+-------------------+------+-------+----+-----+------+------+-------+
| Sort. | Fi- |Starch.|Veg.|Gum. |Acids |Water.| Ana- |
| |brine.| |al- | | and | | lyst. |
| | | |bum.| |Salts.| | |
+-------------------+------+-------+----+-----+------+------+-------+
|Red potatos | 7·0 | 15·0 |1·4 | 4·1 | 5·1 |75·0 |Einhof.|
|Id. germinated | 6·8 | 15·2 |1·3 | 3·7 | |73·0 | |
|Potato sprouts | 2·8 | 0·4 |0·4 | 3·3 | |93·0 | |
|Kidney potatos | 8·8 | 9·1 |0·8 | | |81·3 | |
|Large red do. | 6·0 | 12·9 |0·7 | | |78·0 | |
|Sweet do. | 8·2 | 15·1 |0·8 | | |74·3 | |
| | | | | \_______/ | | |
|Potato of Peru | 5·2 | 15·0 |1·9 | 1·9 |76·0 |Lampad.|
| . . England | 6·8 | 12·9 |1·1 | 1·7 |77·5 | |
|Onion potato | 8·4 | 18·7 |0·9 | 1·7 |70·3 | |
| . . Voigtland | 7·1 | 15·4 |1·2 | 2·0 |74·3 | |
| . . cultivated in| | | | | | | |
|the environs of | | | | | | | |
|Paris | 6·79 | 13·3 |0·92| 3·3 | 1·4 |73·12 |Henry. |
+-------------------+------+-------+----+-----+------+------+-------+
POTASH, or POTASSA. (_Potasse_, Fr.; _Kali_, Germ.) This substance was
so named from being prepared for commercial purposes by evaporating in
iron pots the lixivium of the ashes of wood fuel. In the crude state
called potashes, it consists, therefore, of such constituents of burned
vegetables as are very soluble in water, and fixed in the fire. The
potash salts of plants which originally contained vegetable acids, will
be converted into carbonates, the sulphates will become sulphites,
sulphurets, or even carbonates, according to the manner of incineration;
the nitrates will be changed into pure carbonates, while the muriates or
chlorides will remain unaltered. Should quicklime be added to the
solution of the ashes, a corresponding portion of caustic potassa will
be introduced into the product, with more or less lime, according to the
care taken in decanting off the clear lye for evaporation.
In America, where timber is in many places an incumbrance upon the soil,
it is felled, piled up in pyramids, and burned, solely with a view to
the manufacture of potashes. The ashes are put into wooden cisterns,
having a plug at the bottom of one of the sides under a false bottom; a
moderate quantity of water is then poured on the mass, and some
quicklime is stirred in. After standing for a few hours, so as to take
up the soluble matter, the clear liquor is drawn off; evaporated to
dryness in iron pots, and finally fused at a red heat into compact
masses, which are gray on the outside, and pink-coloured within.
Pearlash is prepared by calcining potashes upon a reverberatory hearth,
till the whole carbonaceous matter, and the greater part of the sulphur,
be dissipated; then lixiviating the mass, in a cistern having a false
bottom covered with straw, evaporating the clear lye to dryness in flat
iron pans, and stirring it towards the end into white lumpy
granulations.
I find the best pink Canadian potashes, as imported in casks containing
about 5 cwts., to contain pretty uniformly 60 per cent. of absolute
potassa; and the best pearlashes to contain 50 per cent.; the alkali in
the former being nearly in a caustic state; in the latter, carbonated.
All kinds of vegetables do not yield the same proportion of potassa. The
more succulent the plant, the more does it afford; for it is only in the
juices that the vegetable salts reside, which are converted by
incineration into alkaline matter. Herbaceous weeds are more productive
of potash than the graminiferous species, or shrubs, and these than
trees; and for a like reason, twigs and leaves are more productive than
timber. But plants in all cases are richest in alkaline salts when they
have arrived at maturity. The soil in which they grow also influences
the quantity of saline matter.
The following TABLE exhibits the average product in potassa of several
plants, according to the researches of Vauquelin, Pertuis, Kirwan, and
De Saussure:--
In 1000 parts. Potassa.
