Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/829

Rh place in them, and that with a rapidity almost instantaneous. This is also a more efficient plan than that of employing a rheotome for changing the direction of the current, so as to bring wires connected with one or more charges successively into the circuit. The liuhmkorff coil is, however, objec tionable for its delicacy and the maintenance of batteries in connection with it. In experiments made by Messrs Wheatstone and Ab3l, a powerful magneto-electric machine was found very limited in its power of igniting several charges arranged in succession in one circuit (it only ignited three at most, with certainty) ; but on M. Savare s plan of arranging the charges in divided circuits, the simul taneous ignition of twenty-five charges was repeatedly effected ; on several occasions as many as forty By this plan each charge was connected with a separate branch attached to the main line, and their connection with earth established by means of uncovered copper wire wound round an iron stake driven in the ground. Another form of instrument^ devised by Wheatstone, consists of six small magnets, to the poles of which are fixed soft iron bars surrounded by coils of insulated wire ; the coils of all the magnets are united together, so as to form, with the external conducting wire and the earth, a single circuit A.n axis carries six soft iron armatures, in succession, before each of the coils. With this apparatus twenty-five charges were frequently fired in divided circuit, .so rapidly that the effect on the ear was as of one explosion, only of slightly longer duration than when the large magnet was employed. The Markus apparatus, largely used in Germany, is on the same principle. Siemens s dynamo-electric machine, in which electro magnets are employed, is a very useful machine for simultaneous firing It is found that the residual magnetism left in the coils of electro-magnets, after a current from even a single Voltaic cell has been once sent through them, is always sufficient to have the necessary inductive action on the armature. This inductive action, though very weak at first, generates slight alternating currents in the armature, which are by means of a com mutator caused to flow always in one direction through the coils of the electro-magnet, thus increasing the magnetism in the core, which, in its turn, increases the inductive action, causing stronger and stronger currents to be generated in the armature. This action and reaction goes on till the limit of magnetic capacity of the core is reached, and if the coil of the armature be then suddenly connected with the line leading to a fuse, a very powerful current is transmitted. In Breguet s exploder (in which a bar of soft iron is suddenly separated from the armature of a magnet bearing two induction coils) a special arrangement gives rise to an extra-current, and considerably increases the intensity of the current. M. Breguet has lately utilized in this apparatus the new and powerful laminated magnets constructed by M. Jamin. Gramme s machines are also effective in exploding charges, but their volume and high price are against a large use of them industrially. For more detailed information on the recent develop ments of blasting, reference may be made to Spon s Dictionary of Engineering, art. &quot; Boring and Blasting ;&quot; Professional Papers of the Corps of Royal Engineers, vols. vii., x., xxii. ; Transactions of the Society of Engineers, 1869 and 1871 ; Proceedings of South Wales Institute of Mining Engineers, vol. viii., No. 5, vol. ix., Noa. 1 and 2 ; Dingler s Polytechnisches Journal, Oct. 1, 1874 ; Annales de Chimie et de Physique, May 1875 ; Journal of the. Society of Arts, May 28, 1875.  

     LEACHING is the process of whitening or depriving objects of colour, an operation incessantly in activity in nature by the influence of light, air, and moisture. The art of bleaching, of which we have here to treat, consists in inducing the rapid operation of whitening agencies, and as an industry it is mostly directed to cotton, linen, silk, wool, and other textile fibres, but it is also applied to the whitening of paper-pulp, bees -wax, and some oils and other substances The term bleaching is derived from the Anglo- Saxon bla?ca/t to bleach, or to fade, from which also comes the cognate German word bleichen, to whiten or render pale. Bleachers, down to the end of last century, were known in England as &quot; whitsters,&quot; a name obviously derived from the nature of their calling. The operation of bleaching must from its very nature be of the same antiquity as the work of washing textures of linen, cotton, or other vegetable fibres. Clothing repeatedly washed, and exposed in the open air to dry, gradually assumes a whiter and whiter hue, and our ancestors cannot have failed to notice and take advantage of this fact. Scarcely anything is known with certainty of the art of bleaching as practised by the nations of antiquity. Egypt in early ages was the great centre of textile manufactures, and her white and coloured linens were in high repute among contemporary nations. As a uniformly well-bleached basis is necessary for the production of a satisfactory dye on cloth, it may be assumed that the Egyptians were fairly proficient in bleaching, and that still more so were the Phoenicians with their brilliant and famous purple dyes. We learn, from Pliny, that different plants, and likewise the ashes of plants, which no doubt contained alkali, were employed as detergents. He men tions particularly the Strut/iiuni as much used for bleaching in Greece, a plant which has been identified by some with Gypsophila Struthium. But as it does not appear from Sibthorp s Flora Grceca, published by Sir James Smith, that this species is a native of Greece, Dr Sibthorp s conjec ture that iheStmthium of the ancients was the Saponaria offi- cinalis, a plant common in Greece, is certainly more probable. In modern times, down to the middle of the 18th century, the Dutsh possessed almost a monopoly of the bleaching trade, although we find mention of bleach-works at South- wark near London as early as the middle of the 17th century. It was customary to send all the brown linen, then largely manufactured in Scotland, to Holland to be bleached. It was sent away in the month of March, and not returned till the end of October, being thus out of the hands of the merchant more than half a year. The Dutch mode of bleaching, which was mostly con ducted in the neighbourhood of Haarlem, was to steep the linen first in a waste lye, and then for about a week in a potash lye poured over it boiling hot. The cloth being taken out of this lye, and washed, was next put into wooden vessels containing butter-milk, in which it lay under a pressure for five or six days. After this it was spread upon the grass, and kept wet for several months, exposed to the sunshine of summer. In 1728 James Adair from Belfast proposed to the Scotch Board of Manufactures to establish a bleachfield in Galloway; this proposal the board approved of, and in the same year resolved to devote 2000 as premiums for the establishment of bleachfields throughout the country. In 1732 a method of bleaching with kelp, introduced by R. Holden, also from Ireland, was submitted to the board ; and with their assistance Holden established a bleachfield for prosecuting his process at Pitkerro, near Dundee.