wetting the nostrils with liquid ammonia, in order to neutralize the
vapour as it reaches the nose, its prejudicial effects may be in some
degree prevented. At any rate, however, this mode of disinfection can
hardly be used in places that are inhabited. And as the vapour of nitric
acid, which is scarcely less efficacious for this purpose, is not at all
prejudicial, it is usually preferred on such occasions.
CAROLINE.
You have not told us yet what is Sir H. Davy’s new opinion respecting
the nature of muriatic acid, to which you alluded a few minutes ago?
MRS. B.
True; I avoided noticing it then, because you could not have understood
it without some previous knowledge of the oxy-muriatic acid, which I
have but just introduced to your acquaintance.
Sir H. Davy’s idea is that muriatic acid, instead of being a compound,
consisting of an unknown basis and oxygen, is formed by the union of
oxy-muriatic gas with hydrogen.
EMILY.
Have you not told us just now that oxy-muriatic gas was itself a
compound of muriatic acid and oxygen?
MRS. B.
Yes; but according to Sir H. Davy’s hypothesis, oxy-muriatic gas is
considered as a simple body, which contains no oxygen--as a substance of
its own kind, which has a great analogy to oxygen in most of its
properties, though in others it differs entirely from it. --According to
this view of the subject, the name of _oxy-muriatic acid_ can no longer
be proper, and therefore Sir H. Davy has adopted that of _chlorine_, or
_chlorine gas_, a name which is simply expressive of its greenish
colour; and in compliance with that philosopher’s theory, we have placed
chlorine in our table among the simple bodies.
CAROLINE.
But what was Sir H. Davy’s reason for adopting an opinion so contrary to
that which had hitherto prevailed?
MRS. B.
There are many circumstances which are favourable to the new doctrine;
but the clearest and simplest fact in its support is, that if hydrogen
gas and oxy-muriatic gas be mixed together, both these gases disappear,
and muriatic acid gas is formed.
EMILY.
That seems to be a complete proof; is it not considered as perfectly
conclusive?
MRS. B.
Not so decisive as it appears at first sight; because it is argued by
those who still incline to the old doctrine, that muriatic acid gas,
however dry it may be, always contains a certain quantity of water,
which is supposed essential to its formation. So that, in the experiment
just mentioned, this water is supplied by the union of the hydrogen gas
with the oxygen of the oxy-muriatic acid; and therefore the mixture
resolves itself into the base of muriatic acid and water, that is,
muriatic acid gas.
CAROLINE.
I think the old theory must be the true one; for otherwise how could you
explain the formation of oxy-muriatic gas, from a mixture of muriatic
acid and oxyd of manganese?
MRS. B.
Very easily; you need only suppose that in this process the muriatic
acid is decomposed; its hydrogen unites with the oxygen of the manganese
to form water, and the chlorine appears in its separate state.
EMILY.
But how can you explain the various combustions which take place in
oxy-muriatic gas, if you consider it as containing no oxygen?
MRS. B.
We need only suppose that combustion is the result of intense chemical
action; so that chlorine, like oxygen, in combining with bodies, forms
compounds which have less capacity for caloric than their constituent
principles, and, therefore, caloric is evolved at the moment of their
combination.
EMILY.
If, then, we may explain every thing by either theory, to which of the
two shall we give the preference?
MRS. B.
It will, perhaps, be better to wait for more positive proofs, if such
can be obtained, before we decide positively upon the subject. The new
doctrine has certainly gained ground very rapidly, and may be considered
as nearly established; but several competent judges still refuse their
assent to it, and until that theory is very generally adopted, it may be
as well for us still occasionally to use the language to which chemists
have long been accustomed. --But let us proceed to the examination of
salts formed by muriatic acid.
Among the compound salts formed by muriatic acid, the _muriat of soda_,
or common salt, is the most interesting.* The uses and properties of
this salt are too well known to require much comment. Besides the
pleasant flavour it imparts to the food, it is very wholesome, when not
used to excess, as it assists the process of digestion.
Sea-water is the great source from which muriat of soda is extracted by
evaporation. But it is also found in large solid masses in the bowels of
the earth, in England, and in many other parts of the world.
