during the
summer solstice of 1884. It will be noticed that the temperature of the
large heater is proportionally higher than that of the small heater, a
fact showing that the latter, owing to its higher temperature, loses more
heat by radiation and convection than the former. Besides, the rate of
cooling of heated bodies increases more rapidly than the augmentation of
temperature.
The loss occasioned by the imperfect reflection of the mirrors, as before
stated, is 0.235 of the energy transmitted by the direct solar rays acting
on the polygonal reflector, hence the temperature which the solar rays are
capable of imparting to the large heater will be 200.5 deg. x 1.235 =
247.617 deg.; but the energy of the solar rays acting on the _reflector_ is
reduced 0.207 by atmospheric absorption, consequently the ultimate
temperature which the sun's radiant energy is capable of imparting to the
heater is 1.207 x 247.617 deg. = 298.87 deg. F. It is hardly necessary to observe
that this temperature (developed by solar radiation diffused fully
ten-thousandfold) must be regarded as an _actual_ temperature, since a
perfectly transparent atmosphere, and a reflector capable of transmitting
the whole energy of the sun's rays to the heater, would produce the same.
The result of the experimental investigation carried out during the summer
solstice of 1884 may be thus briefly stated. The diffusion of the solar
rays acting on the 20 inch heater being in the ratio of 1 to 10,241, the
temperature of the solar surface cannot be less than 298.87 deg. x 10,241 =
3,060,727 deg. F. This underrated computation must be accepted unless it can
be shown that the temperature produced by radiant heat is not inversely as
the diffusion of the rays. Physicists who question the existence of such
high solar temperature should bear in mind that in consequence of the
great attraction of the solar mass, hydrogen on the sun's surface raised
to a temperature of 4,000 deg. C. will be nearly twice as heavy as hydrogen on
the surface of the earth at ordinary atmospheric temperatures; and that,
owing to the immense depth of the solar atmosphere, its density would be
so enormous at the stated low temperature that the observed rapid
movements within the solar envelope could not possibly take place. It
scarcely needs demonstration to prove that extreme tenuity can alone
account for the extraordinary velocities recorded by observers of solar
phenomena. But _extreme tenuity_ is incompatible with low temperature and
the pressure produced by an atmospheric column probably exceeding 50,000
miles in height subjected to the sun's powerful attraction, diminished
only one-fourth at the stated elevation. These facts warrant the
conclusion that the high temperature established by our investigation is
requisite to prevent undue density of the solar atmosphere.
It is not intended at present to discuss the necessity of tenuity with
reference to the functions of the sun as a radiator; yet it will be proper
to observe that on merely dynamical grounds the enormous density of the
solar envelope which would result from low temperature presents an
unanswerable objection to the assumption of Pouillet, Vicaire,
Sainte-Claire Deville, and other eminent _savants_, that the temperature
of the solar surface does not reach 3,000 deg. C.
J. ERICSSON.
* * * * *
CHEMICAL NATURE OF STARCH GRAINS.
Dr. Brukner has contributed to the _Proceedings_ of the Vienna Academy of
Sciences a paper on the "Chemical Nature of the Different Varieties of
Starch," especially in reference to the question whether the granulose of
Nageli, the soluble starch of Jessen, the amylodextrin of W. Nageli, and
the amidulin of Nasse are the same or different substances. A single
experiment will serve to show that under certain conditions a soluble
substance maybe obtained from starch grains.
If dried starch grains are rubbed between two glass plates, the grains
will be seen under the microscope to be fissured, and if then wetted and
filtered, the filtrate will be a perfectly clear liquid showing a strong
starch reaction with iodine. Since no solution is obtained from uninjured
grains, even after soaking for weeks in water, Brukner concludes that the
outer layers of the starch grains form a membrane protecting the interior
soluble layers from the action of the water.
The soluble filtrate from starch paste also contains a substance identical
with granulose. Between the two kinds of starch, the granular and that
contained in paste, there is no chemical but only a physical difference,
depending on the condition of aggregation of their micellae.
