usually thrown to the dogs; and as we cannot expect them to enter into
chemical considerations, it is in some degree excusable. Besides, it
requires a prodigious quantity of fuel to dissolve bones and obtain the
gelatine from them.
The solution of bones in water is greatly promoted by an accumulation of
heat. This may be effected by means of an extremely strong metallic
vessel, called _Papin’s digester_, in which the bones and water are
enclosed, without any possibility of the steam making its escape. A heat
can thus be applied much superior to that of boiling water; and bones,
by this means, are completely reduced to a pulp. But the process still
consumes too much fuel to be generally adopted among the lower classes.
CAROLINE.
And why should not a manufacture be established for grinding or
macerating bones, or at least for reducing them to the state of
shavings, when I suppose they would dissolve as readily as hartshorn
shavings?
MRS. B.
They could not be collected clean for such a purpose, but they are not
lost, as they are used for making hartshorn and sal ammoniac; and such
is the superior science and industry of this country, that we now send
sal ammoniac to the Levant, though it originally came to us from Egypt.
EMILY.
When jelly is made of isinglass, does it leave no sediment?
MRS. B.
No; nor does it so much require clarifying, as it consists almost
entirely of pure gelantine, and any foreign matter that is mixed with
it, is thrown off during the boiling in the form of scum. --These are
processes which you may see performed in great perfection in the
culinary laboratory, by that very able and most useful chemist the cook.
CAROLINE.
To what an immense variety of purposes chemistry is subservient!
EMILY.
It appears, in that respect, to have an advantage over most other arts
and sciences; for these, very often, have a tendency to confine the
imagination to their own particular object, whilst the pursuit of
chemistry is so extensive and diversified, that it inspires a general
curiosity, and a desire of enquiring into the nature of every object.
CAROLINE.
I suppose that soup is likewise composed of gelatine; for, when cold, it
often assumes the consistence of jelly?
MRS. B.
Not entirely; for though soups generally contain a quantity of gelatine,
the most essential ingredient is a mucous or extractive matter,
a peculiar animal substance, very soluble in water, which has a strong
taste, and is more nourishing than gelatine. The various kinds of
portable soup consist of this extractive matter in a dry state, which,
in order to be made into soup, requires only to be dissolved in water.
Gelatine, in its solid state, is a semiductile transparent substance,
without either taste or smell. --When exposed to heat, in contact with
air and water, it first swells, then fuses, and finally burns. You may
have seen the first part of this operation performed in the carpenter’s
glue-pot.
CAROLINE.
But you said that gelatine had no smell, and glue has a very
disagreeable one.
MRS. B.
Glue is not pure gelatine; as it is not designed for eating, it is
prepared without attending to the state of the ingredients, which are
more or less contaminated by particles that have become putrid.
Gelatine may be precipitated from its solution in water by alcohol. --We
shall try this experiment with a glass of warm jelly. --You see that the
gelatine subsides by the union of the alcohol and the water.
EMILY.
How is it, then, that jelly is flavoured with wine, without producing
any precipitation?
MRS. B.
Because the alcohol contained in wine is already combined with water,
and other ingredients, and is therefore not at liberty to act upon the
jelly as when in its separate state. Gelatine is soluble both in acids
and in alkalies; the former, you know, are frequently used to season
jellies.
CAROLINE.
Among the combinations of gelatine we must not forget one which you
formerly mentioned; that with tannin, to form leather.
MRS. B.
True; but you must observe that leather can be produced only by gelatine
in a membranous state; for though pure gelatine and tannin will produce
a substance chemically similar to leather, yet the texture of the skin
is requisite to make it answer the useful purposes of that substance.
The next animal substance we are to examine is _albumen_; this, although
constituting a part of most of the animal compounds, is frequently found
insulated in the animal system; the white of egg, for instance, consists
almost entirely of albumen; the substance that composes the nerves, the
serum, or white part of the blood, and the curds of milk, are little
else than albumen variously modified.
In its most simple state, albumen appears in the form of a transparent
viscous fluid, possessed of no distinct taste or smell; it coagulates at
the low temperature of 165 degrees, and, when once solidified, it will
never return to its fluid state.
