proposed; but it was but a rude sketch compared to its present state of
improvement. Sir H. Davy, after a succession of trials, by which he
brought his lamp nearer and nearer to perfection, at last conceived the
happy idea that if the lamp were surrounded with a wire-work or
wire-gauze, of a close texture, instead of glass or horn, the tubular
contrivance I have just described would be entirely superseded, since
each of the interstices of the gauze would act as a tube in preventing
the propagation of explosions; so that this pervious metallic covering
would answer the various purposes of transparency, of permeability to
air, and of protection against explosion. This idea, Sir Humphry
immediately submitted to the test of experiment, and the result has
answered his most sanguine expectations, both in his laboratory and in
the collieries, where it has already been extensively tried. And he has
now the happiness of thinking that his invention will probably be the
means of saving every year a number of lives, which would have been lost
in digging out of the bowels of the earth one of the most valuable
necessaries of life. Here is one of these lamps, every part of which you
will at once comprehend. (See PLATE X. fig. 1.)
[Illustration: Plate X.
Fig. 1.
A. the cistern containing the Oil
B. the rim or screw by which the gauze cage is fixed to the cistern.
C. apperture for supplying Oil.
E. a wire for trimming the wick.
D. F. the wire gauze cylinder.
G. a double top.
Fig. 2.
A. the reservoir of condensed air.
B. the condensing Syringe.
C. the bladder for Oxygen.
D. the moveable jet.]
CAROLINE.
How very simple and ingenious! But I do not yet well see why an
explosion taking place within the lamp should not communicate to the
external air around it, through the interstices of the wire?
MRS. B.
This has been and is still a subject of wonder, even to philosophers;
and the only mode they have of explaining it is, that flame or ignition
cannot pass through a fine wire-work, because the metallic wire cools
the flame sufficiently to extinguish it in passing through the gauze.
This property of the wire-gauze is quite similar to that of the tubes
which I mentioned on introducing the subject; for you may consider each
interstice of the gauze as an extremely short tube of a very small
diameter.
EMILY.
But I should expect the wire would often become red-hot, by the burning
of the gas within the lamp?
MRS. B.
And this is actually the case, for the top of the lamp is very apt to
become red-hot. But, fortunately, inflammable gaseous mixtures cannot be
exploded by red-hot wire, the intervention of actual flame being
required for that purpose; so that the wire does not set fire to the
explosive gas around it.
EMILY.
I can understand that; but if the wire be red-hot, how can it cool the
flame within, and prevent its passing through the gauze?
MRS. B.
The gauze, though red-hot, is not so hot as the flame by which it has
been heated; and as metallic wire is a good conductor, the heat does not
much accumulate in it, as it passes off quickly to the other parts of
the lamp, as well as to any contiguous bodies.
CAROLINE.
This is indeed a most interesting discovery, and one which shows at once
the immense utility with which science may be practically applied to
some of the most important purposes.
CONVERSATION VIII.
ON SULPHUR AND PHOSPHORUS.
MRS. B.
SULPHUR is the next substance that comes under our consideration. It
differs in one essential point from the preceding, as it exists in a
solid form at the temperature of the atmosphere.
CAROLINE.
I am glad that we have at last a solid body to examine; one that we can
see and touch. Pray, is it not with sulphur that the points of matches
are covered, to make them easily kindle?
MRS. B.
Yes, it is; and you therefore already know that sulphur is a very
combustible substance. It is seldom discovered in nature in a pure
unmixed state; so great is its affinity for other substances, that it is
almost constantly found combined with some of them. It is most commonly
united with metals, under various forms, and is separated from them by a
very simple process. It exists likewise in many mineral waters, and some
vegetables yield it in various proportions, especially those of the
cruciform tribe. It is also found in animal matter; in short, it may be
discovered in greater or less quantity, in the mineral, vegetable, and
animal kingdoms.
EMILY.
I have heard of _flowers of sulphur_, are they the produce of any plant?
MRS. B.
By no means: they consist of nothing more than common sulphur, reduced
to a very fine powder by a process called _sublimation_. --You see some
of it in this phial; it is exactly the same substance as this lump of
sulphur, only its colour is a paler yellow, owing to its state of very
minute division.
EMILY.
Pray what is sublimation?
MRS. B.
It is the evaporation, or, more properly speaking, the volatilisation of
solid substances, which, in cooling, condense again in a concrete form.
