materials for heating to incandescence in the Bunsen burner flame,
should find them in certain very rare minerals, containing a group of
elements formerly of interest only to the scientific chemist, and up to
that time devoid of any practical applications. The names of these
elements, the oxides of which are called “earths,” will, of course, be
strange to non-chemical readers, but we give their names, with the
remark that the nearest familiar substance they at all resemble is
aluminium, of which the oxide, or “earth,” is alumina. These rare
metals, the oxides of which are the materials of the Welsbach “mantle,”
are all discoveries of the present century, or nearly so, and they are
called lanthanum, zirconium, thorium, cerium, didymium, yttrium, erbium,
&c. They occur as silicates or phosphates very sparingly, and in a few
localities in Norway; but some of them have now been found more
abundantly in America. The minerals, from which for the most part the
oxides are obtained, are called _monazite_, _orthite_, and _thorite_. It
was found after many trials that a blend of these earths in certain
proportions gives a mantle that yields a pure white light, while any
preponderance of one or another would impart some tint to the light.
This proper blending of the constituents forms a great improvement on
the first mantles, which generally shed a greenish light.
[Illustration:
FIG. 351_c_.
]
[Illustration:
FIG. 351_d_.
]
The _mantles_ are made by an ingenious process, in which a network of
cotton thread is knitted into the form of a tube; this is cut up into
suitable lengths, and a piece attached to form the top. The network is
then saturated with a solution of the nitrates of the rare earths
above-mentioned, and dried on glass rods. After this a loop of asbestos
thread is passed through the top, by which the _mantle_ may be attached
to its support. The mantle is now shaped to the required form, and the
cotton thread burnt off, when a thin skeleton of the oxides is left
reproducing the form of the original network. The mantle is again
strongly heated, and after cooling is dipped into a solution of
collodion, dried, and carefully laid in a box. The collodion serves to
strengthen the mantle sufficiently for transit, for it is very frail,
and would otherwise be liable to fall in pieces by slight shocks. Fig.
351_d_ is a full-sized representation of the completed mantle, and Fig.
351_e_ shows it mounted on the burner, where a rather small flame is
allowed for the first time to play upon it, by which the collodion is
quickly burnt off, and then the chimney-glass is placed over it, as in
Fig. 351_a_. In the earlier forms of lamp the lighting of the gas was a
matter requiring some delicacy of manipulation, for a rude shock, or an
awkward touch might cause the mantle to crumble into ruin, but now the
makers fit their lamps with a by-pass by which a very small flame is
maintained within the lamp ready to light up the gas when that is fully
turned on. (Fig. 351_c_.) The makers have also now made the lamps
available for street lighting, to which the fragility of the mantle was
formerly an obstacle, as it was liable to collapse by the tremor of the
traffic. This risk has been obviated by providing a spring to support
the mantle at the base. (Fig. 351_b_.)
[Illustration:
FIG. 351_e_.
]
The qualities of the Welsbach lamp have been examined by competent
persons, and from the statements they supply, we extract the following
particulars. The light is, for the same gas consumption, seven times
that of an ordinary gas burner; more than four times that of an Argand
burner; more than twice that of a “regenerative” lamp. It follows, of
course, that, light for light, the products of combustion, such as
carbonic acid, heat, &c., amount to only something like ⅙th of those
produced by ordinary burners, and the consumption of the gas is perfect,
there being absolutely no smoke. Though the mantles have to be renewed
about three times a year, when the burners are in constant use, the
total cost, light for light, is only ¼th of that of ordinary burners.
The light of the Welsbach burner is whiter than ordinary gaslight. It is
rich in the blue rays, and, therefore, more like daylight, permitting
well the comparison of shades of colour, and it is excellently suited
for workers with the microscope, &c. This new gas-lighting must also be
a great boon to photographers using artificial illumination, for the
actinic power is, with the same visual illumination, nearly twice that
of the ordinary gas flame.
[Illustration:
FIG. 352.—_Apparatus for making Magenta._
]
COAL-TAR COLOURS.
Coal-tar is an exceedingly complex material, being a mixture of a great
number of different substances. The following table shows the chemical
name of many of the substances obtainable from the coal-tar. It must not
be supposed that these substances exist ready formed in the coal, and
that they are merely expelled by the heat. We can understand better how
heat, acting upon an apparently simple substance like coal, and one
containing so few elements, is able to produce so large a variety of
different bodies, if we remember that heat is the agent most often
employed to effect chemical changes, and that from even two elements,
variously combined, bodies differing entirely from each other are
producible.