Sulphuric acid and alcohol are each of them capable of coagulating
albumen in the same manner as heat, as I am going to show you.
EMILY.
Exactly so. --Pray, Mrs. B., what kind of action is there between
albumen and silver? I have sometimes observed, that if the spoon with
which I eat an egg happens to be wetted, it becomes tarnished.
MRS. B.
It is because the white of egg (and, indeed, albumen in general)
contains a little sulphur, which, at the temperature of an egg just
boiled, will decompose the drop of water that wets the spoon, and
produce sulphurated hydrogen gas, which has the property of tarnishing
silver.
We may now proceed to _fibrine_. This is an insipid and inodorous
substance, having somewhat the appearance of fine white threads adhering
together; it is the essential constituent of muscles or flesh, in which
it is mixed with and softened by gelatine. It is insoluble both in water
and alcohol, but sulphuric acid converts it into a substance very
analogous to gelatine.
These are the essential and general ingredients of animal matter; but
there are other substances, which, though not peculiar to the animal
system, usually enter into its composition, such as oils, acids,
salts, &c.
_Animal oil_ is the chief constituent of fat; it is contained in
abundance in the cream of milk, whence it is obtained in the form of
butter.
EMILY.
Is animal oil the same in its composition as vegetable oils?
MRS. B.
Not the same, but very analogous. The chief difference is that animal
oil contains nitrogen, a principle which seldom enters into the
composition of vegetable oils, and never in so large a proportion.
There are a few animal acids, that is to say, acids peculiar to animal
matter, from which they are almost exclusively obtained.
The animal acids have triple bases of hydrogen, carbon, and nitrogen.
Some of them are found native in animal matter; others are produced
during its decomposition.
Those that we find ready formed are:
The _bombic acid_, which is obtained from silk-worms.
The _formic acid_, from ants.
The _lactic acid_, from the whey of milk.
The _sebacic_, from oil or fat.
Those produced during the decomposition of animal substances by heat,
are the _prussic_ and _zoonic_ acids. This last is produced by the
roasting of meat, and gives it a brisk flavour.
CAROLINE.
The class of animal acids is not very extensive?
MRS. B.
No; nor are they, generally speaking, of great importance. The _prussic
acid_ is, I think, the only one sufficiently interesting to require any
further comment. It can be formed by any artificial process, without the
presence of any animal matter; and it may likewise be obtained from a
variety of vegetables, particularly those of the narcotic kind, such as
poppies, laurel, &c. But it is commonly obtained from blood, by strongly
heating that substance with caustic potash; the alkali attracts the acid
from the blood, and forms with it a _prussiat of potash_. From this
state of combination the prussic acid can be obtained pure by means of
other substances which have the power of separating it from the alkali.
EMILY.
But if this acid does not exist ready formed in blood, how can the
alkali attract it from it?
MRS. B.
It is the triple basis only of this acid that exists in the blood; and
this is developed and brought to the state of acid, during the
combustion. The acid therefore is first formed, and it afterwards
combines with the potash.
EMILY.
Now I comprehend it. But how can the prussic acid be artificially made?
MRS. B.
By passing ammoniacal gas over red-hot charcoal; and hence we learn that
the constituents of this acid are hydrogen, nitrogen, and carbon. The
two first are derived from the volatile alkali, the last from the
combustion of the charcoal.
CAROLINE.
But this does not accord with the system of oxygen being the principle
of acidity.
MRS. B.
The colouring matter of prussian blue is called an acid, because it
unites with alkalies and metals, and not from any other characteristic
properties of acids; perhaps the name is not strictly appropriate. But
this circumstance, together with some others of the same kind, has
induced several chemists to think that oxygen may not be the exclusive
generator of acids. Sir H. Davy, I have already informed you, was led by
his experiments on dry acids to suspect that water might be essential to
acidity. And it is the opinion of some chemists that acidity may
possibly depend rather on the arrangement than on the presence of any
particular principles. But we have not yet done with the prussic acid.
It has a strong affinity for metallic oxyds, and precipitates the
solutions of iron in acids of a blue colour. This is the prussian blue,
or prussiat of iron, so much used in the arts, and with which I think
you must be acquainted.
