Slate
3 of the 7 encyclopedias on this shelf carry an entry for Slate. Both are reproduced below, so you can see where they agree and where they differ.
Collier's New Encyclopedia (1921)
a very remarkable form of clay rock, frequently fossiliferous and not confined to one geological period. Consisting essentially of clay, the particles of slate are so mechanically arranged that the rock splits with perfect facility into almost indefinitely thin layers in one direction only, and in all others either breaks with a jagged edge, or in well-defined joints at some distance from each other. Mineralogically slate is nothing more than a pure clay; nor does there seem any reason to suppose that any approach is made in it toward crystalline structure. As, however, no other rock shows this tendency to split indefinitely, the case is one of great interest. Practically slate is very valuable, owing to its peculiar facility of splitting and the perfectly smooth natural face which it presents. Its hardness and compactby ness preserve it from all weathering by mere exposure, though, when ground down, it easily passes back into fine clay. Slate is always, and properly, regarded as a metamorphic rock. For a long time slate was used almost exclusively for roofing. For this purpose the slates are cut into sizes varying from a few square inches to two square feet, though some are much larger. This is done with very simple tools and extreme rapidity. The rest is in slabs or thick slates, often very large. The use of slate and slabs has increased considerably of late years. Slabs are now used in house fittings; as in strong rooms, powder magazines, larders, partitions, baths, stables, floors, etc. It is very largely used also for enameling; the surface of enameled slate being made to represent marble of all kinds with wonderful accuracy, and resisting almost all wear. Quarries of great magnitude are worked in Cornwall, Wales, Scotland, and Ireland. The product of the Welsh quarries was formerly largely exported to the United States, but this serious check on the business received a opening of valuable quarries in Vermont, New York, Pennsylvania, Maryland, and other states. Adhesive slate is a kind of greenishgray slate, quickly absorbent of water, and remarkable for its adhesiveness. Bituminous slate, a soft species of sectile slate clay impregnated with bitumen; hornblende slate, a slaty rock consisting of hornblende and feldspar with some chlorite, principally used for flagging pavements, etc. SLATIN PASHA (Baron Rudolf Carl), an Austrian soldier, born near Vienna, in 1857. After service in the Austrian Army, he engaged in service in Egypt under General Gordon, and in 1881 was made Governor-General of Darfur. Shortly after, the Mahdi aroused his followers and began the famous religious war which resulted in 1884 in the defeat of Hicks Pasha, and the capture of Slatin by the Mahdists. He was made practically a slave of the Mahdist commander, Abdullahi, who succeeded the Mahdi after the latter's death. He was kept in captivity for 11 years, until 1895, when he succeeded in escaping to Lower Egypt. He was made a pasha by the Khedive, and was appointed in 1900 British Inspector-General of the Sudan. In 1906 he was created a Baron of the Austrian empire. He had already been made an honorary major-general in the British army, but in 1914 renounced this and other British honors.
The Encyclopedia of Founding (1892)
A highly metamorphosed clay rock, consisting ing essentially of clay. The particles are so mechanically arranged that it splits into plates that are independent of the layers of deposit, and are of a blue, green, gray, or black color. Its hardness prevents it from injury when exposed to the weather; it is therefore well adapted for roofs of houses, etc., and is in great demand for enamelling mantels and other objects, being by this means made to imitate the most expensive marbles at slight cost. Extensive quarries of this substance are worked in Cornwall, Wales, Ireland, and Scotland; also in Vermont and other States in this country. Adhesive slate absorbs water readily, and is highly adhesive. Aluminous slate yields alum. Bituminous slate is impregnated with bitumen. Hornblende slate contains feldspar, and is used for flagging. Hones are made from slate, also pencils; and the slate-clay which consists of silica and alumina is suitable for fire-brick. See ALUM. Slicker--sometimes called "sleeker" and "smoother"--is a moulder's tool that usually has some special shape given to it on one side. The other side has a finger-piece or handle, by which means the slicker is worked upon the sand and made to impart smoothness and finish to the surface. These tools are of cast iron, brass, and steel, highly polished on the face side, and are known as corner, elbow, pipe, button, flange, bead, and web slickers, etc. Considerable dexterity of hand and eye must be acquired before the moulder can use these tools creditably, and only the most skilled workmen should be allowed to use them indiscriminately. Inferior artists are apt to linger too long over the work, and their unpractised eye and lack of taste invariably end in producing lines that bear no resemblance whatever to the original pattern; but on such parts as offer few difficulties they will smooth the surface with such frequency that the alumina in the sand is worked into a clayey skin on the surface, which, if it be not already loosened, will more than likely shrink and break away in scabs when the metal covers it. See SCABBED CASTINGS.
Aiton's Encyclopedia (1910)
a sort of flaky rock formed from clay by heat and tremendous pressure in the making. Experiments have demonstrated that if a cube of ordinary clay be squeezed in a press, it will separate into flakes lying parallel to the surface on which pressure is applied. The black color of slate must be attributed to the stain, presumably of iron, and to heat. Slate rocks are found in most mountain regions. The best known slates of this country are quarried in eastern New York , Vermont, Pennsylvania, and other localities of the northern Alleghany region. Ordinary roofing slate is made up of sand, aluminum, iron, potash, magnesia, carbon, and water. Split thin and trimmed to uniform dimensions, slate forms an excellent substitute for shingles. If laid carefully a slate roof lasts practically forever. Blackboards are made from a soft quality of slate free from grit. It is quarried in large sheets, ground by machinery to a smooth surface, then cut by a power saw into squares of the required size. Many substitutes for slate blackboards have been offered. Some of them show an excellent surface to begin with, but all lack durability and prove in the end poor economy. Slate-pencils are made in two ways, either by sawing the pencil out of slabs of the stone, or else by grinding fragments into a powder and molding the pencil in a press. See Talc