SUBSTANCES FOUND IN COAL-TAR.
_a._ COMPOUNDS OF CARBON AND HYDROGEN.
Hydrides of amyl, hexyl, heptyl, nonyl, and decyl.
Amylene, hexylene, heptylene, octylene, nonylene, decylene; (paraffin).
Benzol, toluol, xylol, cumol, cymol.
Naphthalene.
Anthracene.
Pyrene.
Chrysene.
_b._ COMPOUNDS OF CARBON, HYDROGEN, AND OXYGEN.
Phenol, cresol, phlorol.
Rosolic acid, brunolic acid.
_c._ COMPOUNDS OF CARBON, HYDROGEN, AND NITROGEN.
Aniline, toluidine.
Pyridine, picoline, lutidine, collidine, parvoline, coridine, rubidine,
viridine.
Leucoline, lepidine, cryptidine.
Cespitine, pyrrol.
This list contains only the names of substances which have actually been
found in the coal-tar, and it is certain that a number of products must
have escaped notice. It is obvious, too, that by using coal of different
kinds, and by varying the temperature and pressure at which the
operation of distilling the coal is effected, we shall probably be able
to increase the number of possible constituents of coal-tar almost
indefinitely. The list above presents to the non-chemical reader a
string of quite unfamiliar names; but, though the system of nomenclature
in chemistry is far from perfect, yet each of these names has a meaning
for the chemist beyond the mere designation of a substance. The chemical
name aims at showing, or at least suggesting, the composition of a body
and the general class to which it belongs. This may be illustrated by
the names of hydro-carbons in the above list. The five compounds headed
by benzol have many properties in common, and each one is entirely
different in its chemical behaviour to those which follow amylene. The
Greek numerals enter into the names of the latter, in order to express,
in this case, the number of atoms of carbon which are supposed to be
contained in each ultimate particle of the body. We write down in
parallel columns the names of these two classes of bodies, together with
the _symbols_ which represent their composition, reminding the reader
that the letter C represents carbon; the letter alone indicating _one_
atom of that element, but, when followed by a small figure, it implies
that number of carbon atoms; in like manner H, N, and O represent atoms
of hydrogen, nitrogen, and oxygen respectively.
Hexylene C_{6}H_{12}
Heptylene C_{7}H_{14}
Octylene C_{8}H_{16}
Nonylene C_{9}H_{18}
Decylene C_{10}H_{20}
Benzol C_{6}H_{6}
Toluol C_{7}H_{8}
Xylol C_{8}H_{10}
Cumol C_{9}H_{12}
Cymol C_{10}H_{14}
If these lists be carefully examined, it will be observed that there is
a regular progression in the constituent atoms, so that each set of
substances forms a series, the differences being always the same. The
various bodies contained in the coal-tar are separated from each other
by taking advantage of the fact that each substance has its own
boiling-point; that is, there is a certain temperature, different for
each body, at which it will rise into vapour quickly and continuously.
Benzol, for example, boils at 82° C., toluol at 114° C., and phenol at
188° C.; so that, if we apply heat to a mixture of these three
substances, the benzol will boil when the temperature reaches 82°, and
will pass away in vapour, carrying off heat, so that the temperature
will not rise until all the benzol has been driven off; then, when the
temperature reaches 114°, the toluol will begin to come off, but not
until that has all passed over into the receiver will the temperature
rise above 114°; and the phenol remaining will distil only at 188°.
Another mode of separating bodies when mixed together is by treating
them with a liquid which acts on, or dissolves out, some of the
constituents, but not the rest. The coal-tar, as it is received from the
gas-works, is placed in large stills, capable, perhaps, of holding
several thousand gallons, and usually made of wrought iron. Stills
sufficiently good for the purpose are commonly constructed from the
worn-out boilers of steam engines. The application of heat, of course,
causes the more volatile substances to come over first. These are
condensed and collected apart until products begin to come off which are
heavier than water. The first portion of the distillate, containing the
lighter liquids, is termed “coal naphtha.” The process is continued, and
heavier liquids come over, forming what is called in the trade the “dead
oil.” Pitch remains behind in the retort, from which it is usually run
out while hot, but sometimes the distillation is carried a step further.