[Footnote *: According to Sir H. Davy’s views of the nature of the
muriatic and oxy-muriatic acids, dry muriat of soda is a compound
of sodium and chlorine, for it may be formed by the direct
combination of oxy-muriatic gas and sodium. In his opinion,
therefore, what we commonly call muriat of soda contains neither
soda nor muriatic acid.]
EMILY.
I thought that salts, when solid, were always in the state of crystals;
but the common table-salt is in the form of a coarse white powder.
MRS. B.
Crystallisation depends, as you may recollect, on the slow and regular
reunion of particles dissolved in a fluid; common sea-salt is only in a
state of imperfect crystallisation, because the process by which it is
prepared is not favourable to the formation of regular crystals. But if
you dissolve it, and afterwards evaporate the water slowly, you will
obtain a regular crystallisation.
_Muriat of ammonia_ is another combination of this acid, which we have
already mentioned as the principal source from which ammonia is derived.
I can at once show you the formation of this salt by the immediate
combination of muriatic acid with ammonia. --These two glass jars
contain, the one muriatic acid gas, the other ammoniacal gas, both of
which are perfectly invisible--now, if I mix them together, you see they
immediately form an opake white cloud, like smoke. --If a thermometer
was placed in the jar in which these gases are mixed, you would perceive
that some heat is at the same time produced.
EMILY.
The effects of chemical combinations are, indeed, wonderful! --How
extraordinary it is that two invisible bodies should become visible by
their union!
MRS. B.
This strikes you with astonishment, because it is a phenomenon which
nature seldom exhibits to our view; but the most common of her
operations are as wonderful, and it is their frequency only that
prevents our regarding them with equal admiration. What would be more
surprising, for instance, than combustion, were it not rendered so
familiar by custom?
EMILY.
That is true. --But pray, Mrs. B., is this white cloud the salt that
produces ammonia? How different it is from the solid muriat of ammonia
which you once showed us!
MRS. B.
It is the same substance which first appears in the state of vapour, but
will soon be condensed by cooling against the sides of the jar, in the
form of very minute crystals.
We may now proceed to the _oxy-muriats_. In this class of salts the
_oxy-muriat of potash_ is the most worthy of our attention, for its
striking properties. The acid, in this state of combination, contains a
still greater proportion of oxygen than when alone.
CAROLINE.
But how can the oxy-muriatic acid acquire an increase of oxygen by
combining with potash?
MRS. B.
It does not really acquire an additional quantity of oxygen, but it
loses some of the muriatic acid, which produces the same effect, as the
acid which remains is proportionably super-oxygenated.*
If this salt be mixed, and merely rubbed together with sulphur,
phosphorus, charcoal, or indeed any other combustible, it explodes
strongly.
[Footnote *: According to Sir H. Davy’s new views, just explained,
oxy-muriat of potash is a compound of chlorine with oxyd of
potassium.]
CAROLINE.
Like gun-powder, I suppose, it is suddenly converted into elastic
fluids?
MRS. B.
Yes; but with this remarkable difference, that no increase of
temperature, any further than is produced by gentle friction, is
required in this instance. Can you tell me what gases are generated by
the detonation of this salt with charcoal?
EMILY.
Let me consider . . . . . The oxy-muriatic acid parts with its excess of
oxygen to the charcoal, by which means it is converted into muriatic
acid gas; whilst the charcoal, being burnt by the oxygen, is changed to
carbonic acid gas. --What becomes of the potash I cannot tell.
MRS. B.
That is a fixed product which remains in the vessel.
CAROLINE.
But since the potash does not enter into the new combinations, I do not
understand of what use it is in this operation. Would not the
oxy-muriatic acid and the charcoal produce the same effect without it?
MRS. B.
No; because there would not be that very great concentration of oxygen
which the combination with the potash produces, as I have just
explained.
I mean to show you this experiment, but I would advise you not to repeat
it alone; for if care be not taken to mix only very small quantities at
a time, the detonation will be extremely violent, and may be attended
with dangerous effects. You see I mix an exceedingly small quantity of
the salt with a little powdered charcoal, in this Wedgwood mortar, and
rub them together with the pestle--
CAROLINE.
Heavens! How can such a loud explosion be produced by so small a
quantity of matter?
MRS. B.
You must consider that an extremely small quantity of solid substance
may produce a very great volume of gases; and it is the sudden evolution
of these which occasions the sound.