The process, in this instance, must be performed in a closed vessel,
both to prevent combustion, which would take place if the access of air
were not carefully precluded, and likewise in order to collect the
substance after the operation. As it is rather a slow process, we shall
not try the experiment now; but you will understand it perfectly if I
show you the apparatus used for the purpose. (PLATE XI. fig. 1.) Some
lumps of sulphur are put into a receiver of this kind, which is called a
_cucurbit_. Its shape, you see, somewhat resembles that of a pear, and
is open at the top, so as to adapt itself exactly to a kind of conical
receiver of this sort, called the head. The cucurbit, thus covered with
its head, is placed over a sand-bath; this is nothing more than a vessel
full of sand, which is kept heated by a furnace, such as you see here,
so as to preserve the apparatus in a moderate and uniform temperature.
The sulphur then soon begins to melt, and immediately after this,
a thick white smoke rises, which is gradually deposited within the head,
or upper part of the apparatus, where it condenses against the sides,
somewhat in the form of a vegetation, whence it has obtained the name of
flowers of sulphur. This apparatus, which is called an _alembic_, is
highly useful in all kinds of distillations, as you will see when we
come to treat of those operations. Alembics are not commonly made of
glass, like this, which is applicable only to distillations upon a very
small scale. Those used in manufactures are generally made of copper,
and are, of course, considerably larger. The principal construction,
however, is always the same, although their shape admits of some
variation.
[Illustration: Plate XI. Vol. I. p. 237.
Fig. 1. Sublimation of Sulphur.
A Alembic.
B Sand-bath.
C Furnace.
Fig. 2. Eudiometer.
Fig. 3. Decomposition of water by Carbon.
A Retort containing water.
B Lamp to heat the water.
C.C Porcelain tube containing Carbone.
D Furnace through which the tube passes.
E Receiver for the gas produced.
F Water bath.]
CAROLINE.
What is the use of that neck, or tube, which bends down from the upper
piece of the apparatus?
MRS. B.
It is of no use in sublimations; but in distillations (the general
object of which is to evaporate, by heat, in closed vessels, the
volatile parts of a compound body, and to condense them again into a
liquid,) it serves to carry off the condensed fluid, which otherwise
would fall back into the cucurbit. But this is rather foreign to our
present subject. Let us return to the sulphur. You now perfectly
understand, I suppose, what is meant by sublimation?
EMILY.
I believe I do. Sublimation appears to consist in destroying, by means
of heat, the attraction of aggregation of the particles of a solid body,
which are thus volatilised; and as soon as they lose the caloric which
produced that effect, they are deposited in the form of a fine powder.
CAROLINE.
It seems to me to be somewhat similar to the transformation of water
into vapour, which returns to its liquid state when deprived of caloric.
EMILY.
There is this difference, however, that the sulphur does not return to
its former state, since, instead of lumps, it changes to a fine powder.
MRS. B.
Chemically speaking, it is exactly the same substance, whether in the
form of lump or powder. For if this powder be melted again by heat, it
will, in cooling, be restored to the same solid state in which it was
before its sublimation.
CAROLINE.
But if there be no real change, produced by the sublimation of the
sulphur, what is the use of that operation?
MRS. B.
It divides the sulphur into very minute parts, and thus disposes it to
enter more readily into combination with other bodies. It is used also
as a means of purification.
CAROLINE.
Sublimation appears to me like the beginning of combustion, for the
completion of which one circumstance only is wanting, the absorption of
oxygen.
MRS. B.
But that circumstance is every thing. No essential alteration is
produced in sulphur by sublimation; whilst in combustion it combines
with the oxygen, and forms a new compound totally different in every
respect from sulphur in its pure state. --We shall now _burn_ some
sulphur, and you will see how very different the result will be. For
this purpose I put a small quantity of flowers of sulphur into this cup,
and place it in a dish, into which I have poured a little water: I now
set fire to the sulphur with the point of this hot wire; for its
combustion will not begin unless its temperature be considerably raised.
--You see that it burns with a faint blueish flame; and as I invert over
it this receiver, white fumes arise from the sulphur, and fill the
vessel. --You will soon perceive that the water is rising within the
receiver, a little above its level in the plate. --Well, Emily, can you
account for this?
EMILY.