EMILY.
Yes, I am; it is much used in painting, both in oil and in water
colours; but it is not reckoned a permanent oil-colour.
MRS. B.
That defect arises, I believe, in general, from its being badly
prepared, which is the case when the iron is not so fully oxydated as to
form a red oxyd. For a solution of green oxyd of iron (in which the
metal is more slightly oxydated), makes only a pale green, or even a
white precipitate, with prussiat of potash; and this gradually changes
to blue by being exposed to the air, as I can immediately show you.
CAROLINE.
It already begins to assume a pale blue colour. But how does the air
produce this change?
MRS. B.
By oxydating the iron more perfectly. If we pour some nitrous acid on
it, the prussian blue colour will be immediately produced, as the acid
will yield its oxygen to the precipitate, and fully saturate it with
this principle, as you shall see.
CAROLINE.
It is very curious to see a colour change so instantaneously.
MRS. B.
Hence you perceive that prussian blue cannot be a permanent colour,
unless prepared with red oxyd of iron, since by exposure to the
atmosphere it gradually darkens, and in a short time is no longer in
harmony with the other colours of the painting.
CAROLINE.
But it can never become darker, by exposure to the atmosphere, than the
true prussian blue, in which the oxyd is perfectly saturated?
MRS. B.
Certainly not. But in painting, the artist not reckoning upon partial
alterations in his colours, gives his blue tints that particular shade
which harmonises with the rest of the picture. If, afterwards, those
tints become darker, the harmony of the colouring must necessarily be
destroyed.
CAROLINE.
Pray, of what nature is the paint called _carmine_?
MRS. B.
It is an animal colour prepared from _cochineal_, an insect, the
infusion of which produces a very beautiful red.
CAROLINE.
Whilst we are on the subject of colours, I should like to learn what
_ivory black_ is?
MRS. B.
It is a carbonaceous substance obtained by the combustion of ivory.
A more common species of black is obtained from the burning of bone.
CAROLINE.
But during the combustion of ivory or bone, the carbon, I should have
imagined, must be converted into carbonic acid gas, instead of this
black substance?
MRS. B.
In this, as in most combustions, a considerable part of the carbon is
simply volatilised by the heat, and again obtained concrete on cooling.
This colour, therefore, may be called the soot produced by the burning
of ivory or bone.
CONVERSATION XXIV.
ON THE ANIMAL ECONOMY.
MRS. B.
We have now acquired some idea of the various materials that compose the
animal system; but if you are curious to know in what manner these
substances are formed by the animal organs, from vegetable, as well as
from animal substances, it will be necessary to have some previous
knowledge of the nature and functions of these organs, without which it
is impossible to form any distinct idea of the process of
_animalisation_ and _nutrition_.
CAROLINE.
I do not exactly understand the meaning of the word animalisation?
MRS. B.
Animalisation is the process by which the food is _assimilated_, that is
to say, converted into animal matter; and nutrition is that by which the
food thus assimilated is rendered subservient to the purposes of
nourishing and maintaining the animal system.
EMILY.
This, I am sure, must be the most interesting of all the branches of
chemistry!
CAROLINE.
So I think; particularly as I expect that we shall hear something of the
nature of respiration, and of the circulation of the blood?
MRS. B.
These functions undoubtedly occupy a most important place in the history
of the animal economy. --But I must previously give you a very short
account of the principal organs by which the various operations of the
animal system are performed. These are:
The _Bones_;
_Muscles_,
_Blood vessels_,
_Lymphatic vessels_,
_Glands_, and
_Nerves_.
The _bones_ are the most solid part of the animal frame, and in a great
measure determine its form and dimensions. You recollect, I suppose,
what are the ingredients which enter into their composition?
CAROLINE.
Yes; phosphat of lime, cemented by gelatine.
MRS. B.
