to a transparent, amber-colored substance, known as barley-sugar. If the melted sugar be heated to 420°C., it changes to a dark-brown mixture of several different bodies, which is termed caramel, and is much used for coloring sirups and liquors. 12 22 11 528. Lactose (Milk-Sugar), C1H„O,, + H2O, is obtained only from the milk of the mammalia, to which it gives the sweetish taste. It is obtained by evaporating clarified whey till it crystallizes. It is much less soluble, and therefore much less sweet, than cane-sugar, and its crystals are hard and gritty. It is much used in the preparation of homoeopathic medicines. 12 11 529. Gum, C,,H,,O,, (Arabin).—These terms are applied to a class of substances which are often seen exuding in globular masses from the bark of trees, as the plum and cherry. Gum is translucent, tasteless, inodorous, and either dissolves in water, or swells up and forms with it a thick mucilage. It exists in small proportion in the cereal grains, but its chief source is certain tropical trees, from the bark of which it flows in such quantity as to be gathered for commercial purposes. Gum-arabic, the product of a species of acacia, is a hard, brittle substance, and is, perhaps, the best known of the gums. Its solution being very adhesive, is used as a substitute for paste or glue. Mucilage or bassorin is a kind of gum insoluble in water, but which swells into a gelatinous mass when moistened. It abounds in gum-tragacanth, and also in quince-seeds and linseed. Pectin, or the jelly of fruits, is in its physical properties closely allied to the gums, but its exact chemical composition has not yet been established with certainty. Balsams are complex substances which exude from the bark of certain trees; they consist for the most part of an essential oil which holds in solution peculiar substances known as resins. Gum-copal, mastic, and shellac, belong to this class; they are insoluble in water, but dissolve in alcohol, naphtha, and oil of turpentine. The solutions thus formed constitute varnishes. Caoutchouc, or India-rubber, is a compound of hydrogen and carbon, of great value to the chemist. It is the hardened juice of several tropical trees, and when pure is white. Combined with sulphur in variable proportions it forms the vulcanized caoutchouc or vulcanite of commerce. 530. Starch, C.H,,O,. 10 This substance is found univer- which vary in size from 1 to FIG 195. 531. Properties and Uses.-Starci is insoluble in cold water, alcohol, and ether, but swells up and is converted into a paste in water containing 2 per cent. of alkali. If heated in water to 140° F., the grains swell and burst, producing a jelly-like mass (gelatinous starch, or amadin), which is used to impart a gloss to textile fabrics. The test of starch is iodine, which combines with it, forming a blue compound. The uses of starch are varied; the most important, however, is that of nutrition, the comparative value of different articles of vegetable diet depending largely on the proportion of this proximate principle which they contain. When vegetable food is prepared for the table by the various processes of cooking, the starch is slightly modified. 532. Dextrine.-When commercial starch is heated under pressure to 400° F. for some hours, it becomes soluble in cold water, and is changed into a gummy substance called dextrine, which, under the name of British gum, has been successfully substituted for gum-arabic by calicoprinters in thickening their colors. Dextrine is also produced when starch-paste is boiled for a few minutes with weak sulphuric acid. It is a transparent, brittle solid, isomeric with starch, soluble in water, incapable of fermentation, and produces right-handed rotation in a ray of polarized light; hence its name. When solutions of dextrine are boiled with dilute acids for some hours, the dextrine is converted into glucose. Dextrine is also formed from starch, by the action of animal secretions such as saliva, bile, and pancreatic juice. 10 Glycogen, CH,O,, was first obtained by Bernard from the livers of animals. It is a white, amorphous, starch-like substance, odorless and tasteless, and is converted into glucose by boiling with dilute acids, or by contact with blood, saliva, or pancreatic juice. Inulin is a substance obtained from the roots of many plants, among which are the dahlia, dandelion, and chiccory. It has the same composition as common starch, but differs from it in some important particulars. Inulin may be obtained by washing the rasped roots on a sieve, when it will settle to the bottom of the liquid. It is a white, tasteless substance, which is not soluble in cold water, but freely dissolves by the aid of heat. It exists in the plant in a liquid form, and is converted by the action of dilute acids into lævulose. Iodine does not turn inulin blue. 181 30 15 533. Cellulose, C1,H,,O1s (Woody Fibre, Lignine).—This is the most abundant product of vegetation. Besides forming the chief bulk of all trees, it exists in the straw and stalks of grain, in the membrane which envelops the kernel (bran), in the husk and skin of seeds, and in the rinds, cores, and stones of fruit. Wood consists of slender fibres, or tubes closely packed together. When first formed these tubes are hollow and serve to convey the sap, but in the heart-wood of trees they become filled up and consolidated, the circulation of fluids taking place in the white external sap-wood (alburnum). Upon the density with which the fibres are imbedded depends the property of hardness or softness of wood. Cellulose is the fibrous portion of the woody tissue. 534. Properties and Uses.-The properties of cellulose may be conveniently studied in fine linen and cotton, which are almost entirely composed of it. When pure it is tasteless, insoluble in water and alcohol, and not sensibly affected by boiling water. By cold concentrated sulphuric acid, it is converted into dextrine. The uses of cellulose are almost numberless. It forms the chief bulk of the wood we burn, of the linen and cotton fabrics we wear, and of the paper we write and print upon. Besides these uses of unaltered cellulose, a multitude of useful chemical bodies are derived from its decomposition; charcoal, illuminating gas, tar, wood-spirit, wood-vinegar, creosote, and oxalic acid, being among the most important. Grape-sugar and alcohol have also been made from it. 535. Pyroxylene.-If pure cellulose (C,,H,,O,,), as cot ton, linen, sawdust, or paper, be steeped for a few minutes in a mixture of nitric and sulphuric acids, then squeezed, washed, and dried by gentle heat, a part of the hydrogen of the cellulose is oxidized by the action of the nitric acid, and eliminated as water, while its place in the compound is occupied by a corresponding quantity of the radicle nitryl (NO). This quantity is greater in proportion to the con 18 21 centration of the acid. When a mixture of concentrated sulphuric acid with nitric acid of 1.52 specific gravity is used, pyroxyline, C,,H,, (N,O,), O,,, a highly-explosive compound, is produced, which constitutes the well-known guncotton, discovered a few years ago by Prof. Schönbein. It ignites at 400° F. (200° below gunpowder), and disappears in an instantaneous flash, leaving hardly a trace of residue. Authorities vary in estimating its explosive force, but the latest make it about three times that of gunpowder. The extreme suddenness of the propulsive force overstrains the gun and produces less effect upon the ball than gunpowder. Collodion is formed by dissolving gun-cotton in ether, containing a small proportion of alcohol. On evaporating the ether, a transparent, adhesive film is left, which is insoluble in water and is used in surgery for protecting wounds from the air. The chief use of collodion, however, is in photography. §4. Fermentation. 536. When certain compound substances, derived chiefly from plants and animals, are exposed to the action of air and water, at a given temperature, they undergo decomposition, which, when involving the formation of useful products, is generally known as fermentation, but when resulting in the production of useless and ill-flavored bodies is distinguished as putrefaction. These changes all agree in having a peculiar self-sustaining and contagion-like character. The true nature of these processes is not yet thoroughly understood. But careful investigation has shown |