Pine or fir 0·45
Poplar 0·75
Trefoil 0·75
Beechwood 1·45
Oak 1·53
Boxwood 2·26
Willow 2·85
Elm and maple 3·90
Wheat straw 3·90
Barb of oak twigs 4·20
Thistles 5·00
Flax stems 5·00
Small rushes 5·08
Vine shoots 5·50
Barley straw 5·80
Dry beech bark 6·00
Fern 6·26
Large rush 7·22
Stalk of maize 17·5
Bastard chamomile (_Anthemis cotula_, L.) 19·6
Bean stalks 20·0
Sunflower stalks 20·0
Common nettle 25·03
Vetch plant 27·50
Thistles in full growth 35·37
Dry straw of wheat before earing 47·0
Wormwood 73·0
Fumitory 79·0
Stalks of tobacco, potatos, chesnuts, chesnut husks, broom, heath,
furze, tansy, sorrel, vine leaves, beet leaves, orach, and many other
plants, abound in potash salts. In Burgundy, the well-known _cendres
gravelées_ are made by incinerating the lees of wine pressed into cakes,
and dried in the sun; the ashes contain fully 16 per cent. of potassa.
The purification of pearlash is founded upon the fact of its being more
soluble in water than the neutral salts which debase it. Upon any given
quantity of that substance, in an iron pot, let one and a half times its
weight of water be poured, and let a gentle heat be applied for a short
time. When the whole has again cooled, the bottom will be encrusted with
the salts, while a solution of nearly pure carbonate of potash will be
found floating above, which may be drawn off clear by a syphon. The
salts may be afterwards thrown upon a filter of gravel. If this lye be
diluted with 6 times its bulk of water mixed with as much slaked lime as
there was pearlash employed, and the mixture be boiled for an hour, the
potash will become caustic, by giving up its carbonic acid to the lime.
If the clear settled lixivium be now siphoned off, and concentrated by
boiling in a covered iron pan, till it assumes the appearance of oil, it
will constitute the common caustic of the surgeon, the _potassa fusa_ of
the shops. But to obtain potassa chemically pure, recourse must be had
to the bicarbonate, nitrate, or tartrate of potassa, salts which, when
carefully crystallized, are exempt from any thing to render the potassa
derived from them impure. The bicarbonate having been gently ignited in
a silver basin, is to be dissolved in 6 times its weight of water, and
the solution is to be boiled for an hour, along with one pound of slaked
lime for every pound of the bicarbonate used. The whole must be left to
settle without contact of air. The supernatant lye is to be drawn off by
a syphon, and evaporated in an iron or silver vessel provided with a
small orifice in its close cover for the escape of the steam, till it
assumes, as above, the appearance of oil, or till it be nearly redhot.
Let the fused potassa be now poured out upon a bright plate of iron, cut
into pieces as soon as it concretes, and put up immediately in a bottle
furnished with a well-ground stopper. It is hydrate of potassa, being
composed of 1 atom of potassa 48, + 1 atom of water 9, = 57.
A pure carbonate of potassa may be also prepared by fusing pure nitre in
an earthen crucible, and projecting charcoal into it by small bits at a
time, till it ceases to cause deflagration. Or a mixture of 10 parts of
nitre and 1 of charcoal may be deflagrated in small successive portions
in a redhot deep crucible. When a mixture of 2 parts of tartrate of
potassa, or crystals of tartar, and 1 of nitre, is deflagrated, pure
carbonate of potassa remains mixed with charcoal, which by lixiviation,
and the agency of quicklime, will afford a pure hydrate. Crystals of
tartar calcined alone yield also a pure carbonate.
Caustic potassa, as I have said, after being fused in a silver crucible
at a red heat, retains 1 prime equivalent of water. Hence its
composition in 100 parts is, potassium 70, oxygen 14, water 16.
Anhydrous potassa, or the oxide free from water, can be obtained only by
the combustion of potassium in the open air. It is composed of 83-1/3 of
metal, and 16-2/3 of oxygen. Berzelius’s numbers are 83·05 and 16·95.
