It is said that asphalt exists in this vicinity in large quantities, and under a favorable government thousands of tons might be mined each year.

A sample of the Hasbaya product is thus described: It is black with a bright jetlike lustre, making a blackish-brown streak on unsized paper. It is so brittle that pieces may easily be broken off with the fingers. It is very combustible, but a splinter held in the flames will melt before igniting. Its specific gravity is 1.104.

Egyptian Asphalt.

No natural asphalt is found in Egypt except in very small quantities above Suakim near Abyssinia, where it cannot be worked profitably, and some small deposits on the east coast of the Red Sea.

It is said, however, that two firms in Egypt manufacture artificial asphalt, importing material for their use from Italy, France, and England. What their process was, or to what uses their product was put, could not be learned.

CHAPTER IV.

BRICK-CLAYS AND THE MANUFACTURE OF PAVING-BRICK.

THE word clay as ordinarily used means any earthy substance which can be worked up with water into a plastic mass that when dried will retain any shape into which it may have been formed. Strictly speaking, the term applies to a single mineral, hydrated silicate of alumina, or kaolin. It is not, however, a natural mineral, but is the product of the decomposition of feldspar.

Beds of feldspar have often been found covered by the kaolin formed by the decomposition of a portion of its mass. This occurs when the feldspar is exposed to the action of water containing carbonic acid gas, which acts upon the alkaline base of the mineral and carries it away in solution, leaving the silicate of alumina behind. As, however, feldspar is seldom found in large quantities by itself, so deposits of pure kaolin are very rarely found. mercially they are of corsiderable value.

When pure, kaolin is composed of:

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This is represented chemically by the formula Al,0,2SiO22H2O. It is the base of all the substances known as clays, and as they are formed by the decomposition of rocks, so their chemical composition varies with that of the rocks from which they are derived.

Quartz and feldspar are the two minerals found in the greatest abundance in the earth's crust, and, very naturally, it is expected to find sand and clay as the most common of the products of the decomposition of rocks.

Feldspars are divided into three separate varieties: orthoclase, or potash feldspar; albite, or soda feldspar; and anorthite, or lime. feldspar, each of these varieties being minerals more or less complex. These, too, are at all times in the same mineral, which must be named by one of the terms used in the classification, the one in greatest abundance giving the character to the compound.

All feldspars are acted upon by the atmosphere. The oxygen, carbonic acid, and water contained in it, when taken together, form a solvent that is hard for rocks to resist, especially when supplemented by soil-waters containing more or less acids derived from decaying vegetable products.

Under these influences granites and other rocks containing feldspar, especially the potash variety, are rapidly decomposed. The feldspar having lost its cementing property, the rock falls into pieces. The carbonate of potash is dissolved in the water and borne away. The particles of quartz, mica, and other accessory minerals remain and become assimilated with silicate of alumina from the feldspar, all together making up the product commonly called clay. It can be readily seen that it cannot be a pure mineral and that its composition must vary greatly.

Kaolin has a specific gravity of from 1.5 to 2.2 and is white in color. It is soft to the touch when dry, and very plastic when wet. It has two marked chemical characteristics, insolubility and infusibility. It being the product of a soluble body, the former might be expected. It is not affected by ordinary chemical agents, nor by temperatures that have thus far been produced in the arts. It is consequently of greatest value in the manufacture of crucibles and other refractory utensils used in chemical research.

While this infusibility is true of kaolin, it is not true of clay. For the addition of different minerals found in nature often forms a compound that is easily fused. These minerals when thus used are called fluxes. Naming them in the order of their effectiveness, they are potash, soda, iron, lime, and magnesia. Very small amounts of one or more of these substances are required in any clay to destroy its value as a refractory material.

But on the other hand the finely divided silica of the original rock which is always found in a greater or less amount in most kaolin detracts not at all from its heat-resisting qualities, the silica

itself being practically infusible. For this reason free silica is practically the only impurity that is permissible in kaolin without detracting from its refractory material.

Feldspar and mica are found in nearly all clays, the latter often being discernible to the naked eye. The former, however, cannot be thus distinguished from free silica. These two minerals both contain alkalies in combination with silica and alumina, and so it is understood how alkalies can be discovered in clays by analysis, when it would not be expected to find them existing in a free state in a mineral whose origin was due to the action of water and other solvents.

The oxides and other compounds of iron are generally found in clays. The sesquioxide and the protoxide are the most common forms, but carbonates are not uncommon, and sulphides are occasional as well as injurious impurities. Iron gives the color to clays. The tints vary from buff to red, and from drab to blue or green, the amount of iron not seeming to determine the degree of color. The effect, too, of iron is very much heightened and changed by heat. The colors produced by burning vary from cream to perfectly black, with nearly all the intervening tints and shades, though the reds, browns, and greens are most common. A handsome cream-colored brick is made at Milwaukee, and others of pink color in certain parts of Canada.

Organic matter is frequently found in clays, but it is of little importance. It is generally caused by the presence of decomposing carbonaceous matter. It gives a color to the clay, but when subjected to even a comparatively low heat it is easily driven off. It is very seldom, therefore, that its presence is detrimental.

Clay can then be called a compound of a clay base with sand, feldspar, mica, and other silicates colored by iron oxides or organic matter.

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The properties of clays by which their values are determined are: plasticity, so that when wet it is possible to shape it into any desirable form; the maintenance of this form, while it is being burnt, to such a degree that its shape is permanent; and its refractoriness, so that it is able to withstand great and long-continued heats without fusing.

Plasticity is a property that is shared by practically all clays.

As a rule they all tend towards crystallization, and some kaolins are made up of masses of unattached scales. These are slightly plastic and can be made more so by grinding and kneading in water, when an examination shows that the crystalline structure has been broken up. Naturally, plastic clays do not show this structure, indicating that a clay's plasticity depends upon the extent to which this structure has been destroyed.

In several places clays are found that are entirely free from plasticity, even after being ground and treated with water. Frost and the action of water disintegrate them and a fine sand is formed, but a chemical analysis shows them to be almost pure kaolin.

Permanence of form in clay ware is caused by heat. In ancient times and in dry climates bricks that were only dried in the sun have lasted for a considerable time, but they could not be called permanently shaped.

Generally speaking, if heat has been applied only sufficiently to drive out the water mechanically mixed, the mass will be porous, somewhat shrunken in form, and readily disintegrated under the action of the elements. If, however, the heat be increased and continued, the clay will shrink farther and harden, until, when the proper point is reached, a new material has been formed which is practically indestructible. If the heat be continued still further, the clay will become harder, more brittle, and often deformed. Other clays will melt and become glassy and lavalike, as is so often seen in arch-bricks of an old-fashioned wood-burning kiln.

Argillaceous matter as a whole is divided into two classes, clays and shales. Chemically they are often the same. Physically the shales can be detected by their stratified or laminated structure. They are hard and compact, and require considerable work to prepare them for use. Like the different kinds of granite, clays merge into shales and shales into clays, so that the line separating them. must be an arbitrary one.

Shales must not be confounded with slates, which they very much resemble. Slates have been formed by the action of heat combined with great pressure. They are hard and durable rocks, while shales will rapidly disintegrate when exposed to the action of the atmosphere.

As a rule shales are formed in deeper water than clays. Their