"to grass," or to the surface. Almost all of them are too much mingled with foreign matter to be at once advantageously treated in the smelting furnace, for the production of metal. They require in the first instance to be washed, picked, broken up, and divided according to their character, both as mineral substances and as regards percentage of metal; and upon this the more or less elaborate processes of dressing, or mechanical preparation, are commenced, which with the majority of ores are indispensable, in order that they may be fitted for the operations of the assayer and the smelter.

9. DRESSING OF ORES.-Över a large portion of the earth's surface, natural causes have been at work to prepare, by mechanical means, certain minerals for the use of man. This is particularly exemplified by the heavy and indestructible substances, gold, platinum, and tin-stone. The upper part of the lodes in which these minerals were at first formed has been worn down, along with the parent rock in which they were encased, by atmospheric action exerted through long geological periods. The fragments have been washed down by winter floods, by streams and rivers, and spread at a distance over the lower land or along the beds of water-courses. The lighter and smaller particles have by this process been carried furthest, and the heavier and larger left nearer to the place of original deposit, or accumulated in cavities, whence the force of the stream has not been powerful enough to wash them

out.

In this manner have two operations to a certain extent been effected: the once solid masses have been broken up into small pieces, and the heavier substances more or less separated from the lighter.

10. When ores are broken from the solid rock by mining, they must be submitted to processes more or less akin to the natural ones above mentioned; they have to be washed from the impurities which prevent their character being seen, to be broken up into small pieces, and separated, by taking advantage of their greater weight, from the refuse with which they are associated. A very great number of ingenious contrivances are

employed for those purposes, but differently carried out, according to the degree of purity in which it is desirable for the subsequent treatment by smelting that the ores of different metals should be obtained. Thus

copper ores need only to be dressed until the resulting mixture contains 5, 6, or 8 parts of copper per cent., whilst tin and lead ores should be so far freed from impurity, that the smelter should receive them containing 60 or 70 parts of metal per cent.

There are some few ores in which several of the useful metals are so intimately mingled, that much greater complexity than usual must be introduced both into the methods of dressing and of smelting.

11. The first process, when the rough material is brought out of the mine, is to wash it with a stream of water, coupled with a plan of sorting it into different sizes, by means of a grate or riddle. The large masses must be spalled or broken up with a sledge-hammer, and the best portions picked out by hand, since it is an axiom that whatever can thus at once be selected as sufficiently pure is saved from great loss in the subsequent processes.

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Next follows the reduction in size of the inferior kinds. Nothing should be broken smaller than absolutely necessary, since all powdered material suffers loss when brought in contact with water. Copper, lead, and zinc ores are generally ground in a crusher," which reduces them to a size varying from that of a pin's head to that of a hazel nut. The machine for this purpose consists of two rollers of hard cast-iron, about two feet in diameter, revolving towards one another, and which can be kept closer or further asunder by a lever urged by a heavy weight.

12, Gold and tin ore, which occur most frequently in very small disseminated particles, require that the stone in which they are enclosed should be brought to a finer powder, and are therefore stamped. The stamps" is a series of beams of wood or bars of iron placed upright, and having at their lower extremity each a "head" of cast iron, weighing from 300 to 500 lbs. These are raised by machinery in regular

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succession, about a foot at a time, and then fall with their full weight upon the mineral which is placed beneath, and which cannot escape from the strong box containing it till it is pounded fine enough to pass through a grating with holes of a regulated size, by aid of a stream of water which continually flows through.

A large "stamps" with 100 such heads in a row, worked by a steam-engine or water-wheel, as in Cornwall or Hungary, produces so continuous a thunder that the visitor must escape to some distance before he can hear any explanation.

Fig. 15 is a small stamps as employed for tin ores. It is only in some particular cases, as where silver ores are to be prepared for amalgamation with mercury, that some other plan is employed to reduce them to impalpable powder.

13. When the ore is thus broken up, and has been by other means sorted, more or less, according to the size

of the grains, the separation of the good from the bad, as it were, of the wheat from the chaff, is effected by taking advantage of the difference in specific gravity.

If the fragments are large, a sieve containing a small quantity at a time, is immersed in water and rapidly jerked up and down; all the contents being thus suspended for a moment, at each stroke the heavier settle first, and the process of jigging thus enables three or four divisions to be made, of which the uppermost layer, or the lightest, is generally discarded at once as refuse. When the grains are small or almost impalpable, as in "slime" ores, they are made to flow with a current of water over an inclined plane, so regulated that the richest or heaviest portion is deposited at the upper end, then the inferior, and lastly the worst at the lower end of the apparatus, and a frequent repetition of such "buddling," or some analogous process, brings the ore at last to the desired degree of purity.-WARINGTON SMYTH.

Vegetable Physiologq.

BY EDWIN LANKESTER.

LESSON I.

ON THE ORGANS OF WHICH PLANTS ARE
COMPOSED.

"And he shall be like a tree planted by the rivers of water, that bringeth forth his fruit in his season; his leaf also shall not wither; and whatsoever he doeth shall prosper."

1. PLANTS and animals are called organic beings, because, they possess parts which are unlike each other, and which, performing different actions in relation to the life of the plant or animal, are called organs. Thus all the higher plants have leaves and a stem which have a distinct structure and perform different functions, and are hence called organs. Even the minutest plants and animals possess this difference of structure in their various parts, and present claims to the term organic. The small plants which consist of a single cell, and

hundreds of which are contained in a drop of water, are often covered with small hair-like organs called cilia, by means of which they move.

2. If we submit any organ or portion of an organ of a plant to the microscope, we shall find that, however uniform may be its external appearance, it consists of several different parts. These parts will at last be found to consist of little hollow spherical bodies, which are called cells. If the organ of an animal be examined in the same way, it will be found to present the same ultimate structure, the parts being all evidently made up of cells. It is the fact, of animals and plants being made up of these minute organs, that constitutes one of the great differences between organic and inorganic bodies.

3. As all the organs of plants are made up of cells, it is necessary, in order that we should understand the nature of these organs, that the structure and nature of these cells should be studied. Although from the time that the microscope was first used, it was seen that the organs of plants were composed of cells, it has not been till very recently that the importance of studying these organs has been recognised. We are indebted to two living German observers, Schleiden and Schwann, for drawing attention to the universal presence of cells as the bases of all parts of plants and animals. Schleiden was the first to investigate this subject, and he has not only shown how every part of a plant is made up of cells, but discovered the origin of these bodies.

In the cuts 4, 5, 6, and 7 of Fig. 1, will be seen a dark spot in the centre of the cells there represented. This spot was observed by our own great countryman Robert Brown, and called a "nucleus," but Schleiden pointed out the fact that this nucleus, or "cytoblast," as he called it, was the first part of the cell which was formed, and that all cells took their origin in this central mass. Although in many cases, and especially in the animal cells, this central part or "endoblast," as it has been termed by Mr. Huxley, assumes the form of a central nucleus-this is not always the case, as perhaps in the majority of instances in plants, it is diffused over the