I suppose that the sulphur has absorbed the oxygen from the
atmospherical air within the receiver, and that we shall find some
oxygenated sulphur in the cup. As for the white smoke, I am quite at a
loss to guess what it may be.
MRS. B.
Your first conjecture is very right: but you are mistaken in the last;
for nothing will be left in the cup. The white vapour is the oxygenated
sulphur, which assumes the form of an elastic fluid of a pungent and
offensive smell, and is a powerful acid. Here you see a chemical
combination of oxygen and sulphur, producing a true gas, which would
continue such under the pressure and at the temperature of the
atmosphere, if it did not unite with the water in the plate, to which it
imparts its acid taste, and all its acid properties. --You see, now,
with what curious effects the combustion of sulphur is attended.
CAROLINE.
This is something quite new; and I confess that I do not perfectly
understand why the sulphur turns acid.
MRS. B.
It is because it unites with oxygen, which is the acidifying principle.
And, indeed, the word _oxygen_ is derived from two Greek words
signifying _to produce an acid_.
CAROLINE.
Why, then, is not water, which contains such a quantity of oxygen, acid?
MRS. B.
Because hydrogen, which is the other constituent of water, is not
susceptible of acidification. --I believe it will be necessary, before
we proceed further, to say a few words of the general nature of acids,
though it is rather a deviation from our plan of examining the simple
bodies separately, before we consider them in a state of combination.
Acids may be considered as a peculiar class of _burnt_ bodies, which
during their combustion, or combination with oxygen, have acquired very
characteristic properties. They are chiefly discernible by their sour
taste, and by turning red most of the blue vegetable colours. These two
properties are common to the whole class of acids; but each of them is
distinguished by other peculiar qualities. Every acid consists of some
particular substance, (which constitutes its basis, and is different in
each,) and of oxygen, which is common to them all.
EMILY.
But I do not clearly see the difference between acids and oxyds.
MRS. B.
Acids were, in fact, oxyds, which, by the addition of a sufficient
quantity of oxygen, have been converted into acids. For acidification,
you must observe, always implies previous oxydation, as a body must have
combined with the quantity of oxygen requisite to constitute it an oxyd,
before it can combine with the greater quantity that is necessary to
render it an acid.
CAROLINE.
Are all oxyds capable of being converted into acids?
MRS. B.
Very far from it; it is only certain substances which will enter into
that peculiar kind of union with oxygen that produces acids, and the
number of these is proportionally very small; but all burnt bodies may
be considered as belonging either to the class of oxyds, or to that of
acids. At a future period, we shall enter more at large into this
subject. At present, I have but one circumstance further to point out to
your observation respecting acids: it is, that most of them are
susceptible of two degrees of acidification, according to the different
quantities of oxygen with which their basis combines.
EMILY.
And how are these two degrees of acidification distinguished?
MRS. B.
By the peculiar properties which result from them. The acid we have just
made is the first or weakest degree of acidification, and is called
_sulphureous acid_; if it were fully saturated with oxygen, it would be
called _sulphuric acid_. You must therefore remember, that in this, as
in all acids, the first degree of acidification is expressed by the
termination in _ous_; the stronger, by the termination in _ic_.
CAROLINE.
And how is the sulphuric acid made?
MRS. B.
By burning sulphur in pure oxygen gas, and thus rendering its combustion
much more complete. I have provided some oxygen gas for this purpose; it
is in that bottle, but we must first decant the gas into the glass
receiver which stands on the shelf in the bath, and is full of water.
CAROLINE.
Pray, let me try to do it, Mrs. B.
MRS. B.
It requires some little dexterity--hold the bottle completely under
water, and do not turn the mouth upwards, till it is immediately under
the aperture in the shelf, through which the gas is to pass into the
receiver, and then turn it up gradually. --Very well, you have only let
a few bubbles escape, and that must be expected at a first trial. --Now
I shall put this piece of sulphur into the receiver, through the opening
at the top, and introduce along with it a small piece of lighted tinder
to set fire to it. --This requires being done very quickly, lest the
atmospherical air should get in, and mix with the pure oxygen gas.
EMILY.
How beautifully it burns!
CAROLINE.
But it is already buried in the thick vapour. This, I suppose, is
sulphuric acid?
EMILY.
Are these acids always in a gaseous state?
MRS. B.