The chief colour-producing substances contained in coal-tar are benzol,
toluol, phenol, naphthalene, and anthracene. The aniline which is
present in the tar is very small in amount, and if this ready-formed
aniline were our only supply, it would be impossible to make colours
from it on an industrial scale. The first of the above-named substances,
benzol, was discovered by Faraday, in 1825, in liquid produced by
strongly compressing gas obtained from oil. He called it bicarburet of
hydrogen; but afterwards another chemist, having procured the same body
by distilling benzoic acid with lime, termed it _benzine_. It readily
dissolves fats and oils; and is used domestically for removing
grease-spots, cleaning gloves, &c., and in the arts as a solvent of
india-rubber and gutta-percha. It is a very limpid, colourless liquid,
very volatile, and, when pure, is of a peculiar but not disagreeable
odour. It boils at 82° C., and, cooled to the freezing-point of water,
it solidifies into beautiful transparent crystals, a property which is
sometimes taken advantage of to separate it in a state of purity from
other liquids which do not so solidify.
Benzol is very inflammable, and its vapour produces an explosive mixture
with air. The vapour, which is invisible, will run out of any leak in
the apparatus, like water, and flow along the ground. Accidents have
occurred from this cause, and a case is on record in which the vapour
having crept along the floor of the works, was set on fire by a furnace
forty feet away from the apparatus, the flame, of course, running back
to the spot from which the vapour was issuing. Benzol is a dreadful
substance for spreading fire should it become ignited, for, being
lighter than water, it floats upon its surface, and therefore the flames
cannot be extinguished in the ordinary way. The discovery of the
presence of benzol in coal-tar was made by Hofman in 1845. It is
obtained from the light oil of coal-tar by first purifying this liquid
by alternately distilling it with steam and treating with sulphuric acid
several times. The product so obtained is a colourless liquid, sold as
“rectified coal naphtha,” which, however, has again to be several times
re-distilled with a careful regulation of the temperature, so that the
benzol may be distilled off from other substances, boiling at a somewhat
higher temperature, with which it is mixed. Even then the resulting
liquid (commercial benzol) contains notable quantities of toluol. If
benzol be added in small quantities at a time to very strong and warm
nitric acid, a brisk action takes place, and when after some time water
is added, a yellow oily-looking liquid falls to the bottom of the
vessel. The benzol will have disappeared, for nitric acid under such
circumstances acts upon it by taking out of each particle an atom of
hydrogen, which it replaces by a _group_ of atoms of nitrogen and
oxygen, and, instead of benzol, we have the yellow oil, _nitro-benzol_.
Chemists are accustomed to represent actions of this kind by what is
called a _chemical equation_, the left-hand side showing the symbols
representing the constitution of the bodies which are placed together,
and the right hand the symbols of the bodies which result from the
chemical action. Here is the equation representing the action we have
described:
C_{6}H_{6} + NO_{2}OH = C_{6}H_{5}(NO_{2}) + HOH
Benzol. Nitric acid. Nitro-benzol. Water.
[Illustration:
FIG. 353.—_Iron Pots for making Nitro-Benzol._
]
[Illustration:
FIG. 354.—_Section of Apparatus for making Nitro-Benzol._
]
Nitro-benzol has a sweet taste and a fragrant odour. It is known in
commerce under the names of artificial oil of bitter almonds and essence
of mirbane, and it has been used for perfuming soap. The chemical action
between benzol and concentrated nitric acid is so violent that, when
nitro-benzol first had to be manufactured on the large scale, great
difficulty was experienced on account of the serious explosions which
occurred. The apparatus now used in making nitro-benzol on the large
scale is represented in Fig. 353, which shows some of the cast-iron
pots, of which there is usually a long row. These pots are about 4½ ft.
in diameter, and the same in depth. Each is provided with a stirrer,
which is made to revolve by a bevil-wheel, _c_, on its spindle, working
with a pinion on a shaft, _b_, driven by a steam engine. A layer of
water is kept on the tops of the lids, the water being constantly passed
in and drawn off through the pipes, _d_, in order to keep it cool. For
the chemical action is, as usual, attended with heat, which vaporizes
some of the benzol, but the cold lid re-condenses the vapour, which
would otherwise escape with the nitrous fumes that pass off by the pipe,
_a_. There is at _e_ an opening, through which the material may be
introduced, and in the bottom of the vessel is an aperture through which
the products may be drawn off. Fig. 354 shows a section of one of the
cast-iron vessels, and exhibits the mode in which the spindle of the
stirrer passes through the lid. In the cup, _a_, filled with a liquid, a
kind of inverted cup, which is attached to the spindle, turns round
freely. It would not do to choose water for the liquid in this cup, for
water would, by absorbing the nitrous fumes, form an acid capable of
attacking and destroying the spindle. Nothing has been found to answer
better for this purpose than nitro-benzol itself. The charge introduced
into these vessels is a mixture of nitric and sulphuric acids together
with the benzol. During the action, which may last twelve or fourteen
days, no heat is applied, for the mixture becomes hot spontaneously, and
in fact care must be taken that it does not become too hot. The
nitro-benzol thus obtained is purified by washing with water and
solution of soda.
