bodies was, however, the only thing noticed in connection with electricity by the ancients, and hence no more is found respecting it in books of science till the seventeenth century. The strange fact was recorded and duly handed down, but no theory was framed to account for it. An English philosopher was the first to add to our knowledge in this direction, by discovering that other bodies besides amber have this power of attraction and repulsion-such, for example, as the diamond and other precious stones, rock-crystal, glass, sulphur, and resin. From this time the study of electricity began to make real progress.

3. Among the earliest steps in this advance was the discovery of the fact that, if a stick of sealing-wax be rubbed for a short time with a woollen cloth, it will presently attract light bodies, such as morsels of paper or straw, with some force, and then, after a moment, repel them as energetically as it had attracted them. A rod of glass was found to possess the same power, but it needed to be rubbed with silk instead of with wool. This power, whatever it might prove to be, was called electricity, because it had first been observed in amber, which in Greek is called electron.

4. It will help you to understand more about this wonderful subject if I tell you some interesting experiments in connection with it. Let us take a light ball made of the pith of the elder, and having passed a silk thread through it, hang it from the end of a glass tube, where it can move freely. If now we bring the rubbed stick of sealing-wax, of which I have spoken, near it, the ball will instantly fly towards the wax; but the moment it touches it, it will be driven off again just as quickly. If we now try to bring the sealing-wax near it, strange to say, it moves away from it. Indeed, rather than come near the wax, it will fly round in a circle as wide as the thread from which it hangs allows. But if, in the meantime, we touch the ball with our finger, we shall find that this dislike to the scaling-wax, if I may call it so, is removed, and that it now flies to it as readily as it flew from it before. We may repeat this again and again, and the same results will always be noticed.

5. If we try this experiment in the dark, we find what causes such strange effects. If you watch, you will see that at the moment the ball touches the sealing-wax, a small spark of whitish blue flame leaps from the wax to the ball, and that from that instant the ball is driven off from the wax.

6. In explanation of this we are told that the passing of the spark from the sealing-wax shows the passing of a degree of electrical force from it to the ball. Of course this electricity must be of the same kind as that which is in the sealing-wax, and it is in the fact that it is so that the explanation is found of the two-the wax and the ball-forthwith repelling, instead of, as before, attracting each other. This also

explains how the ball is once more drawn to the wax after we have touched it with our finger, for just as it could receive electricity from the wax, it can give off the same electricity again, and this it does, to the finger, when touched by it. But as soon as the ball has thus lost the electricity it received from the wax, it is in the same position towards it as at first, and, therefore, is once more attracted by it.

7. If a glass rod, rubbed with a silk cloth, be used instead of a stick of sealing-wax, the same phenomena will be seen. The ball will be first attracted, then repelled, and then attracted again as soon as it has been deprived of its electricity by contact with the finger. When it has the same electricity in it as that of the glass rod, it will fly from the rod; when this is withdrawn, it will fly to it. From these facts the natural law has been rightly laid down, that-ELECTRICITY OF THE SAME KIND DRIVES ASUNDER, OR REPELS FROM EACH OTHER, BODIES CHARGED WITH IT.

8. A second advance in the knowledge of the science was made by using the stick of sealing-wax and the glass rod at the same time. An entirely new fact came to light. When the sealing-wax was brought near the pith ball, it flew to it, and then presently flew off from it. But, strange to say, the moment the glass rod was brought near it, instead of flying from it, it was at once attracted to it. Thus, what the sealing-wax repelled, the glass attracted.

From this it was naturally concluded that the electricity in the two bodies could not be alike; that that of the sealing-wax must be of one kind, and that of the glass rod another; and this conjecture has been proved, by a wide range of experiments, to be correct. In what the difference between the two kinds consists is not so clear, and, indeed, is still a mystery; but there is no question that there is a difference, and for want of power to do more, it has been distinguished by giving each kind the name of the substance which specially shows it, The one kind is called resinous, the other vitreous or glass electricity. They are also respectively known as NEGATIVE and POSITIVE electricity.

9. If you keep clearly in mind that the pith ball, after it is filled with positive or glass electricity, is repelled by the glass rod, and attracted by the stick of sealing-wax, which is charged with resinous or negative electricity, you will have the reason before you for another great law of electricity, of universal force, that-LIKE ELECTRICITIES

REPEL EACH OTHER, BUT UNLIKE ELECTRICITIES ATTRACT EACH OTHER.

V.-STRUCTURE OF PLANTS.

1. The Vegetable Cell.-If you take a little of the green mould which grows on the remains of food, &c., and put it under the microscope,*

* microscope, Gr. mikros, little, and skopeo, to look at.

you would almost think you had a grove of minute trees before you. Each plant seems to be made of seed-pearls strung together, rising on a single stem, and spreading into little branches, like the trees of jewels in the "Arabian Nights." The least touch, however, shakes all this beauty asunder, and these seeming pearls are seen to be so many little round vessels with a tender skin enclosing something within it. These are what are called cells; and of such cells, at least in certain periods of its growth, every plant is made up. Everything that grows, indeed, is just a multitude of cells, which have been gradually built up from a single mother-cell,— -some of them remaining unchanged, others turned into the different fibres and vessels of the plant.