Sulphureous acid, as we have already observed, is a permanent gas, and
can be obtained in a liquid form only by condensing it in water. In its
pure state, the sulphureous acid is invisible, and it now appears in the
form of a white smoke, from its combining with the moisture. But the
vapour of sulphuric acid, which you have just seen to rise during the
combustion, is not a gas, but only a vapour, which condenses into liquid
sulphuric acid, by losing its caloric. But it appears from Sir H. Davy’s
experiments, that this formation and condensation of sulphuric acid
requires the presence of water, for which purpose the vapour is received
into cold water, which may afterwards be separated from the acid by
evaporation.
Sulphur has hitherto been considered as a simple substance; but Sir H.
Davy has suspected that it contains a small portion of hydrogen, and
perhaps also of oxygen.
On submitting sulphur to the action of the Voltaic battery, he observed
that the negative wire gave out hydrogen; and the existence of hydrogen
in sulphur was rendered still more probable by his observing that a
small quantity of water was produced during the combustion of sulphur.
EMILY.
And pray of what nature is sulphur when perfectly pure?
MRS. B.
Sulphur has probably never been obtained perfectly free from
combination, so that its radical may possibly possess properties very
different from those of common sulphur. It has been suspected to be of a
metallic nature; but this is mere conjecture.
Before we quit the subject of sulphur, I must tell you that it is
susceptible of combining with a great variety of substances, and
especially with hydrogen, with which you are already acquainted.
Hydrogen gas can dissolve a small portion of it.
EMILY.
What! can a gas dissolve a solid substance?
MRS. B.
Yes; a solid substance may be so minutely divided by heat, as to become
soluble in a gas: and there are several instances of it. But you must
observe, that, in this case, a chemical union or combination of the
sulphur with the hydrogen gas is produced. In order to effect this, the
sulphur must be strongly heated in contact with the gas; the heat
reduces the sulphur to such a state of extreme division, and diffuses it
so thoroughly through the gas, that they combine and incorporate
together. And as a proof that there must be a chemical union between the
sulphur and the gas, it is sufficient to remark that they are not
separated when the sulphur loses the caloric by which it was
volatilized. Besides, it is evident, from the peculiar fetid smell of
this gas, that it is a new compound totally different from either of its
constituents; it is called _sulphuretted hydrogen gas_, and is contained
in great abundance in sulphureous mineral waters.
CAROLINE.
Are not the Harrogate waters of this nature?
MRS. B.
Yes; they are naturally impregnated with sulphuretted hydrogen gas, and
there are many other springs of the same kind, which shows that this gas
must often be formed in the bowels of the earth by spontaneous processes
of nature.
CAROLINE.
And could not such waters be made artificially by impregnating common
water with this gas?
MRS. B.
Yes; they can be so well imitated, as perfectly to resemble the
Harrogate waters.
Sulphur combines likewise with phosphorus, and with the alkalies, and
alkaline earths, substances with which you are yet unacquainted. We
cannot, therefore, enter into these combinations at present. In our next
lesson we shall treat of phosphorus.
EMILY.
May we not begin that subject to-day; this lesson has been so short?
MRS. B.
I have no objection, if you are not tired. What do you say, Caroline?
CAROLINE.
I am as desirous as Emily of prolonging the lesson to-day, especially as
we are to enter on a new subject; for I confess that sulphur has not
appeared to me so interesting as the other simple bodies.
MRS. B.
Perhaps you may find phosphorus more entertaining. You must not,
however, be discouraged when you meet with some parts of a study less
amusing than others; it would answer no good purpose to select the most
pleasing parts, since, if we did not proceed with some method, in order
to acquire a general idea of the whole, we could scarcely expect to take
interest in any particular subjects.
PHOSPHORUS.
PHOSPHORUS is considered as a simple body; though, like sulphur, it has
been suspected of containing hydrogen. It was not known by the earlier
chemists. It was first discovered by Brandt, a chemist of Hamburgh,
whilst employed in researches after the philosopher’s stone; but the
method of obtaining it remained a secret till it was a second time
discovered both by Kunckel and Boyle, in the year 1680. You see a
specimen of phosphorus in this phial; it is generally moulded into small
sticks of a yellowish colour, as you find it here.
CAROLINE.
I do not understand in what the discovery consisted; there may be a
secret method of making an artificial composition, but how can you talk
of _making_ a substance which naturally exists?
MRS. B.