EMILY.
Would not oxy-muriat of potash make stronger gunpowder than nitrat of
potash?
MRS. B.
Yes; but the preparation, as well as the use of this salt, is attended
with so much danger, that it is never employed for that purpose.
CAROLINE.
There is no cause to regret it, I think; for the common gunpowder is
quite sufficiently destructive.
MRS. B.
I can show you a very curious experiment with this salt; but it must
again be on condition that you will never attempt to repeat it by
yourselves. I throw a small piece of phosphorus into this glass of
water; then a little oxy-muriat of potash; and, lastly, I pour in (by
means of this funnel, so as to bring it in contact with the two other
ingredients at the bottom of the glass) a small quantity of sulphuric
acid--
CAROLINE.
This is, indeed, a beautiful experiment! The phosphorus takes fire and
burns from the bottom of the water.
EMILY.
How wonderful it is to see flame bursting out under water, and rising
through it! Pray, how is this accounted for?
MRS. B.
Cannot you find it out, Caroline?
EMILY.
Stop--I think I can explain it. Is it not because the sulphuric acid
decomposes the salt by combining with the potash, so as to liberate the
oxy-muriatic acid gas by which the phosphoric is set on fire?
MRS. B.
Very well, Emily; and with a little more reflection you would have
discovered another concurring circumstance, which is, that an increase
of temperature is produced by the mixture of the sulphuric acid and
water, which assists in promoting the combustion of the phosphorus.
I must, before we part, introduce to your acquaintance the
newly-discovered substance IODINE, which you may recollect we placed
next to oxygen and chlorine in our table of simple bodies.
CAROLINE.
Is this also a body capable of maintaining combustion like oxygen and
chlorine?
MRS. B.
It is; and although it does not so generally disengage light and heat
from inflammable bodies, as oxygen and chlorine do, yet it is capable of
combining with most of them; and sometimes, as in the instance of
potassium and phosphorus, the combination is attended with an actual
appearance of light and heat.
CAROLINE.
But what sort of a substance is iodine: what is its form, and colour?
MRS. B.
It is a very singular body, in many respects. At the ordinary
temperature of the atmosphere, it commonly appears in the form of
blueish black crystalline scales, such as you see in this tube.
CAROLINE.
They shine like black lead, and some of the scales have the shape of
lozenges.
MRS. B.
That is actually the form which the crystals of iodine often assume. But
if we heat them gently, by holding the tube over the flame of a candle,
see what a change takes place in them.
CAROLINE.
How curious! They seem to melt, and the tube immediately fills with a
beautiful violet vapour. But look, Mrs. B., the same scales are now
appearing at the other end of the tube.
MRS. B.
This is in fact a sublimation of iodine, from one part of the tube to
another; but with this remarkable peculiarity, that, while in the
gaseous state, iodine assumes that bright violet colour, which, as you
may already perceive, it loses as the tube cools, and the substance
resumes its usual solid form. --It is from the violet colour of the gas
that iodine has obtained its name.
CAROLINE.
But how is this curious substance obtained?
MRS. B.
It is found in the ley of ashes of sea-weeds, after the soda has been
separated by crystallisation; and it is disengaged by means of sulphuric
acid, which expels it from the alkaline ley in the form of a violet gas,
which may be collected and condensed in the way you have just seen.
--This interesting discovery was made in the year 1812, by M. Courtois,
a manufacturer of saltpetre at Paris.
CAROLINE.
And pray, Mrs. B., what is the proof of iodine being a simple body?
MRS. B.
It is considered as a simple body, both because it is not capable of
being resolved into other ingredients; and because it is itself capable
of combining with other bodies, in a manner analogous to oxygen and
chlorine. The most curious of these combinations is that which it forms
with hydrogen gas, the result of which is a peculiar gaseous acid.
CAROLINE.
Just as chlorine and hydrogen gas form muriatic acid? In this respect
chlorine and iodine seem to bear a strong analogy to each other.
MRS. B.
That is indeed the case; so that if the theory of the constitution of
either of these two bodies be true, it must be true also in regard to
the other; if erroneous in the one, the theory must fall in both.
But it is now time to conclude; we have examined such of the acids and
salts as I conceived would appear to you most interesting. --I shall not
enter into any particulars respecting the metallic acids, as they offer
nothing sufficiently striking for our present purpose.
