should be lowered from 190° to 90°, which completes the drying and tempering process.

It is not often that bricks require or receive much care and for so extended a period; but crucibles and retorts demand it, and the common and rapid destruction of them is largely owing to the fact that they do not get it. That this slow process of tempering greatly improves the refractory nature of the article, and that there is great economy in it, have been clearly proved by long experience. From actual experiments in crucible works it has been found. that crucibles produced from exactly the same mixture, and carefully tempered for a period of from seven to nine months, last fully four times as long as those receiving only about two months of tempering.

All of which clearly demonstrates that the older and more carefully all articles of this class are tempered, the longer and more effective will be the period of their use.

An extended and thorough period of drying is highly desirable for many employments of this class of wares in zinc distilleries; as for zinc retorts, the material is simply dried, and not burned in the kiln.

Sometimes a compound composed of 36 granite, 40 white lead, 15 flint, and 5 glass is applied to the one exposed face of the ware, and converted into a glaze.

The unsuitable qualities and careless preparation of clay intended for use in this class of retorts have done much to hinder advancement, and destroy the results of experiments in the production of zinc.

Dr. Isaac Lawson had many drawbacks in his native

country, Scotland, from this cause, in experiments for the manufacture of zinc from calamine; but he finally succeeded in his invention, and by 1737 had it in successful operation in England.

The first zinc manufactured in the United States was produced one century later, in 1838, at the U. S. Arsenal at Washington, D. C., from the red oxide of New Jersey.

The zinc was for the manufacture of the brass designed for use in the standard weights and measures ordered by Congress; but the experiment was discouraging as well as expensive.

The New Jersey Zinc Co., in 1850, regularly commenced its manufacture from the ore; but from the chemical action of the ore upon the clay of the retorts, the Belgian method, which was the first one they adopted, proved a complete failure.

In the franklinite, the oxide of iron, associated with the zinc ores, formed a fusible silicate with the silex of the clay, which was extremely injurious. From about the same cause, in 1856, Matthiessen and Hegeler made as great a failure of the Silesian plan at the works of the Lehigh Co.

The experiments of John Watson, at Camden, N. J., as well as a patent obtained by John Wetherill, of Bethlehem, Penna., miscarried, mainly from the same cause.

Mr. Wetherill finally obtained a mixture of suitable clay, and by very careful preparation, drying, and tempering, upright retorts were constructed, which proved sufficiently refractory.

This stimulated the Lehigh Zinc Co.; they took a new

lease of life, returned to the Belgian furnace, and their works at Bethlehem, Penna., have since been in successful

operation.

The material used in the construction of the arches, as well as walls of large glass ovens, is best produced from the Stourbridge or similar clay, which is carefully shaped into large slabs, and faithfully dried for more than a year; but it is not burned in the kiln.

As the plasticity and shrinking of clay are of importance in this line of pottery productions, probably no better explanation of these properties can be made than that given. by Bischof in regard to the plasticity, and that by Aron in regard to the latter quality, and I shall embody them here:

The plasticity of clay, or its power of yielding with water a mass that may be moulded, is of great importance in a practical point of view, and interesting as a subject of scientific inquiry.

Aluminium hydrate, like silicic acid, is capable of assuming the gelatinous form, in which, owing to the peculiar arrangement of the atoms, these compounds are able to take up a large quantity of water, swelling out to an extraordinary degree, and thus enveloping or binding together sandy or earthy matters in a fine state of division. On removing the water by drying, the original plastic mass shrivels up; this is termed shrinking.

Either on drying in the air, or on burning, the atoms of clay approach one another more closely, the accompanying admixed constituents also at the same time being drawn together.

An increase of density and diminution of bulk thus

occur.

The capacity for absorbing water in different clays varies as greatly as their plasticity, which increases with their power or tendency to crumble (possibly with the formation of aluminium hydrate). Meagre clays readily absorb water, and attain the desired degree of plasticity; "fat" clays, on the contrary, become very friable. The former become softer by working, the fat clays stiffer. Many fat clays exhibit the phenomenon technically known as "water stiffness," i. e., when softened with a certain quantity of water, they have no inclination readily to absorb more.

Shortness or meagreness depends more upon the presence of undisintegrated mineral particles than on that of sand; a clay rich in sand may, however, be fat, but one rich in unreduced mineral matter never can be.

By gradual drying at a temperature increasing to 130°, the weighed portion of clay being placed upon a glass plate, and two parallel marks cut upon it, and the distance between the marks repeatedly measured, it was found that the shrinking did not continue until the clay was quite dry, but ceased before this point was attained.

To a certain point, the shrinking exactly expressed the loss of water; at this point, it suddenly stopped, just as the clay particles came into contact. Aron terms this point the "limit of shrinking," and distinguishes the water dissipated to this point as the "water of shrinking," and that subsequently driven off as "water of porosity." The sum of the two is total water.

The cubical amounts of shrinking of a pasty mass of clay were found to be equal to the volumes of water evaporated. The proportion of pores in the dry clay is constant, i. e., independent of the water originally contained. From the fact that the proportion of pores in several chemically different clays is nearly equal, it may be inferred that the smaller atoms of clay have a regular spherical shape. This view is confirmed by microscopic observations.

In a plastic mass of clay there is thus a vast number of these little spheres at equal distances, suspended in water. The distance between these particles is so small that the attraction between them is considerable, and so a system of capillary tubes is formed, in which the expulsion of water by pressure is so opposed, that neither the power of attraction of the spherical atoms for one another, nor their vertical downward pressure, will permit the water to penetrate through the tubes. Plasticity commences with increase of the distance between clay atoms, and ceases when that increase has attained a certain amount. In shrinking, as water evaporates on the surface, a fresh supply is drawn from the interior of the mass through the fine capillary tubes mentioned above, the particles approximating throughout the whole mass, in obedience to their power of attraction; and this process continues until the atoms come into contact, and then room for water is afforded only in the spaces between the particles (water of porosity). In meagre clays these fine spherical atoms envelop the irregular - shaped particles of foreign matter. On trying the effect of additions of very fine sand to some washed clay, it was found