"China clay" is an example, and fusible clays, of which the common clays for earthenware manufacture afford examples. To these may be added the colored clays used for the brick and terra-cotta industry. The difference in chemical character of these three groups may be seen from the subjoined analyses :

From kaolin, to which has been added a so-called flux, usually feldspar, is made porcelain. The materials are thoroughly admixed in a finely-ground and levigated condition with the aid of water, and after the articles are shaped on the potter's wheel or by hand they are dried and burned in the pottery kiln. This is shown in sectional view in Fig. 8o. The ware is placed in vessels known as "saggers," which are arranged one above the other in columns in the compartments of the kiln as shown in Fig. 81. The baked ware is known as "porcelain biscuit," and is then to be given a glaze. This is a mixture of silicate of alumina and alkalies, and is applied in a thin cream, dried on, and then burned again in the kiln at a temperature sufficient to cause uniform fusion on the surface.

Cements. The natural calcareous clays and artificial mixtures of similar composition are used in the manufacture of hydraulic mortars or cements. Common or Roman cements are prepared by burning natural calcareous clays or cement-rock at a temperature short of that required to cause the sintering of the mass. Portland cement, on the other hand, is made from mixtures of limestone and clay or natural cement-rock, of such composition that when burned the product will contain from 55 to 60 per cent. of lime, from 22 to 25 per cent. of silica, and some 7 per cent. of alumina. This variety is also burned until the mass is sintered together. It is then ground, allowed several months to season, and packed in barrels. These cements are essentially basic silicates of lime and alumina, and they "set" by virtue of their power of absorbing and combining with water. The production

of common hydraulic cement in the United States for the year 1893 amounted to 7,503,385 bbls. of 300 lbs., and of Portland cement for the same year to 596,531 bbls.

Ultramarine is a blue coloring matter, consisting of silica, alumina, soda, and sulphur, first found in the rare and costly mineral lapis lazuli, but since 1828 made artificially. A mixture of kaolin, charcoal, and sodium sulphate or carbonate is heated

to redness in closed fire-clay crucibles, and the green mass thus formed is ground to powder and then roasted in thin layers with flowers of sulphur until the required blue shade is obtained. FIG. 81.

Part of the sodium in ultramarine is combined as a double silicate with alumina, and part is present as sulphide. Hence, when ultramarine is heated with hydrochloric acid, hydrogen sulphide is evolved and the blue color is destroyed.

Violet and red ultramarines are also prepared at present by conducting dry hydrochloric acid gas and air over common ultramarine at 100°-150°. The present annual production of ultramarine is about 9000 tons, of which Germany produces 6500 tons.

RARE EARTHS AND METALS.

In some relatively rare minerals, like cerite, gadolinite, allanite, monazite, and samarskite, occur a number of metals, the oxides of which are very analogous to alumina in chemical characters. They will be briefly enumerated, and their relationships in the list of elements pointed out.

Scandium (Sc= 43.97) is contained in euxenite and gadolinite. Its oxide, Sc2O3, is a white, infusible powder like magnesia. It forms a hydrate, Sca(OH), and a double sulphate or alum, Sc2(SO4)3 + 3K2SO4

It is interesting as having been discovered since the announcement of the periodic-system theory, and as filling a place pointed out in that system. Yttrium (Yt=88.9) is mostly obtained from gadolinite. It forms a potassium double sulphate, soluble in potassium sulphate solution, and thus separable from cerium, lanthanum, and didymium.

Lanthanum (La 138.2) has been obtained by the electrolysis of its chloride as a metal resembling iron as regards color and lustre, and with a specific gravity 6.16.

Ytterbium (Yb=172.6) is obtained from the so-called erbium earths by fractional decomposition of the mixed nitrates by heating. Its oxide, YbO, is a white, infusible powder, of specific gravity 9.17.

These four elements form a sub-group of basic elements, attaching to aluminum in the third group of the periodic system (see p. 286).

Cerium (Ce = 139.9) occurs in cerite to the amount of 60 per cent. of the oxide. The metal has been obtained by the electrolysis of the chloride. It is very similar to lanthanum, but burns more readily. Its specific gravity is 6.72. It forms two oxides, CeOg and CeO2. The salts of the sesquioxide are colorless, while those obtained from the dioxide are yellow or brown.

Cerium Oxalate, Се(С2O4)3.9H2O (Cerii Oxalas, U. S. P.), "forms a white, granular powder, without odor or taste, and permanent in the air. Insoluble in water, alcohol, ether, or in solutions of potassium or sodium hydrate; soluble in diluted sulphuric or hydrochloric acid. When heated to redness it is decomposed, leaving a residue of reddish-yellow ceric oxide." Cerium nitrate has also been employed in medicine, although not official. The cerium salts are nervine tonics, and useful in cases of vomiting and dyspepsia.

Didymium (Di = 142) forms two oxides, Di2O, and Di2O. The salts of didymium by fractional crystallization can be broken up into salts of what are apparently two distinct bases, neo-didymium and praseodidymium; the salts of the first class are rose-red and of the second yellowish-green in color.

Samarium (Sm=149.62) was discovered in samarskite. It is very similar to didymium.

Erbium (Er = 166) forms an oxide, Er2Og, of reddish color, and has not certainly been obtained as yet pure.

Terbium (Tb = 159.1) occurs in large amount in samarskite. The oxide has an orange-yellow color, and otherwise resembles the oxide of erbium.

These elements all form sesquioxides, but are not as closely related to aluminum, as they seem to form higher oxides also and several series of salts.

Three heavy metals also form a sub-group belonging in the same group of the periodic system as aluminum,-viz., gallium, indium, and thallium.

Gallium (Ga=69.9) was discovered in zinc-blende in 1875 by means of the spectroscope. It also exactly corresponded with one of the hypothetical elements indicated as possible by Mendelejeff in his periodic system. It has been obtained by the electrolysis of an ammoniacal solution of its sulphate as a white, hard metal of sp. gr. 5.9 and fusing at 30°. It forms a true alum, Ga(SO4)3. (NH4)2SO4 + 24H2O.

Indium (In=113.6) was discovered in 1863 by the aid of spectrum analysis in zinc-blendes from Freiberg. It is a silvery-white, soft, and tenacious metal of sp. gr. 7.42. It fuses at 176°, and when heated burns with a blue flame. The vapor-density of the chloride corresponds to the formula InClg. It forms, however, a true alum with ammonium sulphate.

Thallium (T1=203.7) is rather widely distributed in nature, being found in traces with potassium in carnallite, in mineral springs, and in some pyrites and zinc-blendes. It was discovered in the chamber sludge of the sulphuric-acid works by means of the spectroscope. It is a white metal, as soft as sodium, and with a sp. gr. 11.8. It fuses at 290°, and oxidizes rapidly in moist air. It burns with a beautiful green flame, whose spectrum shows an intense green line, whence the name, from valλos, green.

It forms two series of salts, derived from Tl, and