W. Nageli maintains that granulose, or soluble starch, differs from
amylodextrin in the former being precipitated by tannic acid and acetate
of lead, while the latter is not. Brukner fails to confirm this
difference, obtaining a voluminous precipitate with tannic acid and
acetate of lead in the case of both substances. Another difference
maintained by Nageli, that freshly precipitated starch is insoluble,
amylodextrin soluble in water, is also contested; the author finding that
granulose is soluble to a considerable extent in water, not only
immediately after precipitation, but when it has remained for twenty-four
hours under absolute alcohol. Other differences pointed out by W. Nageli,
Brukner also maintains to be non-existent, and he regards amidulin and
amylodextrin as identical. Brucke gave the name erythrogranulose to a
substance nearly related to granulose, but with a stronger affinity for
iodine, and receiving from it not a blue but a red color. Brukner regards
the red color as resulting from a mixture of erythrodextrin, and the
greater solubility of this substance in water.
If a mixture of filtered potato starch paste and erythrodextrin is dried
in a watch glass covered with a thin pellicle of collodion, and a drop of
iodine solution placed on the latter, it penetrates very slowly through
the pellicle, the dextrin becoming first tinctured with red, and the
granulose afterward with blue. If, on the other hand, no erythrodextrin is
used, the diffusion of the iodine causes at once simply a blue coloring.
With regard to the iodine reaction of starch, Brukner contests Sachsse's
view as to the loss of color of iodide of starch at a high temperature. He
shows that the iodide may resist heat, and that the loss of color depends
on the greater attraction of water for iodine as compared with starch, and
the greater solubility of iodine in water at high temperatures.
The different kinds of starch do not take the same tint with the same
quantity of (solid) iodine. That from the potato _arum_ gives a blue, and
that from wheat and rice a violet tint; while the filtrate from starch
paste, from whatever source, always gives a blue color.
* * * * *
THE AMALGAMATION OF SILVER ORES.
DESCRIPTION OF THE FRANCKE "TINA" OR VAT PROCESS FOR THE AMALGAMATION OF
SILVER ORES.
[Footnote: Paper read before the Institution of Mechanical Engineers at
the Cardiff meeting.--_Engineering_.]
By Mr. EDGAR P. RATHBONE, of London.
In the year 1882, while on a visit to some of the great silver mines in
Bolivia, an opportunity was afforded the writer of inspecting a new and
successful process for the treatment of silver ores, the invention of Herr
Francke, a German gentleman long resident in Bolivia, whose acquaintance
the writer had also the pleasure of making. After many years of tedious
working devoted to experiments bearing on the metallurgical treatment of
rich but refractory silver ores, the inventor has successfully introduced
the process of which it is proposed in this paper to give a description,
and which has, by its satisfactory working, entirely eclipsed all other
plans hitherto tried in Bolivia, Peru, and Chili. The Francke "tina"
process is based on the same metallurgical principles as the system
described by Alonzo Barba in 1640, and also on those introduced into the
States in more recent times under the name of the Washoe process.[1]
[Footnote 1: Transactions of the American Institute of Mining Engineers,
vol. ii., p. 159.]
It was only after a long and careful study of these two processes, and by
making close observations and experiments on other plans, which had up to
that time been tried with more or less success in Bolivia, Peru, and
Chili--such as the Mexican amalgamation process, technically known as the
"patio" process; the improved Freiberg barrel amalgamation process; as
used at Copiapo; and the "Kronke" process--that Herr Francke eventually
succeeded in devising his new process, and by its means treating
economically the rich but refractory silver ores, such as those found at
the celebrated Huanchaca and Guadalupe mines in Potosi, Bolivia. In this
description of the process the writer will endeavor to enter into every
possible detail having a practical bearing on the final results; and with
this view he commences with the actual separation of the ores at the
mines.