During the earliest period of animal life, they consist almost entirely
of gelatinous membrane having the form of the bones, but of a loose
spongy texture, the cells or cavities of which are destined to be filled
with phosphat of lime; it is the gradual acquisition of this salt which
gives to the bones their subsequent hardness and durability. Infants
first receive it from their mother’s milk, and afterwards derive it from
all animal and from most vegetable food, especially farinaceous
substances, such as wheat-flour, which contain it in sensible
quantities. A portion of the phosphat, after the bones of the infant
have been sufficiently expanded and solidified, is deposited in the
teeth, which consist at first only of a gelatinous membrane or case,
fitted for the reception of this salt; and which, after acquiring
hardness within the gum, gradually protrude from it.
CAROLINE.
How very curious this is; and how ingeniously nature has first provided
for the solidification of such bones as are immediately wanted, and
afterwards for the formation of the teeth, which would not only be
useless, but detrimental in infancy!
MRS. B.
In quadrupeds the phosphat of lime is deposited likewise in their horns,
and in the hair or wool with which they are generally clothed.
In birds it serves also to harden the beaks and the quills of their
feathers.
When animals are arrived at a state of maturity, and their bones have
acquired a sufficient degree of solidity, the phosphat of lime which is
taken with the food is seldom assimilated, excepting when the female
nourishes her young; it is then all secreted into the milk, as a
provision for the tender bones of the nursling.
EMILY.
So that whatever becomes superfluous to one being, is immediately wanted
by another; and the child acquires strength precisely by the species of
nourishment which is no longer necessary to the mother. Nature is,
indeed, an admirable economist!
CAROLINE.
Pray, Mrs. B., does not the disease in the bones of children, called the
rickets, proceed from a deficiency of phosphat of lime?
MRS. B.
I have heard that this disease may arise from two causes; it is
sometimes occasioned by the growth of the muscles being too rapid in
proportion to that of the bones. In this case the weight of the flesh is
greater than the bones can support, and presses upon them so as to
produce a swelling of the joints, which is the great indication of the
rickets. The other cause of this disorder is supposed to be an imperfect
digestion and assimilation of the food, attended with an excess of acid,
which counteracts the formation of phosphat of lime. In both instances,
therefore, care should be taken to alter the child’s diet, not merely by
increasing the quantity of aliment containing phosphat of lime, but also
by avoiding all food that is apt to turn acid on the stomach, and to
produce indigestion. But the best preservative against complaints of
this kind is, no doubt, good nursing: when a child has plenty of air and
exercise, the digestion and assimilation will be properly performed, no
acid will be produced to interrupt these functions, and the muscles and
bones will grow together in just proportions.
CAROLINE.
I have often heard the rickets attributed to bad nursing, but I never
could have guessed what connection there was between exercise and the
formation of the bones.
MRS. B.
Exercise is generally beneficial to all the animal functions. If man is
destined to labour for his subsistence, the bread which he earns is
scarcely more essential to his health and preservation than the
exertions by which he obtains it. Those whom the gifts of fortune have
placed above the necessity of bodily labour are compelled to take
exercise in some mode or other, and when they cannot convert it into an
amusement, they must submit to it as a task, or their health will soon
experience the effects of their indolence.
EMILY.
That will never be my case: for exercise, unless it becomes fatigue,
always gives me pleasure; and, so far from being a task, is to me a
source of daily enjoyment. I often think what a blessing it is, that
exercise, which is so conducive to health, should be so delightful;
whilst fatigue, which is rather hurtful, instead of pleasure, occasions
painful sensations. So that fatigue, no doubt, was intended to moderate
our bodily exertions, as satiety puts a limit to our appetites.
MRS. B.
Certainly. --But let us not deviate too far from our subject. --The
bones are connected together by ligaments, which consist of a white
thick flexible substance, adhering to their extremities, so far as to
secure the joints firmly, though without impeding their motion. And the
joints are moreover covered by a solid, smooth, elastic, white
substance, called _cartilage_, the use of which is to allow, by its
smoothness and elasticity, the bones to slide easily over one another,
so that the joints may perform their office without difficulty or
detriment.
Over the bones the _muscles_ are placed; they consist of bundles of
fibres which terminate in a kind of string, or ligament, by which they
are fastened to the bones. The muscles are the organs of motion; by
their power of dilatation and contraction they put into action the
bones, which act as levers, in all the motions of the body, and form the
solid support of its various parts. The muscles are of various degrees
of strength or consistence in different species of animals. The
mammiferous tribe, or those that suckle their young, seem in this
respect to occupy an intermediate place between birds and cold-blooded
animals, such as reptiles and fishes.