Caustic potassa may be crystallized; but in general it occurs as a white
brittle substance of spec. grav. 1·708, which melts at a red heat,
evaporates at a white heat, deliquesces into a liquid in the air, and
attracts carbonic acid; is soluble in water and alcohol, forms soft
soaps with fat oils, and soapy-looking compounds with resins and wax;
dissolves sulphur, some metallic sulphurets, as those of antimony,
arsenic, &c., as also silica, alumina, and certain other bases; and
decomposes animal textures, as hair, wool, silk, horn, skin, &c. It
should never be touched with the tongue or the fingers.
The following TABLE exhibits the quantity of _Fused Potassa_ in 100
parts of _caustic lye_, at the respective densities:--
+----+-------+
|Sp. | Pot. |
|gr. |in 100.|
+----+-------+
|1·58| 53·06 |
|1·56| 51·58 |
|1·54| 50·09 |
|1·52| 48·46 |
|1·50| 46·45 |
|1·48| 44·40 |
|1·46| 42·31 |
|1·44| 40·17 |
|1·42| 37·97 |
|1·40| 35·99 |
|1·38| 34·74 |
|1·36| 33·46 |
|1·34| 32·14 |
|1·32| 30·74 |
|1·30| 29·34 |
|1·28| 27·86 |
|1·26| 26·34 |
|1·24| 24·77 |
|1·22| 23·14 |
|1·20| 21·25 |
|1·18| 19·34 |
|1·16| 17·40 |
|1·14| 15·38 |
|1·12| 13·30 |
|1·10| 11·28 |
|1·08| 9·20 |
|1·06| 7·02 |
|1·04| 4·77 |
|1·02| 2·44 |
|1·00| 0·00 |
+----+-------+
The only certain way of determining the quantity of free potassa in any
solid or liquid, is from the quantity of a dilute acid of known strength
which it can saturate.
The hydrate of potassa, or its lye, often contains a notable quantity of
carbonate, the presence of which may be detected by lime water, and its
amount be ascertained by the loss of weight which it suffers, when a
weighed portion of the lye is poured into a weighed portion of dilute
sulphuric acid poised in the scale of a balance.
There are two other oxides of potassium; the suboxide, which consists,
according to Berzelius, of 90·74 of metal, and 9·26 oxygen; and the
hyperoxide, an orange-yellow substance, which gives off oxygen in the
act of dissolving in water, and becomes potassa. It consists of 62 of
metal, and 38 of oxygen.
Carbonate of potassa is composed of 48 parts of base, and 22 of acid,
according to most British authorities; or, in 100 parts, of 68·57 and
31·43; but according to Berzelius, of 68·09 and 31·91.
Carbonate of potassa, as it exists associated with carbon in calcined
tartar, passes very readily into the _Bicarbonate_, on being moistened
with water, and having a current of carbonic acid gas passed through it.
The absorption takes place so rapidly, that the mass becomes hot, and
therefore ought to be surrounded with cold water. The salt should then
be dissolved in the smallest quantity of water at 120° F., filtered, and
crystallized.
POTASSIUM (Eng. and Fr.; _Kalium_, Germ.); is a metal deeply
interesting, not only from its own marvellous properties, but from its
having been the first link in the chain of discovery which conducted Sir
H. Davy through many of the formerly mysterious and untrodden labyrinths
of chemistry.
The easiest and best mode of obtaining this elementary substance, is
that contrived by Brunner, which I have often practised upon a
considerable scale. Into the orifice of one of the iron bottles, as A,
_fig._ 889., in which mercury is imported, adapt, by screwing, a piece
of gun-barrel tube, 9 inches long; having brazed into its side, about 3
inches from its outer end, a similar piece of iron tube. Fill this
retort two-thirds with a mixture of 10 parts of cream of tartar,
previously calcined in a covered crucible, and 1 of charcoal, both in
powder; and lay it horizontally in an air-furnace, so that while the
screw orifice is at the inside wall, the extremity of the straight or
nozzle tube may project a few inches beyond the brickwork, and the tube
brazed into it at right angles may descend pretty close to the outside
wall, so as to dip its lower end a quarter of an inch beneath the
surface of some rectified naphtha contained in a copper bottle
surrounded by ice-cold water. By bringing the condenser-vessel so near
the furnace, the tubes along which the potassium vapour requires to
pass, run less risk of getting obstructed. The horizontal straight end
of the nozzle tube should be shut by screwing a stopcock air-tight into
it. By opening the cock momentarily, and thrusting in a hot wire, this
tube may be readily kept free, without permitting any considerable waste
of potassium. The heat should be slowly applied at first, but eventually
urged to whiteness, and continued as long as potassuretted hydrogen
continues to be disengaged. The retort, and the part of the nozzle tube
exposed to the fire, should be covered with a good refractory lute, as
described under the article PHOSPHORUS. The joints must be perfectly
air-tight; and the vessel freed from every trace of mercury, by
ignition, before it is charged with the tartar-ash.
