Riddle's Report of the Great Exhibition.

Scientific American 21, 5.2.1853

(continued from page 158.)

Crystal of Copperas, or Sulphate of Iron. - This substance, so largely used in the arts for dyeing, &c., and also in chemistry and pharmacy, is obtained, as a natural product, from aluminous chalybeate springs, as well as by the spontaneous decomposition of certain native sulphurets of iron, or iron pyrites; and is prepared in large quantity by the action of air and water. The sulphur and iron are thus both oxidated; and sulphate of iron, or copperas, is obtained by crystallizing the lixiviated masses.

Crystallization, and the circumstances under which it takes place, form an interesting subject of inquiry. Not having the operations of nature open to our inspection, our only sources of information relative to the formation of crystals are those afforded by the process of crystallization; and here, until very recently, out experiments were circumscribed by a very few modes of operation; that of the deposit of crystals from solution in some fluid; their production while gradually cooling from a state of fusion; and their volatilization by heat, or otherwise. Latterly, however, by the aid of that universal agent - electricity - new methods of producing crystals have been pursued; and there can now be little doubt that all the phenomena of crystallization are governed, in a greater of lesser degree, by electric influence.


Ferrocyanide of Potassium, used ofr Dyeing Blue, in place of Indigo. - This is perhaps one of the most important chemicals used in the art of dyeing, and calico printing. Its preparation consists in projecting a mixture of pearl ashes with hoofs, horns, and other animal matters, in the proportion of two to five, into a red-hot iron crucible, and stirring diligently the pasty mass thus formed until fetid vaports cease to arise from it. - When the product has cooled, it is lixiviated with cold water, filtered and concentrated, upon which yellow crystals of ferrocyadine of potassium are formed. By the addition of a salt of iron to ferrocyadine of potassium, that most beautiful blue, called Prussian blue, is produced.


Refined Indigo. - This substance is the innoxious and beautiful product of an interessting tribe of tropical plants, and is very extensively employed in dyeing and calico printing; being esteemed the most useful and substantial of all dyes. When the plant is in full flower it contains most coloring matter.It is then cut, dried, and put into vats, and covered with water; fermentation takes place, accompanied with the evolution of carbonic acid, and other gaseous products, and the yellow liquor is covered with a froth. This froth, in a little time, becomes of a violet color, and a substance is evolved which is rendered blue by absorbing oxygen of the air, and, being thus rendered insoluble, is precipitated. This, when, collected and dried, is indigo.

Specimen of Ultramarine. - This is a well known blue pigment of extraordinary beauty. Until within the last few years it was entirely prepared from the lapis lazuli, or lazulite, and from the great costliness attending its preparation, its use was confined to the artist.
It is now prepared, artificially, at a very moderate rate, and equal in beauty to that obtained from the lazulite. It is stated that by adding freshly precipitated silica and alumina, mixed with sulphur, to a solution of caustic soda, evaporating the mixture to dryness, and placing the residue in a covered crucible, and exposed to a white heat, where the air has a partial access to it, a pure ultramarine is obtained. The product is then reduced to impalpable powder. The proportions of materials to be used are about 36 silica, 36 alumina, 24 soda, and 3 sulphur. Since this discovery, its value has become very much reduced, and it is now used extensively in the arts.

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