II.-DYES.

"Dyeing," says Wagner, "is distinguished from painting by the fact that the pigments are fixed to the animal and vegetable textile fabrics according to certain physico-chemical principles, and are not, as in painting, simply fixed by adhesion to the surface, although painters and artists occasionally use the same pigments. Printing consists in the duplication of coloured patterns, and is a very important part of dyeing. In the art of dyeing, some colouring matters are applied by immersing the tissue to be coloured in the decoction or solution of the pigment. Some substances are applied to the surface of the woven fabric by the intervention of what is termed a ‘mordant,' which secures the adhesion, fixing, and permanency of the colours."

The great majority of dyes and dye-stuffs are obtained from the vegetable kingdom; some from the animal; and, till recently, only a few from the mineral. Though these are chiefly of organic origin, the "mordants," or substances employed in fixing or striking their colours, whether in woollen, silk, cotton, or linen, are all, or nearly all, of inorganic origin-such as acetates of iron, lead, and alumina, sulphates of iron and alumina, aluminate of soda, and alum. In this way the art of dyeing comes within the range of Economic Geology. Of recent years, however, the relationship has become more intimate, and by the researches of modern chemistry we now derive from the inorganic world a variety of dyes of unsurpassed beauty and brilliancy. Strange as it may seem, these are chiefly obtained from coal-tar-a dark, dingy, and uninviting by-product of our gas-works. Chemically treated, by a number of ingenious processes this substance yields the aniline or coal-tar colours of commerce-fuchsin, magenta, aniline blue and violet, Manchester yellow, aniline orange and aniline brown, coralline, alizarine, Magdala red, and aniline black. Few triumphs of chemistry have been more marvellous than the production of these beautiful colours-no substances so unlike as a mass of pitchy coal-tar, and the brilliant flush of roseine, mauve, and magenta.

III.-DETERGENTS.

Fuller's Earth.

One of the best-known and abundant of mineral detergents is fuller's earth or fuller's clay, so called from its being em

ployed in the fulling of woollens. In composition it is somewhat varied; but all the varieties are soft, unctuous, hydrous silicates of alumina-that of Reigate, from the greensand of Surrey, consisting of 53 silica, II alumina, 24 water, and 9 iron oxide, with traces of magnesia and lime. Good fuller's earth is usually massive, opaque, soft, dull, with a greasy feel and an earthy fracture; scarcely adheres to the tongue; and when placed in water, falls down to an impalpable powder without forming a paste with it. It occurs abundantly in the oolitic and cretaceous systems of England, in beds from one to several feet in thickness, and of a greenish or greyish-green colour. So important at one time was this earth to the woollen manufacture of England, that its exportation was prohibited by Act of Parliament. Its place is now mainly supplied by soap and other chemical detergents, though considerable quantities are said to be still dug and prepared for the fuller in Surrey, Gloucestershire, and Bedfordshire. Besides being used by the fuller, under the names of fuller's earth, Walker's earth or walkerite, and smectite (Gr. smectes, a cleaner) it is also employed in paper-making, and as an addition to artificial ultramarine.

Nowadays the principal detergents, whether employed in woollen, silk, cotton, linen, or leather manufacture, are of chemical preparation-soaps, leys of soda and potash, chlorine, chloride of lime, &c.—and as far as the limes, alkaline salts, and soluble silica (which enters into the composition of some soaps), are concerned, come under the cognisance of Geology. The operations of washing, bleaching, and tanning are all more or less facilitated by preparations obtained wholly, or in part, from the mineral kingdom. The same may be said of sugarrefining, in which lime, gypsum, and baryta are now successfully employed, not only in producing a purer article, but in facilitating the operation.

As explained in the preceding pages, most of the pigments, several of the dyes, and many of the detergents, are obtained either directly or indirectly from the mineral kingdom, and in this way come within the scope of Economic Geology. The elaboration of these substances belongs more especially to chemistry; but to the working geologist is left the discovery of the raw materials, their modes of occurrence, abundance, and the facility with which they can be procured. Many of them have long been known; but some are the results of recent research, and hold out the hope that others may yet reward the skill and industry of the diligent inquirer. The earth is a

vast storehouse of mineral and metallic wealth, only awaiting the requirements of man, and the skill to utilise them; and the geologist best performs his function, when in conjunction with the chemist and technologist, he is ever on the watch for new products and more advantageous appliances. Numerous, beautiful, and useful as our pigments, dyes, and detergents undoubtedly are, it cannot for a moment be supposed that we have either exhausted the field of their variety, or the sources from which they are derived. The artificial preparation of ultramarine, and the discovery of the coal-tar or aniline colours, are apposite cases in illustration, and hold out the incentive alike to chemist and geologist to persevere in their researches for results equally successful and satisfactory. The discovery of mineral substances has not been exhausted, any more than the limits of invention have been reached, by those who have gone before us.