Tartar skilfully treated in this way will afford 3 per cent. of
potassium; and when it is observed to send forth green fumes, it has
commenced the production of the metal. Instead of the construction above
described, the following form of apparatus may be employed.
[Illustration: 889 890]
A. _fig._ 889., represents the iron bottle, charged with the incinerated
tartar; and B is a fire-brick support. A piece of fire-tile should also
be placed between the bottom of the bottle and the back wall of the
furnace, to keep the apparatus steady during the operation. Whenever the
moisture is expelled, and the mass faintly ignited, the tube C should be
screwed into the mouth of the bottle, through a small hole left for this
purpose in the side of the furnace. That tube should be no longer, and
the front wall of the furnace no thicker, than what is absolutely
necessary. As soon as the reduction is indicated by the emission of
green vapours, the receiver must be adapted, _d_, _a_, D, E, shown in a
large scale in _fig._ 890.
This is a condenser, in two pieces, made of thin sheet copper; D, the
upper part, is a rectangular box, open at bottom, about 10 inches high,
by 5 or 6 long, and 2 wide; near to the side _a_, it is divided inside
into two equal compartments, up to two-thirds of its height, by a
partition _b_, _b_, in order to make the vapours that issue from C
pursue a downward and circuitous path. In each of its narrow sides, near
the top, a short tube is soldered, at _d_ and _a_; the former being
fitted air-tight into the end of the nozzle of the retort, while the
latter is closed with a cork traversed by a stiff iron probe _e_, which
passes through a small hole in the partition _b_, _b_, under _c_, and is
employed to keep the tube C clear, by its drill-shaped steel point. In
one of the broad sides of the box D, near the top, a bit of pipe is
soldered on at _e_, for receiving the end of a bent glass tube of
safety, which dips its other and lower end into a glass containing
naphtha. E, the bottom copper box, with naphtha, which receives pretty
closely the upper case D, is to be immersed in a cistern of cold water
containing some lumps of ice.
The chemical action by which potassa is reduced in this process, seems
to be somewhat complicated, and has not been thoroughly explained. A
very small proportion of pure potassium is obtained; a great deal of it
is converted into a black infusible mass, which passes over with the
metal, and is very apt to block up the tube. Should this resist clearing
out with the probe, the fire must be immediately withdrawn from the
furnace, otherwise the apparatus will probably burst or blow up. Care
must be taken to prevent any moisture getting into the nozzle, for it
would probably produce a violent detonation.
When the operation has proceeded regularly, accompanied to the end with
a constant evolution of gas, the retort becomes nearly empty, or
contains merely a little charcoal, or carbonate of potassa, and the
potassium collects in the naphtha at the bottom of the receiver E, in
the form of globules or rounded lumps, of greater or less size, and of a
leaden hue. But the greater part of the metal escapes with the gas, in a
state of combination not well understood. This gaseous compound burns
with a white or reddish-white flame, and deposits potassa. Several
ounces of potassium may be produced in this way at one operation; but,
as thus obtained, it always contains some combined charcoal, which must
be separated by distilling it in an iron retort, having its beak plunged
in naphtha.
