for a few minutes (about five) in a mixture of one volume of concentrated sulphuric acid and two volumes of concentrated nitric acid, washed with water till of neutral reaction, and dried at the ordinary temperature. The explosive force of gun cotton is about four times that of an equal quantity of gunpowder.

If, instead of cold, a warm nitration mixture be employed, or if a large quantity of sulphuric acid be added to the first, and the cotton left in contact with it for some time, the resulting nitrate is then soluble in a mixture of one part of alcohol and four parts of ether to a syrupy liquid, 'collodion.' On evaporation in air the nitro-cellulose is left as a transparent, waterproof membrane. Collodion, therefore, finds much use in surgery and in the preparation of photographic negatives on glass.

If finely divided cellulose be heated to 180° in a sealed glass tube with six to eight times its weight of acetic anhydride, a syrupy solution of triacetyl cellulose in acetic acid is obtained, from which the compound [CH,O2(O.C2H2O)3], can be separated by water in the form of white flocks insoluble in alcohol or ether.

885. Tunicine, or animal cellulose, is isomeric with cellulose, and closely resembles it in chemical character. It is obtained from the mantles of ascidians and cyntheans by long boiling with hydrochloric acid and sodic hydrate, followed by washing with water, alcohol, and ether. It is not attacked by dilute sulphuric acid even on long boiling, but gives dextrose by dissolving in cold concentrated sulphuric acid and pouring into water.

Starch, or Amylum, (C6H1005)3 (?).

886. Starch occurs most widely disseminated in the vegetable kingdom, always in the form of microscopic granules enclosed in the vegetable cells. It occurs in seeds (corn, chestnuts) in the trunk, especially in many palms, in roots (of Jatropha Manihot as tapioca, of Maranta arundinacea as arrowroot in commerce), in tubers (potato), &c. To prepare it the vegetable is finely ground, so as to tear the cell membranes, the starch washed with water, and the milky liquid, after passing through a fine sieve, allowed to stand quietly for some time, when the starch settles to the bottom of the vessel.

From the meal of grain, especially of wheat, it is best obtained by allowing the meal to remain for a long time in contact with water, when the nitrogenous constituents (gluten, &c.) enter into putrefaction and go in great part into solution, whilst the starch remains unchanged and can be purified by washing, &c. It is finally dried in air at the ordinary temperature.

Starch granules possess very different sizes (e.g. 185 mm. long in potato starch and 045 mm. in wheat starch), and, as a rule, show under the microscope a series of concentric markings, of which the innermost nucleus (the hilium) appears to lie at one side. In polarised light starch granules (like the sections of many crystals) show a dark cross, whose point of intersection agrees with the hilium.

The integument of the starch granule, and also of the small layers, appears to consist of a very brittle layer of cellulose.

In mass starch forms a glittering white powder, insoluble in cold water, and from which alcohol and ether remove minute quantities of

wax-like and fatty substances. In water heated to more than 60° the contents of the granules swell, break the integument, and soon diffusing through the whole of the hot liquid, make the mass appear like a solution. On cooling, if too much water has not been employed, it becomes a transparent jelly, termed starch paste, and dries to a hard transparent mass.

The most characteristic reaction for starch, both as granules and as paste, is the deep blue colour which it gives with iodine. The combination between the two is very feeble; it decomposes with loss of colour on heating, but the colour returns on cooling.

On mixing boiling solutions of starch with lime or baryta water, or with basic plumbic acetate, white precipitates are obtained of metallic derivatives of starch.

On long boiling starch paste loses the power of gelatinising on cooling, having been converted into a modification soluble also in cold water, which is strongly dextrorotary, is precipitated by alcohol, and gives a blue coloration with iodine.

The same conversion into soluble starch is effected by long heating to 100° in the dry state, or by gentle heating with solutions of diastase (the ferment of germinating grain), zincic chloride, or dilute sulphuric acid, but soon passes first into maltose and dextrine, and finally into dextrose (comp. § 870). If dry starch be heated to 200° it is converted into dextrin.

On boiling with nitric acid starch yields the same oxidation products as dextrose.

Starch dissolves readily in cold concentrated nitric acid; on diluting largely with water a white powder, xyloidine, separates. This is a nitrate of the formula C,H,(NO2)05 or [CH,O,(O.NO2)(OH)2]x which, after washing and drying, inflames at 180° and burns briskly, leaving some charcoal. If the nitric acid solution be mixed with sulphuric acid, there is then obtained, on dilution with water, a similar white powder of a higher nitrate, which has approximately the formula C6H8(NO2)2O5 or [CH2O2(O.NO2)2OH]、.