CONVERSATION XX.
ON THE NATURE AND COMPOSITION OF VEGETABLES.
MRS. B.
We have hitherto treated only of the simplest combinations of elements,
such as alkalies, earths, acids, compound salts, stones, &c.; all of
which belong to the mineral kingdom. It is time now to turn our
attention to a more complicated class of compounds, that of ORGANISED
BODIES, which will furnish us with a new source of instruction and
amusement.
EMILY.
By organised bodies, I suppose, you mean the vegetable and animal
creation? I have, however, but a very vague idea of the word
_organisation_, and I have often wished to know more precisely what it
means.
MRS. B.
Organised bodies are such as are endowed by nature with various parts,
peculiarly constructed and adapted to perform certain functions
connected with life. Thus you may observe, that mineral compounds are
formed by the simple effect of mechanical or chemical attraction, and
may appear to some to be in a great measure the productions of chance;
whilst organised bodies bear the most striking and impressive marks of
design, and are eminently distinguished by that unknown principle,
called _life_, from which the various organs derive the power of
exercising their respective functions.
CAROLINE.
But in what manner does life enable these organs to perform their
several functions?
MRS. B.
That is a mystery which, I fear, is enveloped in such profound darkness
that there is very little hope of our ever being able to unfold it. We
must content ourselves with examining the effects of this principle; as
for the cause, we have been able only to give it a name, without
attaching any other meaning to it than the vague and unsatisfactory idea
of au unknown agent.
CAROLINE.
And yet I think I can form a very clear idea of life.
MRS. B.
Pray let me hear how you would define it?
CAROLINE.
It is perhaps more easy to conceive than to express--let me consider--
Is not life the power which enables both the animal and the vegetable
creation to perform the various functions which nature has assigned to
them?
MRS. B.
I have nothing to object to your definition; but you will allow me to
observe, that you have only mentioned the effects which the unknown
cause produces, without giving us any notion of the cause itself.
EMILY.
Yes, Caroline, you have told us what life _does_, but you have not told
us what it _is_.
MRS. B.
We may study its operations, but we should puzzle ourselves to no
purpose by attempting to form an idea of its real nature.
We shall begin with examining its effects in the vegetable world, which
constitutes the simplest class of organised bodies; these we shall find
distinguished from the mineral creation, not only by their more
complicated nature, but by the power which they possess within
themselves, of forming new chemical arrangements of their constituent
parts, by means of appropriate organs. Thus, though all vegetables are
ultimately composed of hydrogen, carbon, and oxygen, (with a few other
occasional ingredients,) they separate and combine these principles by
their various organs, in a thousand ways, and form, with them, different
kinds of juices and solid parts, which exist ready made in vegetables,
and may, therefore, be considered as their immediate materials.
These are:
_Sap_,
_Mucilage_,
_Sugar_,
_Fecula_,
_Gluten_,
_Fixed Oil_,
_Volatile Oil_,
_Camphor_,
_Resins_,
_Gum Resins_,
_Balsams_,
_Caoutchouc_,
_Extractive colouring Matter_,
_Tannin_,
_Woody Fibre_,
_Vegetable Acids_, _&c._
CAROLINE.
What a long list of names! I did not suppose that a vegetable was
composed of half so many ingredients.
MRS. B.
You must not imagine that every one of these materials is formed in each
individual plant. I only mean to say, that they are all derived
exclusively from the vegetable kingdom.
EMILY.
But does each particular part of the plant, such as the root, the bark,
the stem, the seeds, the leaves, consist of one of these ingredients
only, or of several of them combined together?
MRS. B.
I believe there is no part of a plant which can be said to consist
solely of any one particular ingredient; a certain number of vegetable
materials must always be combined for the formation of any particular
part, (of a seed for instance,) and these combinations are carried on by
sets of vessels, or minute organs, which select from other parts, and
bring together, the several principles required for the development and
growth of those particular parts which they are intended to form and to
maintain.
EMILY.
And are not these combinations always regulated by the laws of chemical
attraction?
MRS. B.
No doubt; the organs of plants cannot force principles to combine that
have no attraction for each other; nor can they compel superior
attractions to yield to those of inferior power; they probably act
rather mechanically, by bringing into contact such principles, and in
such proportions, as will, by their chemical combination, form the
various vegetable products.