Pure potassium, as procured in Sir H. Davy’s original method, by acting
upon fused potassa under a film of naphtha, with the negative wire of a
powerful voltaic battery, is very like quicksilver. It is semi-fluid at
60° Fahr., nearly liquid at 92°, and entirely so at 120°. At 50° it is
malleable, and has the lustre of polished silver; at 32° it is brittle,
with a crystalline fracture; and at a heat approaching to redness, it
begins to boil, is volatilized, and converted into a green-coloured gas,
which condenses into globules upon the surface of a cold body. Its
specific gravity in the purest state is 0·865 at 60°. When heated in the
air, it takes fire, and burns very vividly. It has a stronger affinity
for oxygen than any other known substance; and is hence very difficult
to preserve in the metallic state. At a high temperature it reduces
almost every oxygenated body. When thrown upon water, it kindles, and
moves about violently upon the surface, burning with a red flame, till
it be consumed; that is to say, converted into potassa. When thrown upon
a cake of ice, it likewise kindles, and burns a hole in it. If a globule
of it be laid upon wet turmeric paper, it takes fire, and runs about,
marking its desultory path with red lines. The flame observed in these
cases is owing chiefly to hydrogen, for it is at the expense of the
water that the potassium burns.
Potassa, even in a pretty dilute solution, produces a precipitate with
muriate of platinum, a phenomenon which distinguishes it from soda. It
forms, moreover, with sulphuric and acetic acids, salts which
crystallize very differently from the sulphates and acetates of soda.
POTTERY, PORCELAIN. (Eng. and Fr.; _Steingut_, _Porzellan_, Germ.) The
French, who are fond of giving far-fetched names to the most ordinary
things, have dignified the art of pottery with the title of _ceramique_,
from the Greek noun κεραμος, an earthen pot, compounded of two words
which signify, in that language, _burned clay_. In reference to chemical
constitution, there are only two genera of baked stoneware. The first
consists of a fusible earthy mixture, along with an infusible, which
when combined are susceptible of becoming semi-vitrified and translucent
in the kiln. This constitutes porcelain or china-ware; which is either
hard and genuine, or tender and spurious, according to the quality and
quantity of the fusible ingredient. The second kind consists of an
infusible mixture of earths, which is refractory in the kiln, and
continues opaque. This is pottery, properly so called; but it
comprehends several sub-species, which graduate into each other by
imperceptible shades of difference. To this head belong earthenware,
stoneware, flintware, _fayence_, delftware, iron-stone china, &c.
The earliest attempts to make a compact stoneware, with a painted glaze,
seem to have originated with the Arabians in Spain, about the 9th
century, and to have passed thence into Majorca, in which island they
were carried on with no little success. In the 14th century, these
articles, and the art of imitating them, were highly prized by the
Italians, under the name of Majolica, and _porcelana_, from the
Portuguese word for a cup. The first fabric of stoneware possessed by
them, was erected at Fayenza, in the ecclesiastical state, whence the
French term _fayence_ is derived. The body of the ware was usually a red
clay, and the glaze was opaque, being formed of the oxides of lead and
tin, along with potash and sand. Bernhard de Pallissy, about the middle
of the 16th century, manufactured the first white _fayence_, at Saintes,
in France; and not long afterwards the Dutch produced a similar article,
of substantial make, under the name of delftware, and delft _porcelain_,
but destitute of those graceful forms and paintings for which the ware
of Fayenza was distinguished. Common fayence may be, therefore, regarded
as a strong, well-burned, but rather coarse-grained kind of stoneware.
It was in the 17th century that a small work for making earthenware of a
coarse description, coated with a common lead glaze, was formed at
Burslem, in Staffordshire, which may be considered as the germ of the
vast potteries now established in that county. The manufacture was
improved about the year 1690, by two Dutchmen, the brothers Elers, who
introduced the mode of glazing ware by the vapour of salt, which they
threw by handfuls at a certain period among the ignited goods in the
kiln. But these were rude, unscientific, and desultory efforts. It is to
the late Josiah Wedgewood, Esq. that this country and the world at large
are mainly indebted for the great modern advancement of the _ceramic_
art. It was he who first erected magnificent factories, where every
resource of mechanical and chemical science was made to co-operate with
the arts of painting, sculpture, and statuary, in perfecting this
valuable department of the industry of nations. So sound were his
principles, so judicious his plans of procedure, and so ably have they
been prosecuted by his successors in Staffordshire, that a population of
60,000 operatives now derives a comfortable subsistence within a
district formerly bleak and barren, of 8 miles long, by 6 broad, which
contains 150 kilns, and is significantly called the Potteries.