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percussion ; (2) the possibility of its spontaneous decomposition when kept for a length of time.

A variety of gun-cotton, containing a smaller proportion of NO, dissolves readily in a mixture of ether and alcohol, and yields a solution termed collodion. This is largely used for the purpose of forming a thin coating on glass to receive silver salts, upon which the photographic image is formed.

164. Vegetable Parchment. — When paper is immersed for a short time in a mixture of two volumes of oil of vitriol and one of water, and thoroughly cleansed by repeated washings with water, and finally with ammonia, it is changed into a substance resembling parchment, and often called vegetable parchment.

“ The alteration which takes place in the paper is of a very remarkable kind. No chemical change is effected, nor is the weight increased; but it appears that a molecular change takes place, and the material is placed in a transition state between the cellulose of woody fibre and dextrine.”

Vegetable parchment is now extensively used instead of parchment for legal and other documents. In some respects it is preferable to the old kind, for insects attack it less.

165. Vegetable Acids. — The acids of vegetable origin are almost innumerable. We shall describe here only a few of the most important.

Tartaric acid, C.H.O., is found, either free or combined with potash or lime, in tamarinds, grapes, pineapples, and many other vegetables. It is usually seen in the form of colorless crystals, which have an agreeable acid taste, and are soluble in water and alcohol. It is used in large quantities in calico printing and dyeing, and also in medicine, especially for making effervescent draughts. Some of its more important compounds are

the familiar substances, Rochelle salts, cream of tartar, and tartar-emetic.

Citric acid, CH,Oy, is usually prepared from lemons. It is also contained in gooseberries, currants, raspberries, strawberries, and other fruits. It is a crystalline solid, readily soluble in water, and has an intensely sour taste. It is used in silk-dyeing, in calico-printing, and in medicine.

Malic acid, CH.05, is one of the most abundant of these vegetable acids. It is found in apples, from which (Latin, malum) it takes its name ; in currants, barberries, rhubarb, and many other fruits and plants. It forms needle-shaped crystals, which are very soluble in water and alcohol. It is not used in the arts or in medicine,

Oxalic anhydride, C202, is not known in a free state. It is found in the juice of wood-sorrel (oxalis acetosella, from which it takes its name), and of many other plants, in combination with potash or lime. It forms a compound with water, H,C,0, which is oxalic acid. This is a crystalline substance, and is very poisonous. It is much used in cotton-printing and straw-bleaching, and is · prepared in very large quantities by the action of caustic potash on sawdust. Crude potassic oxalate (oxalate of potash) is thus formed, which, by means of lime, is converted into the insoluble calcic oxalate (oxalate of lime), and this is decomposed by sulphuric acid.

Tannic acid and tannin are the names given by chemists to a number of compounds of C, H, and O, which are characterized by a well-marked astringent taste. They are soluble in water and alcohol, and form precipitates with most metallic oxides. Ferric salts yield black, or nearly black, precipitates, which are the basis of the ordinary writing inks. Tannic acid also precipitates gelatine (or glue); and it is by a similar process that it converts the gelatinous tissue of raw hides into tanned

leather. The bark and leaves of most forest trees, such as the oak, elm, willow, horse-chestnut, and pine, and of many fruit-trees, as the pear and plum, contain considerable quantities of tannin. Coffee and tea likewise contain modifications of this compound; but the tannin in coffee, unlike that from all the other plants we have named, does not form the black salt with iron. A drop of tea on a knife-blade becomes inky at once, but a drop of coffee does not.

166. Natural Fats and Oils. - The natural oils and fats (fats being merely solid or semi-solid oils) are all compounds of organic acids, called the fatty acids, with a base called glycerine (167), and they are found in both plants and animals. Cocoa-nut oil, palm oil, and nutmeg butter are examples of vegetable fats.