_Ore Dressing, etc._--This consists simply in the separation of the ore by
hand at the mines into different qualities, by women and boys with small
hammers, the process being that known as "cobbing" in Cornwall. The object
of this separation is twofold: first to separate the rich parts from the
poor as they come together in the same lump of ore, otherwise rich pieces
might go undetected; and, secondly, to reduce the whole body of ore coming
from the mine to such convenient size as permits of its being fed directly
into the stamps battery. The reason for this separation not being effected
by those mechanical appliances so common in most ore dressing
establishments, such as stone breakers or crushing rolls, is simply
because the ores are so rich in silver, and frequently of such a brittle
nature, that any undue pulverization would certainly result in a great
loss of silver, as a large amount would be carried away in the form of
fine dust. So much attention is indeed required in this department that it
is found requisite to institute strict superintendence in the sorting or
cobbing sheds, in order to prevent as far as practicable any improper
diminution of the ores. According to the above method, the ores coming
from the mine are classified into the four following divisions:
1. Very rich ore, averaging about six per cent. of silver, or containing
say 2,000 ounces of silver to the ton (of 2,000 lb.).
2. Rich ore, averaging about one per cent. of silver, or say from 300 to
400 ounces of silver to the ton.
3. Ordinary ore, averaging about 1/2 per cent. of silver, or say from 150
oz. to 200 oz. of silver to the ton.
4. Gangue, or waste rock, thrown on the dump heaps.
The first of these qualities--the very rich ore--is so valuable as to
render advantageous its direct export in the raw state to the coast for
shipment to Europe. The cost of fuel in Bolivia forms so considerable a
charge in smelting operations, that the cost of freight to Europe on very
rich silver ores works out at a relatively insignificant figure, when
compared with the cost of smelting operations in that country. This rich
ore is consequently selected very carefully, and packed up in tough
rawhide bags, so as to make small compact parcels some 18 in. to 2 ft.
long, and 8 in. to 12 in. thick, each containing about 1 cwt. Two of such
bags form a mule load, slung across the animal's back.
The second and third qualities of ore are taken direct to the smelting
works; and where these are situated at some distance from the mines, as at
Huanchaca and Guadalupe, the transport is effected by means of strong but
lightly built iron carts, specially constructed to meet the heavy wear and
tear consequent upon the rough mountain roads. These two classes of ores
are either treated separately, or mixed together in such proportion as is
found by experience to be most suitable for the smelting process.
On its arrival at the reduction works the ore is taken direct to the stamp
mill. At the Huanchaca works there are sixty-five heads of stamps, each
head weighing about 500 lb., with five heads in each battery, and crushing
about 50 cwt. per head per twenty-four hours. The ore is stamped dry,
without water, requiring no coffers; this is a decided advantage as
regards first cost, owing to the great weight of the coffers, from 2 to 3
tons--a very heavy item when the cost of transport from Europe at about
50_l_. per ton is considered. As fast as the ore is stamped, it is
shoveled out by hand, and thrown upon inclined sieves of forty holes per
lineal inch; the stuff which will not pass through the mesh is returned to
the stamps.
Dry stamping may be said to be almost a necessity in dealing with these
rich silver ores, as with the employment of water there is a great loss of
silver, owing to the finer particles being carried away in suspension, and
thus getting mixed with the slimes, from which it is exceedingly difficult
to recover them, especially in those remote regions where the cost of
maintaining large ore-dressing establishments is very heavy. Dry stamping,
however, presents many serious drawbacks, some of which could probably be
eliminated if they received proper attention. For instance, the very fine
dust, which rises in a dense cloud during the operation of stamping, not
only settles down on all parts of the machinery, interfering with its
proper working, so that some part of the battery is nearly always stopped
for repairs, but is also the cause of serious inconvenience to the
workmen. At the Huanchaca mines, owing to the presence of galena or
sulphide of lead in the ores, this fine dust is of such an injurious
character as not unfrequently to cause the death of the workmen; as a
precautionary measure they are accustomed to stuff cotton wool into their
nostrils. This, however, is only a partial preventive; and the men find
the best method of overcoming the evil effect is to return to their homes
at intervals of a few weeks, their places being taken by others for the
same periods. In dry stamping there is also a considerable loss of silver
in the fine particles of rich ore which are carried away as dust and
irrevocably lost. To prevent this loss, the writer proposed while at
Huanchaca that a chamber should be constructed, into which all the fine
dust might be exhausted or blown by a powerful fan or ventilator.