Works which may be consulted.

Wagner's Handbook of Chemical Technology'-Crooke's Edition; Ure's Dictionary of Arts and Manufactures-Hunt's Revision; Knapp's 'Chemical Technology'—vol. ii.; Watt's Dictionary of Chemistry.'

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XIV.

SALTS AND SALINE EARTHS.

By the Salts and Saline Earths we mean those substances which, like rock-salt, natron, and nitre, are obtained from the rocky crust, either in a crystallised state comparatively pure, or so associated with earthy matters as to require chemical processes of extraction and purification. These substances play important parts in the arts and industries-in domestic economy, in medicine, agriculture, bleaching, dyeing, glass-making, powder-making, glazing, enamelling, and various other processes. Some, like rock-salt, occur in stratiform masses; some are obtained by the evaporation of saline waters; and others, again, are found commingled with sand, gravel, and earthy debris, in the sites of desiccated lakes, deserted sea-lagoons, and old upraised sea-beaches. Many of these salts are hydrous, —that is, they contain a definite proportion of water of crystalisation; others are destitute of water, and are dry or anhydrous salts. Some attract moisture when exposed to the air, and are said to be deliquescent; others suffer their water to escape, and become opaque and pulverulent, and are said to be efflorescent. They are all more or less soluble in cold water, and much more easily so in boiling water-forming brines and saturated solutions. Whatever their characteristics, they are easily treated, and many of them are produced on the large scale by artificial processes.

I. SALTS OF SODA.

Foremost among these, both in point of bulk and importance, is Rock-salt, common salt, or chloride of sodium, 60.4 chlorine and 39.6 sodium. This mineral occurs as an efflorescence in most of the salt-deserts of the world, in old seareaches, and on the shores of salt-lakes. It is thrown up in solution by saline springs, and forms a principal ingredient in the waters of the ocean. It is also found as a rock-mass

matter

in several formations, often in a state of considerable purity, but more frequently coloured by iron oxides and commingled with earthy impurities. In its purest state it may consist of 98 or 99 of chloride of sodium, with traces of chloride of magnesium, sulphate of lime, chloride of calcium, and insoluble -the rock-salt of Chester yielding to Henry 98.3 chloride of sodium, 0.05 chloride of magnesium, 0.65 sulphate of lime, and I of insoluble ingredients. In its impure condition it may contain from a sixth to even a half of earthy admixtures. Pure chloride of sodium is not liable to deliquesce, but it rapidly attracts moisture from the air when it contains chlorides of magnesium and calcium.

In the British Isles the great repository of rock-salt is the Trias, or Upper New Red Sandstone (Cheshire, Middlesborough, Antrim); but deposits of equal magnitude are found in connection with oolitic strata, as in the Salzburg Alps-with cretaceous greensands, as at Cordova in Spain-with chalk and tertiary rocks in the Valley of Cardona in the district of the Pyrenees-with tertiary marls, as in Sicily and at Wielitscka in Poland; and salt or brine springs are known to issue from carboniferous and older strata. It is thus a product of all epochs, and must have been formed either by the gradual and long-continued desiccation of limited areas of salt water alternately cut off and placed in communication with the ocean, or by precipitation from saturated solutions, brought about, perhaps, by the evaporating power of volcanic or other thermal agency.

The Cheshire deposits of rock-salt, which may be taken as a typical illustration, lie along the line of the valley of the Weaver, in small patches, about Northwich. There are two main beds lying beneath 120 feet of coloured marls, sands, and irregular bands of salt and gypsum, in which no traces of animal or vegetable fossils occur. The upper bed of salt is 75 feet thick; it is separated from the lower one by 30 feet of coloured marls, sands, and salt-bands, similar to the general cover and the lower bed of salt is above 100 feet thick, but has nowhere been perforated. They extend in an irregular oval area, about a mile and a half in length by three-quarters of a mile in breadth. The salt in these deposits is in some portions pure and transparent, and in others of a dirty reddish hue, and mixed to the amount of half its bulk with earthy impurities. It is not stratified nor laminated, but divided into vertical prisms of various forms and magnitudes, sometimes more than a yard in diameter-the outer sides of these rude crystallisations being generally pure and transparent.