The vegetable oils are divided into the fixed oils, which cannot be distilled without decomposition, and the volatile oils, which bear distillation without change. The fixed oils are divided into drying and non-drying oils. The former become dry and solid from oxidation, when spread out thin and exposed to the air; while the latter remain unaltered. The chief drying oils are those of linseed, hemp, poppy, and walnut, all much used in paints and varnishes; and the most important non-drying oils are olive oil, almond oil, and colza oil, which are extensively used in making soap, candles, and illuminating oils, in wool-dressing, and for many other purposes. Castor oil seems to form a connecting link between these two classes of oils, as it gradually becomes hard by long exposure to the air.

The volatile or essential oils are believed to constitute the odorous principles of plants. Most of them are nearly colorless when fresh, but darken on exposure to light and air. When long exposed to the air, they absorb oxygen, and become resins. They are largely used in the manu

facture of perfumery; for flavoring confectionery and liquors, and for various other purposes in the arts; and in medicine.

167. Soap. — When the oils or fats are boiled with an alkali, they undergo thie remarkable change called saponification. The fat is decomposed into a fatty acid and glycerine (166). The acid combines with the alkali to form soap, and the glycerine passes into solution. If the alkali be soda, a hard soap is formed ; if potash, a soft soap. The soaps, then, like the fats, are true salts.

Fats may also be separated into acid and glycerine by distillation with steam, at a temperature between 500° and 600°.

Glycerine (C6H1606), is a colorless viscid liquid, of a. sweet taste (whence its name, which is from a Greek word, meaning sweet), soluble in water and alcohol, but nearly insoluble in ether. It is used in medicine, in making copying-ink, and as a lubricating agent.

The most important of the fatty acids are oleic, stearic, and palmic, or palmitic. They are used on a large scale in the manufacture of candles.

168. Wax. — The various forms of vegetable wax resemble in many respects the fixed oils, but are quite different from them in chemical composition. They are solid or semi-solid substances; easily broken when cold, but soft and pliable when moderately warm, and melting below 212°. They are insoluble in water and cold alcohol, but dissolve readily in ether; they burn with a bright flame, and are not volatile. They are found as exudations on leaves and fruits, where they form the glaucous surface, which repels water. Some fruits, as the bayberry, are thickly coated with wax.

169. Gums and Resins. - In the strict sense, a gum is a substance which dissolves in water, forming a mucilage ; but is insoluble in ether, alcohol, and oils. A resin,

on the other hand, is insoluble in water, and soluble in ether, alcohol, and oils. Gum arabic and gum senegal are products of different species of acacia, and are extensively used in the arts. Gum tragacanth is the product of a shrub which grows in Persia and Asia Minor. It is only partially soluble in water, forming a white paste instead of a mucilage. It is used for paste and cement, and also for stiffening woven fabrics. Dextrine is now much used as a substitute for these and other gums (162).

The resins are divided into the hard resins, the soft resins, and the gum-resins. The hard resins are brittle at ordinary temperatures, and are easily pulverized. The most important are copal, the varieties of lac, mastic, and sandarach. They are much used for varnishes and for other purposes in the arts. The soft resins are easily moulded by the hand, and some of them are semi-fluid, in which case they are called balsams. They consist essentially of hard resins dissolved in oils. On exposure to the air, they are oxidized and converted into hard resins. Under this head are placed turpentine, Canada balsam, etc. The gum-resins are the milky juices of certain plants, solidified by exposure to the air. They consist of a mixture of gums, resins, and essential oils. Caoutchouc, or india rubber, is the most important of the gum-resins. It is obtained from several quite different kinds of trees. The original india rubber of the East Indies is the product of a species of fig-tree. Guttapercha is the dried milky juice of a tree found in Malacca and the neighboring islands. The applications of these two gum-resins in the arts are almost innumerable, and are continually increasing. Many valuable medicines are obtained from gums and resins Amber is a fossil resin.

170. Vegetable Alkaloids. — A great variety of alkaline compounds, known as alkaloids, are produced by plants. They are mostly formed in the bark and in the

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