2. Cells differ in structure from the first. In some, there is a surrounding bag or skin which has chemical powers of its own; but many have no such differently endowed skin, at least for part of their life. In some cells, again, a number of little grains are found which are wanting in many others. All living cells, however, that is, all cells which have yet to do with the growth of the plant-contain a syruplike fluid, to which a name, made up of two Greek words which mean the first principle or the first form, has been given. This name is "protoplasm," which is a hard word, but is the only one in use, else, you may be sure, I would not trouble you with it. It is the first form of the materials out of which the whole plant is developed. You can go back no farther than to it, in trying to find how the life of the plant begins.

3. Only those cells are alive which contain this substance, and no others can grow, or make the different other substances needed by the plant, or form new cells. Cells without it serve only to give strength, like pillars or braces, or as tubes through which water, &c., may circulate, or to give the plant bulk and shape, or to form the protecting shell round it, which we call bark. All living cells, when they have come to their full growth, have gradually built little walls for themselves, out of their own substance, all round and over them, and thus become, as it were, little chambers—sometimes of one shape, sometimes of another, and of very different sizes in different plants. Within these fairy chambers, the wonderful magician, Protoplasm, lives and works.

4. I must now tell you what we know of this mysterious substance. It is a compound of a number of different substances, one of which, however, generally forms the greater part of it, and is never wholly wanting-albumen,* which you will readily imagine to yourselves when I tell you that the white of an egg consists of it. This albumen is also found in the blood of all animals, for it is the principle from

* albumen-the white of eggs (Lat. albus, white).

which flesh is formed. Thus the same substance is, as it were, the vital basis of both plants and living creatures. Protoplasm always contains more or less water, which it absorbs readily, but only to a certain amount, the rest remaining distinct from it in the cell. One might expect that the substance out of which all the organs and vessels of the plant are formed would itself have some traces of both. But the microscope has, as yet, failed to see any trace of either the one or the other. Little specks are found floating in it; and these specks themselves are found, in some plants, to contain other still smaller specks; but beyond this, nothing has been found. It has the power of motion, however, for in some cells, especially when newly formed, it may be seen circulating hither and thither. The size of the larger specks, or kernels, ranges from the 160,000th to the 16,000th part of an inch.

PART II.

1. THE size of cells varies greatly in different plants: except in rare cases, even the largest are scarcely visible to the eye, and most can only be seen by powerful microscopes. As to their shape, it varies so much that I can only refer you to the illustrations, which show a few of their forms out of many. As the cell grows, it begins to form layers within its outer skin, and little pores or ducts open from the centre to the circumference, sometimes in one thin thread, sometimes in branches; and these ducts in different cells often unite, the wall of the cell disappearing that separated them. The final structure of the tubes thus joined is, of course, greatly affected by this, and hence a great many forms of them are found. Some are spiral, some like spindles, and others with many other shapes.

2. The cell-walls consist of a substance called Cellulose, the material out of which cells are finally formed, for the protoplasm turns into cellulose, and this again into other cells. It is by the chemical changes effected by this substance that all the wonderful variety of materials are produced. The cell-walls turn, in part, into wood; in part, when chemically changed in their properties, they are absorbed by the plant for the purposes of growth; while a part, which is not needed, is rejected in the form of gum, &c., so often seen exuding from the bark of trees. 3. The cell is, in fact, the store-house of the plant in which it lays up, till it requires them, all it needs for any of its parts. Besides protoplasm, cells yield water, different gases which the water has absorbed, many substances held in solution in the water, and others in a solid state. It is wonderful how many things are packed away in these next to invisible spaces. All the colours of the leaves and flowers are treasured up in them, and they hide away stores of starch, gum, resin, mineral salts, oil, sugar, albumen, and many things besides,

obtaining them all either by the wonderful chemistry of nature from the simple elements of plant-food, or drinking them in through the roots, in the form of solutions in water. You may think a common plant a very insignificant thing, but there are no greater miracles than the miracle of its growth.

4. A very important element in these cell-stores is what is known as Leaf-green.* It is found in distinct cells, in which it generally floats in the form of small round specks. Its composition is not well-known, but iron is always found in it, which explains why plants growing in a soil that has no iron have not green leaves, and why they begin to have them as soon as iron is added to it. Light, also, is needed, as well as iron, for the formation of leaf-green; for although there are some plants which produce it even in total darkness, and those fitted for shady places require very little light, yet they are the exception; plants growing in darkness, as a rule, remaining white.

5. STARCH is found in the cells even more widely than Leaf-green. The granules+ of it are formed at first within the specks of Leaf-green, but they form walled cells for themselves after a time, growing into a great many different shapes in different plants.

Crystals of different chemical compounds of lime are found in the cells of almost all highly organized plants, and more rarely in those of the lower orders.

ones.

Cells multiply by the division of older ones, or by the formation of new ones within them, but new cells are never made between older When a cell divides into several, new walls are presently formed for each; but when the new cells are formed within the mother-cell, the walls of the mother-cell disappear as those of the daughter-cells grow.

VI.-FOOD OF PLANTS.

1. PLANTS need to be supported by daily food just as much as we ourselves do, but they very seldom get the materials on which they thus live in the state in which they can at once use them. Nearly all that a plant receives comes to it in the form of simple elements, which need to be changed by natural chemistry, in the plant itself, into the various substances required. I have already told you what these substances are let me now briefly tell you the elements from which the plant mysteriously prepares them.

2. Plants require for their different wants a supply of the four gases -carbonic acid, oxygen, nitrogen, and hydrogen-and also of sulphur,

* leaf-green. Chlorophyll, from Gr. chloros, green, and phyllon, a leaf.

† granules. Lat. dim., from granum, a seed.