[Illustration:
FIG. 355.—_Apparatus for making Aniline._
]
If nitro-benzol were brought into contact with ordinary hydrogen gas, no
action whatever would take place. But it is well known to chemists that
gases which are just being liberated from a compound have at the
_instant of their generation_ much more powerful chemical properties
than they possess afterwards. Gases in this condition are said to be in
the _nascent state_. If we submit nitro-benzol to the action of nascent
hydrogen we find a remarkable change is produced. This change consists,
first, in the hydrogen robbing the nitro-benzol of all its oxygen atoms;
second, in the addition of hydrogen to the remainder; third, in some
re-arrangement of the atoms, by which a new body is formed. Not that
these changes are successive, or that we actually know the movement of
atoms, but we are thus able to form ideas which correspond with the
final result. The new substance is named _aniline_. It is regarded by
chemists as a base; that is, a substance capable of neutralizing and
combining with an acid to form a _salt_. Its composition is represented
by the symbols C_{6}H_{5} H_{2}N. Aniline was found in coal-tar in 1834,
and even its colour-producing power was noticed, for its discoverer
named it _kyanol_, in allusion to the blue colour it produced with
chloride of lime. Later it was obtained by distilling indigo with
potash, and hence received its present name from _anil_, the Portuguese
for indigo. The quantity of aniline contained in the tar is quite
insignificant.
Aniline is prepared from nitro-benzol on the large scale by heating it
with acetic acid and iron filings or iron borings, a process which
rapidly changes the nitro-benzol into aniline. The equation representing
the change is—
C_{6}H_{5}NO_{2} + H_{6} = C_{6}H_{5}H_{2}N + 2H_{2}O.
Nitro-benzol. Hydrogen. Aniline. Water.
The operation is effected in the apparatus represented in Fig. 355. It
consists of a large iron cylinder, within which works a paddle on a
vertical revolving spindle, which, being hollow, is also a pipe to
convey high pressure steam within the apparatus. Fig. 356 is a section
of the hollow spindle, in which _f_ is the pivot at the bottom of the
cylinder on which it turns; _d_ is the stirring paddle; _e_ is an
aperture admitting the steam from the pipe, _c_, forming the shaft of
the paddle, which is made to revolve by the bevil-wheel. The steams
enters by the elbow-pipe, which has a nozzle ground to fit the head of
the vertical revolving pipe, upon which it is pressed down by the screw.
When the materials have been introduced into the cylinder, the stirrer
is set in motion, and superheated steam is sent down the pipe; the
aniline is volatilized and passes with the steam through the pipe, which
is connected with a worm surrounded by cold water. The aniline is
purified by another distillation over lime or soda. When pure, aniline
is a colourless, somewhat oily-looking liquid, of a feeble aromatic
odour. Under the influence of light and air it becomes of a brownish
tint, in which condition it usually presents itself in commerce. It
scarcely dissolves in water, but is readily soluble in alcohol, ether,
&c.
[Illustration:
FIG. 356.—_Section of Hollow Spindle,—Aniline Apparatus._
]
It was Mr. Perkin who, in 1856, first obtained from aniline a substance
practically available for dyeing. Let it be noticed that when Mr. Perkin
discovered _aniline purple_, he was not engaged in searching for
dye-stuffs, but was carrying on a purely scientific investigation as to
the possibility of artificially preparing quinine. With this view,
having selected a substance into the composition of which nitrogen,
hydrogen, and carbon enter in exactly the same proportions as they occur
in quinine, but differing from it by containing no oxygen, he thought it
not improbable that by oxidizing this body he might obtain quinine. In
this he was disappointed, for the result was a dirty reddish-brown
powder. Being desirous, however, of understanding more fully the nature
of this reddish powder, he proceeded to try the effects of oxidation on
other similarly constituted but more simple bodies. For this purpose he
fortunately selected _aniline_, which, when treated with sulphuric acid
and bichromate of potash, he found to yield a perfectly black product.