A body may exist in nature so closely combined with other substances, as
to elude the observation of chemists, or render it extremely difficult
to obtain it in its separate state. This is the case with phosphorus,
which is always so intimately combined with other substances, that its
existence remained unnoticed till Brandt discovered the means of
obtaining it free from other combinations. It is found in all animal
substances, and is now chiefly extracted from bones, by a chemical
process. It exists also in some plants, that bear a strong analogy to
animal matter in their chemical composition.
EMILY.
But is it never found in its pure separate state?
MRS. B.
Never, and this is the reason that it has remained so long undiscovered.
Phosphorus is eminently combustible; it melts and takes fire at the
temperature of one hundred degrees, and absorbs in its combustion nearly
once and a half its own weight of oxygen.
CAROLINE.
What! will a pound of phosphorus consume a pound and half of oxygen?
MRS. B.
So it appears from accurate experiments. I can show you with what
violence it combines with oxygen, by burning some of it in that gas. We
must manage the experiment in the same manner as we did the combustion
of sulphur. You see I am obliged to cut this little bit of phosphorus
under water, otherwise there would be danger of its taking fire by the
heat of my fingers. I now put into the receiver, and kindle it by means
of a hot wire.
EMILY.
What a blaze! I can hardly look at it. I never saw any thing so
brilliant. Does it not hurt your eyes, Caroline?
CAROLINE.
Yes; but still I cannot help looking at it. A prodigious quantity of
oxygen must indeed be absorbed, when so much light and caloric are
disengaged!
MRS. B.
In the combustion of a pound of phosphorus, a sufficient quantity of
caloric is set free to melt upwards of a hundred pounds of ice; this has
been computed by direct experiments with the calorimeter.
EMILY.
And is the result of this combustion, like that of sulphur, an acid?
MRS. B.
Yes; phosphoric acid. And had we duly proportioned the phosphorus and
the oxygen, they would have been completely converted into phosphoric
acid, weighing together, in this new state, exactly the sum of their
weights separately. The water would have ascended into the receiver, on
account of the vacuum formed, and would have filled it entirely. In this
case, as in the combustion of sulphur, the acid vapour formed is
absorbed and condensed in the water of the receiver. But when this
combustion is performed without any water or moisture being present, the
acid then appears in the form of concrete whitish flakes, which are,
however, extremely ready to melt upon the least admission of moisture.
EMILY.
Does phosphorus, in burning in atmospherical air, produce, like sulphur,
a weaker sort of the same acid?
MRS. B.
No: for it burns in atmospherical air, nearly at the same temperature as
in pure oxygen gas; and it is in both cases so strongly disposed to
combine with the oxygen, that the combustion is perfect, and the product
similar; only in atmospherical air, being less rapidly supplied with
oxygen, the process is performed in a slower manner.
CAROLINE.
But is there no method of acidifying phosphorus in a slighter manner, so
as to form _phosphorus_ acid?
MRS. B.
Yes, there is. When simply exposed to the atmosphere, phosphorus
undergoes a kind of slow combustion at any temperature above zero.
EMILY.
But is not the process in this case rather an oxydation than a
combustion? For if the oxygen is too slowly absorbed for a sensible
quantity of light and heat to be disengaged, it is not a true
combustion.
MRS. B.
The case is not as you suppose: a faint light is emitted which is very
discernible in the dark; but the heat evolved is not sufficiently strong
to be sensible: a whitish vapour arises from this combustion, which,
uniting with water, condenses into liquid phosphorus acid.
CAROLINE.
Is it not very singular that phosphorus should burn at so low a
temperature in atmospherical air, whilst it does not burn in pure oxygen
without the application of heat?
MRS. B.
So it at first appears. But this circumstance seems to be owing to the
nitrogen gas of the atmosphere. This gas dissolves small particles of
phosphorus, which being thus minutely divided and diffused in the
atmospherical air, combines with the oxygen, and undergoes this slow
combustion. But the same effect does not take place in oxygen gas,
because it is not capable of dissolving phosphorus; it is therefore
necessary, in this case, that heat should be applied to effect that
division of particles, which, in the former instance, is produced by the
nitrogen.
EMILY.
I have seen letters written with phosphorus, which are invisible by
day-light, but may be read in the dark by their own light. They look as
if they were written with fire; yet they do not seem to burn.
MRS. B.
But they do really burn; for it is by their slow combustion that the
light is emitted; and phosphorus acid is the result of this combustion.