To adjust the draught from the gas-burners, there are stopcocks introduced into several of the pipes d; and to regulate the action of the exhausting apparatus, an air vessel o, is suspended by a cord or chain passing over pulleys, and balanced by a weight p. There is also a scraper introduced into the tube c, which is made, by any convenient contrivance, to revolve and slide backwards and forwards, for the purpose of removing any light matter that may arise from the goods singed, and which would otherwise obstruct the air passage. Two of these draught tubes c, may be adapted and united to the exhausting apparatus, when a double row of burners is employed, and the inclination of the flame may be directed upwards, downwards, or sideways, according to the position of the slit in the draft tube, by which means any description of goods may, if required, be singed on both sides at one operation.

The greater part of the bobbin-net lace made in England, is sent to Mr. Hall’s works, at Basford, near Nottingham, to be singed; and at a reduction of price truly wonderful. He receives now only one farthing for what he originally was paid one shilling.

SKIN (Peau, Fr.; Haut, Germ.); the external membrane of animal bodies, consists of three layers: 1. the epidermis, scarf-skin, (Oberhaut, Germ.); 2. the vascular organ, or papillary body, which performs the secretions; and 3. the true skin, (Lederhaut, Germ.), of which leather is made. The skin proper, or dermoid substance, is a tissue of innumerable very delicate fibres, crossing each other in every possible direction, with small orifices between them, which are larger on its internal than on its external surface. The conical channels thus produced, are not straight, but oblique, and filled with cellular membrane; they receive vessels and nerves which pass out through the skin (cutis vera), and are distributed upon the secretory organ. The fibrous texture of the skin is composed of the same animal matter as the serous membranes, the cartilages, and the cellular tissue; the whole possessing the property of dissolving in boiling water, and being, thereby, converted into glue. See [Glue], [Leather], and [Tan].

SLAG (Laitier, Fr.; Schlacke, Germ.); is the vitreous mass which covers the fused metals in the smelting-hearths. In the iron-works it is commonly called cinder. Slags consist, in general, of bi-silicates of lime and magnesia, along with the oxides of iron and other metals; being analogous in composition, and having the same crystalline form as the mineral, pyroxene. See [Copper] and [Iron].

SLATES (Ardoises, Fr.: Schiefern, Germ.) The substances belonging to this class may be distributed into the following species:—

1. Mica-slate.—This is a mountain rock of vast continuity and extent, of a schistose texture, composed of the minerals mica and quartz, the mica being generally predominant.

2. Clay-slate.—This substance is closely connected with mica; so that uninterrupted transitions may be found between these two rocks in many mountain chains. It is a simple schistose mass, of a bluish-gray or grayish-black colour, of various shades, and a shining, somewhat pearly internal lustre on the faces, but of a dead colour in the cross fracture.

Clay-slate is extensively distributed in Great Britain. It skirts the Highlands of Scotland, from Lochlomond by Callender, Comrie, and Dunkeld; resting on, and gradually passing into mica-slate throughout the whole of that territory. Roofing-slate occurs, on the western side of England, in the counties of Cornwall and Devon; in various parts of North Wales and Anglesea; in the north-east parts of Yorkshire, near Ingleton, and in Swaledale; as also in the counties of Cumberland and Westmorland. It is likewise met with in the county of Wicklow and other mountainous districts of Ireland.

All the best beds of roofing-slate improve in quality as they lie deeper under the surface; near to which, indeed, they have little value.