ORGANIC CHEMISTRY.

The chief constituents of organic compounds are hydrogen, oxygen, carbon, and nitrogen, occasionally associated with sulphur and phosphorus. These elements in infinitely varied proportions combine with one another and form innumerable compounds; these compounds unite and form new combinations according to the laws of definite multiple and equivalent proportions.

Complex organic bodies are of a very unstable character; by a slight increase of temperature the carbon combines with the oxygen to form carbonic acid, and the hydrogen with the nitrogen to form ammonia, and a high temperature is attended with the destruction of organic compounds. This unstable character decreases with organic bodies of simpler composition till we arrive at compounds of considerable permanence.

TYPES AND THE LAW OF SUBSTITUTION.

Most elementary and compound bodies may be referred to one or other of several different groups or classes, each class being composed of members possessing a similar constitution, and formed according to a certain pattern or type. Thus we may consider NaCl, HCl, HI, Fei, &c. to belong to the same class or group, the simplest member of which is perhaps HCl, which may therefore be taken as the type of this particular class of bodies.

There are four types recognized by chemists, viz. :-the hydrogen type, the hydrochloric acid type, the water type; and the ammonia type.

To the first class belong the metals and metalloids, and compound bodies acting as elements, as cyanogen (CN), Ethyl (CH), Phenyl (CH), &c.

To the second, or hydrochloric acid type, belong the hydrogen and metallic compounds of the chlorine group, and other bodies having a similar constitution.

The third, or water type, comprises the metallic oxides, the oxygen acids, anhydrides, oxysalts, alcohols, ethers, &c.

The fourth class includes the metallic derivatives of ammonia, amines, urea, &c.

Common salt differs from HCl merely in containing Na instead of H, in combination with Cl. If we act upon Na with HCl, the H is displaced, and the Na substituted for it, the result being common salt, NaCl. All cases of decomposition are merely examples of this displacement of one body, and the substitution for it, of another body.

But, besides this kind of substitution, there is another which is more remarkable. It is found that a great number of organic compounds containing H, may have that H displaced by Cl, Br, I, without suffering any great change in their general chemical characters. By this means one, two, three equivalents of H may be displaced by one, two, three equivalents of CI, I, or Br; and even all the H may thus be removed. For example: aniline, CH,N, may have one equivalent of Cl substituted for one of H, the resulting body chloraniline, C,,H,CIN, very much resembling aniline itself. Dutch liquid, C,H,Cl,, may have its H successively displaced by Cl, yielding the following compounds: CH.C., C‚Í‚C, CHCI, CC. Many bodies allow of their H being displaced by NO, as nitronaphthaline, CH,NO; nitrobenzole, C,H,NO, binitronaphthaline, CH(NO), &c. By this means many new bodies have of late years been discovered by chemists, some of them possessing very interesting properties.

ISOMERIC BODIES.

In organic compounds we have frequent illustration of bodies having the same chemical composition, but differing in properties; and, during the past few years, the number of these isomeric bodies has been greatly increased.

The difference of properties is ascribed to the differing arrangement of the elements. Gum (C12HO11), and cane sugar (CHO); starch (C,H10O10), and cellulose (CH10 O10), are examples of isomerism; acetate of methyle and formic ether are isomeric. The former compound is thought to be made up of acetic acid and wood-spirit (CH.0 ̧ + CH2O CH2O.). While the latter compound is similarly thought to be made up of formic acid and ether, C2HO ̧ + CH,O = CH.O

=

NON-AZOTIZED BODIES.

Starch, CH10O10.-This substance is present in almost all vegetables; it is most abundant in the cereals, peas, beans, lentils, turnips, carrots, and potatoes. Tapioca, sago, and arrowroot are varieties of starch extensively used as articles of diet. Starch.can be obtained by kneading under a stream of water on a sieve the pulp of potatoes. The starch passes through with the water and settles at the bottom of the vessel in the form of a white insoluble powder. When this process is applied to the meal of any of the cereals a tough, elastic substance remains, which is gluten, and the value of wheat and other grain as articles of food depends greatly on the quantity of gluten. Pure flour contains about ten per cent., and bran contains about fifteen per cent. When starch is mixed with water and heated to the boiling point, the granules burst and form a gelatinous mass, which when dried in thin layers forms a yellow horny substance, which again dissolves in boiling water. This is called amidin.

Iodine gives with very weak solutions of starch a deep characteristic blue colour. Under the microscope starch granules vary in form according to the vegetable substance from which they were obtained.

If a solution of starch be boiled with dilute sulphuric acid it is converted into dextrin, a substance identical in composition with starch; and if the boiling be continued for some time the dextrin is converted into glucose or grape sugar.

Cellulose.-True cellulose is the material which encrusts the interior of the cells of vegetable substances; it has the same composition as starch.

Lignin is the basis of all woody fibre, and one of its purest forms is well-bleached flax.

Of the several varieties of sugar the most important are cane sugar, glucose or grape sugar, diabetic sugar, sugar of milk, and amorphous sugar.

Cane Sugar, CHO-This is obtained from the juice of the sugar-cane, and on the Continent it is largely manufactured from beetroot; and in the western parts of America it is obtained from the sap of the sugar maple. It is easily crystallized, and is very soluble in hot water, but insoluble in alcohol and ether. When heated it melts into a black syrup, which solidifies on cooling into an amorphous mass known as barley sugar. When cane sugar is boiled with dilute sulphuric acid it is converted into grape sugar.

Sugar, as an article of diet, is not nutritious; its chief value depends on the support afforded by the carbon in keeping up the animal heat by respiration.

Caramel, a dark brown substance, obtained from cane sugar, is used for colouring spirits. It is C12H,O,.

Grape Sugar, Glucose, Diabetic Sugar, CHO-This is the sweet principle of grapes, raisins, and most fruits, and a product, as we have seen, of the metamorphosis of starch and cane sugar. It may also be obtained from milk sugar

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and woody fibre when either of these bodies is boiled for some time with dilute sulphuric acid; the solution is then neutralized with chalk, filtered, and evaporated. The theory of the change is, that starch, CH,O,o, takes up four equivalents of water, and produces CHO, or glucose. By varying the quantity of water the same theory may be applied to the other bodies. Glucose is less soluble in water than cane sugar, and not so sweet. It is found in considerable quantities in the urine of persons afflicted with diabetes.

Sugar of Milk, CHO-This is obtained by evaporating the whey of milk till it crystallizes. When pure it forms hard white crystals, soluble in water, but insoluble in ether or alcohol. The Tartars make an intoxicating liquor by the fermentation of mares' milk. Sugar of milk is used in preparing the little globules of homoeopathic medicines.

Amorphous Sugar, CHO-This may be obtained from the juices of vegetables and honey.

In the bodies previously described we have several cases of isomerism, and the oxygen and hydrogen usually exist in the proportions necessary to form water. The chief value of these substances as articles of food depends on their supplying those elements of respiration by which the animal heat is maintained.

AZOTIZED BODIES.

Animal Albumen.-The serum of the blood and the white of egg are strong solutions of animal albumen. In its natural state it is soluble in cold water, and is easily coagulated by heat; it is also coagulated by acids, the ferrocyanide of potassium and several metallic salts. Albumen in its purest form always contains small quantities of phosphorus, sulphur, and alkaline phosphates. When dried it becomes a yellow, transparent, horny, and brittle