The Liverpool Standard and General Commercial Advertiser.
Liverpool, Merseyside, England, 1839-03-22 00:00:00

LIVERPOOL MECHANICS' INSTITUTION

LECTURES ON ORGANIC CHEMISTRY.

On Saturday evening, R. H. Brett, Esq., delivered the
third of his interesting lectures to a full audience.
In the last lecture I concluded the consideration of those
substances which belong to the 'acid and basic classes of vege-
table proximate principles. lam now about to consider the
next division of our subject. namely, those vegetable princi-
ples which, possessing neither an acid nor basic property,
come under the denomination of neutral substances. The
vegetable principles occur very abundantly in all plants, and;
like the acid and basic substances to which
.I before alluded,
are constituted for the most part of carbon, °Viten, and hy-
drogen. They may be considered as essentially consisting of
starch, sugar, and gum, and these three principles are cha-
racteris'ed by their possessing no properties which belong
either to acids or to bases. Besides these substances which
occur naturally in plants, we have other neutral bodies of
artificial production, and hence we naturally divide the neu-
tral bodies of vegetable origin into those which are natural
and those which are artificial ; in other words, into those
which occur ready formed in plants and those which we forth
by the action of certain substances upon organic principles.
Amongst those which occur ready formed, as i have before
stated, starch, sugar, and gum stand in the first rank. The
first of these three substances is one 'of the most abundant
proximate principles met with hi vegetable substances. It
occurs in almost every trait of the plant ; we,meet with it in
the root, the stem, 'and we also meet with it in the fruit.
All those plants which belong to the natural order of the
grasses, contain in the fruit a very large quantity of starch.
All the esculent varieties contain a very large quantity of it.
It is met with in many of the plants belonging to the palm
tribe ; and in many of the tuberose roots starch is also abun-
dant. The most fruitful source of this substance, however, ,
is wheat ; a very large portion being obtained also from the
ordinary potato. The mode in which it is separated from
wheat is somewhat complicated. It is found associated in
wheat with another proximate principle eallod gluten. To
separate it from this substance, it 18 macerated in water,
after the grain has been bruised. A peculiar process or spe-
cies of fermentation take§ place, by which the gluten under- -
goes solution, in consequence of the formation of acid, in
which it is soluble, and is washed out with water. By the
same mode starch may be obtained from the ordinary potato.
This substance, as met with in commerce, is formed in co-
lumnar masses, and instead of possessing the siriking white
character which belongs to pure starch, has more or less of
a blue tinge, in consequence of its being coloured with the
blue colouring matter called smelt. Starch, when perfectly
pure, is, as I observed, white. It is insoluble in cold
water and in spirits of wine, but it is soluble in hot water.
In this it differs from gum, which is soluble in cold water.
One of the most striking properties belonging to starch is
the peculiar blue colour which it is capable of producing,
when brought into contact with the simple substance iodine.
If we take a solution of starch, and add to it some of the
simple element called iodine, we shall obtain a deep blue co-
lour. This peculiar action of iodine upon starch is sufficient
to distinguish starch from any other proximate principle of
vegetable or animal origin. lodine combines with starch,
and forms what is called an iodide of starch, which substance
possesses somewhat peculiar properties. When exposed for
any very great length of time to the atmosphere the blue co-
lour is entirely discharged, and a white or whitish brown re-
sidue is obtained. This change is not owing to the mere loss
of its iodine, because we find upon the addition of an
acid to the white or whitish brown substance that the blue
colour is restored. And it is found that the reason why the
starch loses its blue colour is, because the iodide of ammonia
is formed, which is not capable of producing a blue colour with
starch. It may be stated, that when iodine is combined with
any alkali or base, it is incapable of forming a blue colour
with the proximate principle of starch. Here we have the
iodide of potassium, which is incapable of producing a blue
colour with starch, but if I can set the potassium free, the
iodine will unite with the starch. This characteristic is very
important to the detection of starch. We shall find upon
the application of a moderate heat to this flask that a gas
will be evolved, which will cause the liquid in the jar to be-
come blue. The rationale of this experiment is the follow-
ing. In the flask I have the ingredients for generating chlo-
rine. Chlorine, as you are aware, is a simple substance of a
gaseous nature, which has a very strong affinity for the sub-
stance:with which the iodine was combined in the jar, namely,
the potassium. It unites with it, and sets the iodine free.