By heating starch to 140° with much acetic anhydride triacetyl amylum, [CH7O2(O.C2H2O),], is obtained as an amorphous mass, insoluble in water, alcohol, and acetic acid; with dilute alkalies it gives starch and acetates.

Paramylum occurs in the green infusoria Euglena viridis in granules, the mass closely resembling starch; it is insoluble in cold water and dilute acids, swells up in hot water, and is converted into a fermentable sugar by hot hydrochloric acid.

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887. Lichenine, or moss starch, xC6H1005, occurs in many lichens, especially in Iceland moss. Finely divided Iceland moss is washed with alcohol, ether, weak solution of sodic hydrate, dilute hydrochloric acid, and finally with water, and the lichenine then extracted with boiling water. The clear hot solution becomes a colourless jelly or cooling, which after drying forms a hard transparent mass. Iodine gives with lichenine a yellow or brown colour, frequently, however, probably from the presence of some starch, a green or faint blue colour. On boiling with dilute acid lichenine is converted into a fermentable sugar.

Inuline, xC6H1005, occurs in the roots of many composita-e.g. in

Inula Helenium, Helianthus tuberosus, Leontodon Taraxacum, and especially in the tubercules of the dahlia. It is obtained either like starch or by boiling with water, from which it precipitates on cooling as a white powder. It is nearly insoluble in cold water, but is largely dissolved on boiling. Its solution is lævorotary, and on inversion with dilute sulphuric acid gives lævulose. It is coloured yellow by iodine.

888. Glycogen, xC6H10O5, occurs in the animal liver in largest quantity during and shortly after digestion, and has also been recognised in the foetus during the first month of its existence, in yolk of eggs, and in many mollusca. It vanishes from the liver with extraordinary rapidity after death, being converted into sugar by the ferment of that organ.

To prepare it animals are killed quickly whilst in the full act of digestion, the liver removed, coarsely divided, rubbed to a pulp with sand heated to 100°, and then extracted several times with hot boiling water. The filtered opalescent solution is treated with alcohol, which precipitates impure glycogen; the precipitate is boiled with concentrated potassic hydrate as long as ammonia is evolved, in order to destroy admixed albuminoids, and after large dilution again precipitated with alcohol. The glycogen is then repeatedly dissolved in acetic acid and precipitated with alcohol, finally washed with absolute ether, and dried in vacuo.

It is a white amorphous powder, destitute of smell or taste, which gives an opalescent solution with water, and is coloured wine red by iodine. It is very readily converted into dextrose by dilute acids, &c.

Dextrin, xC6H1005.

889. As already mentioned, starch is converted into dextrin by the action of dilute acids, of diastase, or by heating to 200°. In preparing it commercially the starch is moistened with water containing about 2% of nitric acid, allowed to dry in air, and the conversion finally effected by heating to 110°.

The crude dextrin so prepared is a clear yellow powder, from which the pure compound can be obtained by solution in water and precipitation of the filtrate by alcohol. It dissolves largely in water to a gummy, slimy mass, which leaves a transparent, gum-like mass on evaporation and drying. The aqueous solution is strongly dextrorotary ([a] + 138.5°), is not blued by iodine, does not reduce alkaline cupric solutions even on boiling, and is finally completely converted into grape sugar by dilute acids and diastase.

=

On heating dextrin with an excess of acetic anhydride it yields triacetyl dextrin, xCH,(CH30)305, which can also be obtained from the isomeric triacetyl amylum at 160°. It is amorphous, insoluble in water and alcohol, but soluble in acetic acid.

Gums and Mucilages, xC6H1005.

890. Under these names a number of different but closely related bodies are collected, which are widely disseminated in plants; they are partly readily soluble in water (gums), partly only swell up therein. (mucilage). The gums occur dissolved in the juices of plants, but exude

on rupture of the cells, and harden in the air to a transparent mass of vitreous fracture. The natural gums and mucilages are invariably metallic derivatives of peculiar organic compounds, and therefore leave a relatively large quantity of ash, consisting of the carbonates of potassium, calcium, and magnesium.

Arabin is the organic constituent of gum arabic, an exudation on different tropical acacias. In these it occurs mainly as the calcic compound. It is obtained from gum arabic by solution in water, acidulation with hydrochloric or acetic acids, and addition of alcohol, when it precipitates in flocks. By frequent repetition of this process it is obtained free from metal, and can no longer be precipitated from its aqueous solution by addition of alcohol, but again acquires that property on addition of small quantities of basic hydrates or inorganic salts.