Persevering in his experiments by examining this black substance, he
obtained, by digesting it with spirits of wine, the now well-known
“aniline purple.” Mr. Perkin, having determined to make the aniline
purple on the large scale, patented his process, and succeeded in
overcoming the many obstacles incident to the establishment of a new
manufacture requiring as its raw material products not at that time met
with as commercial articles. The process is now carried on the large
scale by mixing sulphuric acid and aniline in the proportions in which
they combine to form the sulphate of aniline, and dissolving by boiling
with water in a large vat. Bichromate of potash is dissolved in water in
another large vat. When both solutions are cold, they are mixed together
in a still larger vessel and allowed to stand a day or two. A fine black
powder settles on the bottom of the vessel in large quantities; this is
collected in filters, washed with water, and dried. This powder is not
aniline purple alone, but a mixture of this with other products,
presenting a very unpromising appearance; but when it has been digested
for some time with diluted methylated spirit, all the colouring matter
is dissolved out, and is obtained from the solution by placing the
latter in a still, where the spirit is distilled off and collected for
future use, while all the colouring matter remains behind, held in
solution by the water. From this aqueous solution the mauve is thrown
down by adding caustic soda. It is collected, washed, and drained until
of a pasty consistence, in which condition it is sent into the market.
It can be obtained in crystals, but the commercial article is seldom
required in this form, as the additional expense is not compensated by
any superiority in the practical applications of the colour. Mauve is
readily soluble in spirits of wine, but not very soluble in water. Its
tinctorial power is so great that _one-tenth of a grain_ suffices to
impart quite a deep colour to a gallon of water. Silk and woollen
fabrics have an extraordinary attraction for this colouring matter,
which attaches itself very firmly to their fibres. If some white wool is
dipped into even a very dilute solution, the colour is quickly absorbed.
Mauve is more permanent than any other coal-tar colour, being little
affected by the prolonged action of light.
Mauve is chemically a salt of a base which has been termed “mauveine.”
Mauveine itself is a nearly black crystalline powder, which forms
solutions of a dull blue-violet tint, but when an acid is added to such
a solution the tint is at once changed to purple. Mauveine is a powerful
base, displacing ammonia from its compounds. The commercial crystallized
mauve is the acetate of mauveine.
The process by which Mr. Perkin originally obtained mauve from aniline
evidently depends upon the well-known oxidizing property of bichromate
of potash, and experiments were accordingly made with other, oxidizing
bodies and aniline; in fact, patents were taken out for the use of
nearly every known oxidizing chemical. Three years after Mr. Perkin’s
discovery of mauve, M. Verguin, of Lyons, obtained, by treating _crude_
aniline with chloride of tin, the bright red colouring matter now known
as magenta. It was found also that crude aniline, when treated with
other metallic chlorides, nitrates, or other salts, which are oxidizing
agents less powerful than bichromate of potash, yields this bright red
colouring matter. A process patented by Medlock, in 1860, in which
arsenic acid is the oxidizing agent, has almost entirely superseded, in
England at least, all the others yet proposed for the manufacture of
magenta. It is not a little remarkable that magenta would not have been
discovered had M. Verguin and others operated on _pure_ aniline instead
of on the ordinary commercial article. For it was found subsequently by
Dr. Hofman that pure aniline cannot be made to yield magenta: the
presence of another body is necessary. A reference to the table of
coal-tar constituents will show that there is a hydro-carbon named
“toluol.” This substance is of a similar nature to benzol, and has a
boiling-point so little above that of benzol, that in the rough methods
of separation usually employed, a notable quantity of toluol is carried
over with the benzol, and is always present in the commercial article.
In the processes which benzol undergoes for conversion into aniline, the
toluol accompanies it in a series of parallel transformations, resulting
in the production of a base termed “toluidine”—similar to aniline—being,
however, in its pure state a solid at ordinary temperatures. We write
down the symbols representing the composition of the bodies formed in
the two cases in order to clearly show this:
Benzol C_{6}H_{6}
Nitro-benzol C_{6}H_{5}(NO_{2})
Aniline C_{6}H_{5}NH_{2}
Toluol C_{7}H_{8}
Nitro-toluol C_{7}H_{7}(NO_{2})
Toluidine C_{7}H_{7}NH_{2}