The iodine being released, immediately acts upon the starch
and renders it blue. This may be considered as the most
striking chemical characteristic belonging to starch, but there
are several physical circumstances connected with it which it
will be necessary to mention. Starch is not a simple homo-
geneous substance like sugar, buf constituted of a number of
small vesicles, which, when examined under the microscope,
assume the appearance of small ovoid masses. They are
made up of integumentous substances, not soluble in water,
which contain the true starch or amidon, and it is this latter
principle which causes the blue colour with iodine.. Starch
suffers a peculiar change when subjected to the action of
heat. If a portion of ordinary starch be roasted, it is con-
verted into a substance resembling gum. It is no longer
rendered blue by iodine, and becomes soluble in cold water.
It loses the characteristics of starch and puts on those of
gum. It is this substance which is called British gum, and it
is obtained the application of heat to starch.
The next substance which I shall notice is gum. This
vegetable substance occurs in great abundance in all plants,
particularly in those belonging to the acacia tribe, and it is
principally from these tribes that it is obtained for the pur-
poses of the arts and manufactures. These substances differ
materially from starch, in being soluble in cold water. It is
not rendered' blue by iodine, and it moreover differs very
much from starch, from the circumstance, that when acted
upon by nitric acid it is converted into mucic acid, and is en-
tirely insoluble in water, whereas, when starch is acted upon
by the same acid, it is very soluble in water. Gum
being soluble in cold water and insoluble in alcohol, is capa-
ble of uniting with earths and metallic oxides, and forming
certain chemical compounds. If, for example, I add to a
solution of gum in water a solution of acetate of lead, we
shall find that the gum will unite with the oxide of lead, and
fall as an insoluble substance. This precipitate is termed by
some chemists the gummate of lead, and is a compound of
gum with oxide of lead in definite proportions. The most
striking combination of gum with a metallic oxide, is that
which we meet with in the oxide of iron and gum. If to a
very saturated solution of gum in water, we add a little of
the solution of the sulphate of iron, we shall find that the
oxide of iron will combine with the fluid mass and convert it
into a solid glutinous , substance, which is called the gum-
mate of iron. This is an exceeding characteristic distinction
between gum and any other vegetable proximate principle.
The next substance which will come under our notice,
namely sugar, is one of very considerable interest. It occurs
in great abundance in a variety of plants. It is extracted,
for the most part, from what is termed the arundo saccha-
rifera, from which the sugar of commerce is obtained. It is
also extracted from the beet root in an extremely white state.
When sugar is properly purified, it is perfectly white. It is
frequently met with in combination with vegetable mucilages,
constituting the soft form of sugar, and it is only by manu-
facturing processes that this principle can be obtained per-
fectly white and pure. The mode in which ordinary cane
sugar is rendered perfectly pure is sufficiently simple in a
theoretical point of view. It consists in separating the brown
substances, with which the sugar is associated, by means of
charcoal, coagulating certain vegetable principles of a muci-
laginous nature by means of albumen, and afterwards clari-
fying'the substance by chalk. In this manner we obtain the
pure white crystalline sugar, and the more crystalline the
sugar is met with, the purer it may be considered. Sugar,
although occurring naturally in plants, is capable of being
produced artificially, and some of the most striking experi-
ments which have ever been made in organic chemistry, have
arisen from the powerful action of certain substances upon
starch, in the production of sugar.' It is a matter of extreme
interest and importance in chemical science, that certain
proximate principles, differing in their chemical constitution,
and more especially in their physical characters, are capable
of being converted, by the action of certain substances, into
sugar. If starch be submitted to the action of sulphuric
acid, it will be converted entirely into sugar. This extraor-
dinary fact was first observed by a Russian chemist. He
was the first to discover that when starch was boiled with
water and very diluted sulphuric acid, that it was converted
into sugar. All the forms of starch with which we are ac-
quainted suffer this change by the action of sulphuric acid.
Common arrowroot is converted into sugar by the same
means, as well as common sago, both of these substances
being almost entirely made up of starch. Ligneous fibre,
such as we recognise in ordinary wood, is converted into
sugar by the action of sulphuric acid. Common paper and
rags, the essential element of which is ligneous fibre, become
converted into sugar from the action of sulphuric acid. But
there occurs in certain plants a substance, to which belongs
the very peculiar property of converting starch into sugar.