After drying at 100° it is a glassy mass of conchoidal fracture and a composition corresponding to the formula:

C12H22O11 or C12H20010,H2O,

which decomposes carbonates, yielding compounds containing 1 equivalent of metal to 18 atoms of carbon. Probably, therefore, the molecular formula of arabin dried at 100° should be C36H66033.

At 120°-130° arabin loses weight until it has the composition nC6H1005 (C36H60030 or C18H30015?). It is then insoluble in water, but swells up enormously in it (metarabin). By addition of potassic hydrate solution or lime water it forms the metallic derivatives of ordinary arabin.

Compounds of arabin with potassium and calcium form the main portion of cherry and plum tree gums, and occur frequently in other plants, e.g. sugar beet.

All solutions of arabin and its metallic derivatives rotate the plane of polarisation, but to very different degrees, and are in part dextro-, in part lævo-rotary. By boiling with dilute sulphuric acid they are all without exception converted into a dextrorotary mixture of partly fermentable sugars, in which arabinose (§ 294) always occurs.

Bassorin. This name was first given to the chief constituent of Bassora gum, which is obtained from a species of cactus. This drug is insoluble in water, but swells up therein to a slimy jelly. Later the name was extended to all vegetable substances of similar nature, which in their totality are termed vegetable mucilages. These occur in the roots of different orchis (salap), in the roots of althæa, quince kernels, Semen Psylii, Carraghen moss, &c., as a rule mixed with gum. Mucilages also occur in cherry and plum tree gums, and in traganth, the hardened juice of some kinds of Astragalus.

Gum or bassorin-like bodies are formed, together with butyric acid and mannite, in the mucous fermentation of sugar.

On heating with nitric acid, gums and mucilage yield much mucic acid and also saccharic acid; as products of further oxidation, tartaric and oxalic acids.

Pectous Substances.

891. Under this name are included a series of bodies which are most widely disseminated in plants, especially in fleshy fruits and

roots, which further stand in very clear relation to the carbo-hydrates, and always occur together with gum and mucilage. The chemistry of these compounds is still very obscure.

A completely insoluble body, pectose, occurs in unripe fleshy fruits and roots, and is converted on ripening or by long boiling into bodies dissolving in water, or in part apparently dissolving and gelatinising on cooling. The products distinguished as pectine, parapectine, metapectine, pectoic acid, pectic acid, and parapectic acid are not sufficiently individualised to require further notice here. The end product of all these metamorphoses, the body termed metapectic acid, has lately been shown to be slightly impure arabine.

PYRIDINE BASES, CnH2n-5N.

892. The group of pyridine bases appears also to belong to the derivatives of the heptavalent hydrocarbon nucleus, CnH2n – 5 They were first obtained in inconsiderable quantity from the oily products of the dry distillation of nitrogenous organic matter, such as turf, coal, and bones. From bone oil they were obtained by shaking with dilute hydrochloric acid, when they go into aqueous solution as hydrochlorides, from which they are separated, together with alkylamines, such as trimethylamine, butylamine, and amylamine, on addition of alkali. The separation of the single homologues from one another is very difficult and tedious. Latterly several of them have been obtained from well-known compounds of other groups.

The structure of these compounds is yet but little known; they appear, however, to belong to the nitril bases, as ethylic iodide unites with them, giving iodides of ammonium radicals, from which the base cannot be separated by alkali, whilst argentic oxides convert the iodide into strongly alkaline bases, corresponding to the tetralkylammonic hydrates.

Pyridine, CH,N, is obtained in small quantity amongst the products of the action of phosphoric anhydride on isoamylic nitrate :

CH11.O.NO2 + 3P2O5 = 6HPO3 + C¿H¿N.

It is a colourless liquid of sharp, nauseous odour, boiling at 116.5°, and has sp. gr. ·986 at 0°. It yields a deliquescent salt with hydrochloric acid of the formula C,H,N.HCl, whose yellow platinochloride, (C,H,N.HCI),PtCl,, is difficultly soluble in water.

5

On long treatment with metallic sodium, especially on heating, it polymerises to solid dipyridine, melting at 108°, crystallising in needles, and is sublimable.

Picoline, CH,N, can be obtained by the dry distillation of acroleïn ammonia (§ 763):

and by heating glyceryl tribromide (§ 681) to 250° with alcoholic ammonia. It is a liquid, is strongly basic and closely resembles pyridine, is soluble in water, boils at 135°, has sp. gr. 961 at 0°, and is polymerised by sodium.

Lutidine, C,H,N, has only been obtained from tar oil. It boils at about 155° and has sp. gr. 946 at 0°.

Collidine, CH,,N, is formed from aldehyde ammonia by dry dis