CAROLINE.
We may then consider each of these organs as a curiously constructed
apparatus, adapted for the performance of a variety of chemical
processes.
MRS. B.
Exactly so. As long as the plant lives and thrives, the carbon,
hydrogen, and oxygen, (the chief constituents of its immediate
materials,) are so balanced and connected together, that they are not
susceptible of entering into other combinations; but no sooner does
death take place, than this state of equilibrium is destroyed, and new
combinations produced.
EMILY.
But why should death destroy it; for these principles must remain in the
same proportions, and consequently, I should suppose, in the same order
of attractions?
MRS. B.
You must remember, that in the vegetable, as well as in the animal
kingdom, it is by the principle of _life_ that the organs are enabled to
act; when deprived of that agent or stimulus, their power ceases, and an
order of attractions succeeds similar to that which would take place in
mineral or unorganised matter.
EMILY.
It is this new order of attractions, I suppose, that destroys the
organisation of the plant after death; for if the same combinations
still continued to prevail, the plant would always remain in the state
in which it died?
MRS. B.
And that, you know, is never the case; plants may be partially preserved
for some time after death, by drying; but in the natural course of
events they all return to the state of simple elements; a wise and
admirable dispensation of Providence, by which dead plants are rendered
fit to enrich the soil, and become subservient to the nourishment of
living vegetables.
CAROLINE.
But we are talking of the dissolution of plants, before we have examined
them in their living state.
MRS. B.
That is true, my dear. But I wished to give you a general idea of the
nature of vegetation, before we entered into particulars. Besides, it is
not so irrelevant as you suppose to talk of vegetables in their dead
state, since we cannot analyse them without destroying life; and it is
only by hastening to submit them to examination, immediately after they
have ceased to live, that we can anticipate their natural decomposition.
There are two kinds of analysis of which vegetables are susceptible;
first, that which separates them into their immediate materials, such as
sap, resin, mucilage, &c.; secondly, that which decomposes them into
their primitive elements, as carbon, hydrogen, and oxygen.
EMILY.
Is there not a third kind of analysis of plants, which consists in
separating their various parts, as the stem, the leaves, and the several
organs of the flower?
MRS. B.
That, my dear, is rather the department of the botanist; we shall
consider these different parts of plants only, as the organs by which
the various secretions or separations are performed; but we must first
examine the nature of these secretions.
The _sap_ is the principal material of vegetables, since it contains the
ingredients that nourish every part of the plant. The basis of this
juice, which the roots suck up from the soil, is water; this holds in
solution the various other ingredients required by the several parts of
the plant, which are gradually secreted from the sap by the different
organs appropriated to that purpose, as it passes them in circulating
through the plant.
_Mucus_, or _mucilage_, is a vegetable substance, which, like all the
others, is secreted from the sap; when in excess, it exudes from trees
in the form of gum.
CAROLINE.
Is that the gum so frequently used instead of paste or glue?
MRS. B.
It is; almost all fruit-trees yield some sort of gum, but that most
commonly used in the arts is obtained from a species of acacia-tree in
Arabia, and is called _gum arabic_; it forms the chief nourishment of
the natives of those parts, who obtain it in great quantities from
incisions which they make in the trees.
CAROLINE.
I did not know that gum was eatable.
MRS. B.
There is an account of a whole ship’s company being saved from starving
by feeding on the cargo, which was gum senegal. I should not, however,
imagine, that it would be either a pleasant or a particularly eligible
diet to those who have not, from their birth, been accustomed to it. It
is, however, frequently taken medicinally, and considered as very
nourishing. Several kinds of vegetable acids may be obtained, by
particular processes, from gum or mucilage, the principal of which is
called the _mucous acid_.
_Sugar_ is not found in its simple state in plants, but is always mixed
with gum, sap, or other ingredients; this saccharine matter is to be met
with in every vegetable, but abounds most in roots, fruits, and
particularly in the sugar-cane.
EMILY.
If all vegetables contain sugar, why is it extracted exclusively from
the sugar-cane?
MRS. B.
Because it is both most abundant in that plant, and most easily obtained
from it. Besides, the sugars produced by other vegetables differ a
little in their nature.