_Roasting_.--From the stamps the stamped ore is taken in small ore cars to
the roasting furnaces, which are double bedded in design, one hearth being
built immediately above the other. This type of furnace has proved, after
various trials, to be that best suited for the treatment of the Bolivian
silver ores, and is stated to have been found the most economical as
regards consumption of fuel, and to give the least trouble in labor.
At the Huanchaca mines these furnaces cost about 100_l_. each, and are
capable of roasting from 2 to 21/2 tons of ore in twenty-four hours, the
quantity and cost of the fuel consumed being as follows:
Bolivian dollars at 3s. 1d.
Tola (a kind of shrub), 3 cwt., at 60 cents. 1.80
Yareta (a resinous moss), 4 cwt., at 80 cents. 3.20
Torba (turf), 10 cwt., at 40 cents. 4.00
----
Bolivian dollars. 9.00, say 28s.
One man can attend to two furnaces, and earns 3s. per shift of twelve
hours.
Probably no revolving mechanical furnace is suited to the roasting of
these ores, as the operation requires to be carefully and intelligently
watched, for it is essential to the success of the Francke process that
the ores should not be completely or "dead" roasted, inasmuch as certain
salts, prejudicial to the ultimate proper working of the process, are
liable to be formed if the roasting be too protracted. These salts are
mainly due to the presence of antimony, zinc, lead, and arsenic, all of
which are unfavorable to amalgamation.
The ores are roasted with 8 per cent. of salt, or 400 lb. of salt for the
charge of 21/2 tons of ore; the salt costs 70 cents, or 2s. 2d. per 100 lb.
So roasted the ores are only partially chlorinized, and their complete
chlorination is effected subsequently, during the process of amalgamation;
the chlorides are thus formed progressively as required, and, in fact, it
would almost appear that the success of the process virtually consists in
obviating the formation of injurious salts. All the sulphide ores in
Bolivia contain sufficient copper to form the quantity of cuprous chloride
requisite for the first stages of roasting, in order to render the silver
contained in the ore thoroughly amenable to subsequent amalgamation.
_Amalgamating_.--From the furnaces the roasted ore is taken in ore cars to
large hoppers or bins situated immediately behind the grinding and
amalgamating vats, locally known as "tinas," into which the ore is run
from the bin through a chute fitted with a regulating slide. The tinas or
amalgamating vats constitute the prominent feature of the Francke process;
they are large wooden vats, shown in Figs. 1 and 2, page 173, from 6 ft.
to 10 ft. in diameter and 5 ft. deep, capacious enough to treat about 21/2
tons of ore at a time. Each vat is very strongly constructed, being bound
with thick iron hoops. At the bottom it is fitted with copper plates about
3 in. thick, A in Fig. 1; and at intervals round the sides of the vat are
fixed copper plates, as shown in Figs. 3 and 4, with ribs on their inner
faces, slightly inclined to the horizontal, for promoting a more thorough
mixing. It is considered essential to the success of the process that the
bottom plates should present a clear rubbing surface of at least 10 square
feet.
[Illustration: THE FRANCKE "TINA" PROCESS FOR THE AMALGAMATION OF SILVER
ORES.]
Within the vat, and working on the top of the copper plates, there is a
heavy copper stirrer or muller, B, Figs. 1 and 2, caused to revolve by the
shafting, C, at the rate of 45 revolutions per minute. At Huanchaca this
stirrer has been made with four projecting radial arms, D D, Figs. 1 and
2; but at Guadalupe it is composed of one single bell-shaped piece, Figs.
3 and 4, without any arms, but with slabs like arms fixed on its
underside; and this latter is claimed to be the most effective. The
stirrer can be lifted or depressed in the vat at will by means of a worm
and screw at the top of the driving shaft, Fig. 3.
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