EMILY.
The different degrees of firmness and solidity in the muscles of these
several species of animals proceed, I imagine, from the different nature
of the food on which they subsist?
MRS. B.
No; that is not supposed to be the case: for the human species, who are
of the mammiferous tribe, live on more substantial food than birds, and
yet the latter exceed them in muscular strength. We shall hereafter
attempt to account for this difference; but let us now proceed in the
examination of the animal functions.
The next class of organs is that of the _vessels_ of the body, the
office of which is to convey the various fluids throughout the frame.
These vessels are innumerable. The most considerable of them are those
through which the blood circulates, which are of two kinds: the
_arteries_, which convey it from the heart to the extremities of the
body, and the _veins_, which bring it back into the heart.
Besides these, there are a numerous set of small transparent vessels,
destined to absorb and convey different fluids into the blood; they are
generally called the _absorbent_ or _lymphatic_ vessels: but it is to a
portion of them only that the function of conveying into the blood the
fluid called _lymph_ is assigned.
EMILY.
Pray what is the nature of that fluid?
MRS. B.
The nature and use of the lymph have, I believe, never been perfectly
ascertained; but it is supposed to consist of matter that has been
previously animalised, and which, after answering the purpose for which
it was intended, must, in regular rotation, make way for the fresh
supplies produced by nourishment. The lymphatic vessels pump up this
fluid from every part of the system, and convey it into the veins to be
mixed with the blood which runs through them, and which is commonly
called venous blood.
CAROLINE.
But does it not again enter into the animal system through that channel?
MRS. B.
Not entirely; for the venous blood does not return into the circulation
until it has undergone a peculiar change, in which it throws off
whatever is become useless.
Another set of absorbent vessels pump up the _chyle_ from the stomach
and intestines, and convey it, after many circumvolutions, into the
great vein near the heart.
EMILY.
Pray what is chyle?
MRS. B.
It is the substance into which food is converted by digestion.
CAROLINE.
One set of the absorbent vessels, then, is employed in bringing away the
old materials that are no longer fit for use; whilst the other set is
busy in conveying into the blood the new materials that are to replace
them.
EMILY.
What a great variety of ingredients must enter into the composition of
the blood?
MRS. B.
You must observe that there is also a great variety of substances to be
secreted from it. We may compare the blood to a general receptacle or
storehouse for all kinds of commodities, which are afterwards fashioned,
arranged, and disposed of as circumstances require.
There is another set of absorbent vessels in females which is destined
to secrete milk for the nourishment of the young.
EMILY.
Pray is not milk very analogous in its composition to blood; for, since
the nursling derives its nourishment from that source only, it must
contain every principle which the animal system requires?
MRS. B.
Very true. Milk is found, by its analysis, to contain the principal
materials of animal matter, albumen, oil, and phosphat of lime; so that
the suckling has but little trouble to digest and assimilate this
nourishment. But we shall examine the composition of milk more fully
afterwards.
In many parts of the body numbers of small vessels are collected
together in little bundles called _glands_, from a Latin word meaning
acorn, on account of the resemblance which some of them bear in shape to
that fruit. The function of the glands is to _secrete_, or separate
certain matters from the blood.
The secretions are not only mechanical, but chemical separations from
the blood; for the substances thus formed, though contained in the
blood, are not ready combined in that fluid. The secretions are of two
kinds, those which form peculiar animal fluids, as bile, tears, saliva,
&c.; and those which produce the general materials of the animal system,
for the purpose of recruiting and nourishing the several organs of the
body; such as albumen, gelatine, and fibrine; the latter may be
distinguished by the name of _nutritive secretions_.
CAROLINE.
I am quite astonished to hear that all the secretions should be derived
from the blood.
EMILY.
I thought that the bile was produced by the liver?
MRS. B.
So it is; but the liver is nothing more than a very large gland, which
secretes the bile from the blood.
The last of the animal organs which we have mentioned are the _nerves_;
these are the vehicles of sensation, every other part of the body being,
of itself, totally insensible.