Barley, which has been suffered to germinate, contains a por-
tion of this substance, which is of an organic nature. If we
suffer barley to germinate, so as to cause the developement
of the plumula, which ultimately becomes the plant, and
then stop the process of vegetation, we shall find that the
starch in the plant has been converted into sugar, by a cer-
tain principle called diastase, which has the property of con-
verting starch into sugar, to such an extent, that the one-
thousandth portion of it, mingled with starch, has the power
of converting it into sugar. And this is one of the Most im-
portant facts in organic chemistry, because it has the effect
of explaining certain circumstances which would otherwise
appear ery extraordinary. It explains the conversion of
starch into sugar, and sugar into gum, because the agent
which is capable of effecting the one is also capable of pro-
ducing the other. Starch is composed of 12 carbon, 10 hy-
drogen, and 10 oxygen. Gum differs from starch only in
containing one proportion more of water. Sugar, perfectly
free from water, has precisely the same chemical constitution
as starch, and therefore may be considered as isomeric, being
a substance containing the same elements, in the same pro-
portions, as starch, but differing materially in its physical
properties. Crystalline sugar and gum are isomeric. Grape
sugar, which is that form of sugar into which starch is con-
verted by the action of sulphuric acid or diastase, differs from
crystalline sugar in containing three more proportions of
water. We, therefore, can conceive how starch is converted
into grape sugar, as we find that the sulphuric acid, in acting
upon the starch, has undergone no change whatever, and may
be recovered. The only substance acted upon by the starch
has been the water. Sugar, like gum, possesses certain pe-
culiar properties in its relation to alkalies and metallic oxides.
If we mix a solution of sugar and a salt of copper, we shall
find that the sugar will combine with the oxide of copper,
and prevent it from being thrown down, forming a soluble
compound. If to an ordinary solution of the sulphate of
copper I add a little potass, we shall find that the oxide of
copper will be thrown down in the form of a blue bulky de-
posit. If, on the other hand, I mix a salt of copper with
sugar, we shall find that the same alkaline agent will
be incapable of throwing down the same amount of pre-
cipitate. This is entirely owing to the circumstance that
the sugar combines with the oxide of copper, and forms a
combination, which, being soluble, remains diffused through
the fluid. ugar is capable of uniting with oxide of lead,
and the sugar is found under these circumstances to be en-
tirely free from water. These are the only means we possess
of freeing sugar from water. When combined with lead or
sugar, it becomes anlydrous.
Having thus briefly considered these three different princi-
ples, starch, gum, and sugar, I shall now direct your atten-
tion to a substance of more interest, namely, alcohol, or
spirits of wine. This substance comes 'under the class of
artificial productions, seeing that it is not met with in a
natural state. Alcohol forms a very important feature in
organic chemistry, not only in consequence of its peculiar
properties, but in consequence of the many combinations it
forms with other substances, constituting the neutral class of
bodies called ethers. We find it in almost all of those fluids
which are recognised as spirituous. Alcohol occurs in malt
liquors as well as in the vinous. It is produced by what is
termed the vinous fermentation by a peculiar chemical
change which sugar undergoes under certain circumstances.
There are certain conditions necessary for establishing what
is termed the vinous fermentation. These are the presence
of sugar, water, and the existence of a certain temperature
varying between 50 degrees and 70 degrees Fahrenheit. But
these conditions being observed, it is necessary that a sub-
stance called ferment should be present. This substance
causes the elements, entering into the composition of sugar,
to be re-arranged, and the consequence of their re-arrange-
ment is the production of alcohol. During the time this
fermentation is going on, a quantity of gas is evolved, which
is carbonic acid, and that such is the case may be very readily
proved by collecting the gas produced in the alcoholic or
vinous fermentation, when, by causing it to pass through
lime-water, we obtain an insoluble precipitate of carbonate
of lime. Sugar is composed of twelve carbon, ten hydrogen,
and ten oxygen, equal to 160 parts. We shall find that if
we separate from one portion of sugar, four of carbonic
acid, we shall have upon the condition of two proportions of
water, the elements in the proper propoitions to form four
proportions of alcohol. Alcohol is a combination of nothing
more than (defiant gas and water. Olefiaiit gas is constituted