as in steam-engine boilers, the whole 1,000° of latent heat being then given off again by the steam, as sensible heat. * 3. Two causes oppose the change of water, when heated, into steam or vapour-first, the force with which its own particles hold together, that is, their cohesion; and, second, the pressure or weight of the atmosphere. It requires water to be heated to 212° before these two hindrances to its turning into vapour are overcome, and its particles, now free to separate, begin to do so. As soon as 212° of heat are reached, bells of vapour will be seen forming on the bottom of the vessel, and rising, by their lightness, to the surface, to which they give the bubbling motion known as boiling. 4. The boiling-point of water has been said to be 212°, but this is at the level of the sea. It is different if we boil it at a great height above the sea, as, for instance, on a high mountain. On the high table-land of Quito, in South America, where you are nearly nine thousand feet above the sea, water boils at 194°, which is not hot enough to boil an egg to hardness. But this is not the lowest heat at which water may be made to boil, for if some be put under a glass vessel from which the air has been drawn as perfectly as possible by an air-pump, it will boil with the heat of the hand! Let me explain how these things come about. 5. As is shown in paragraph third, water, when heated, has to overcome two hindrances before it can turn into vapour, one of these being the pressure, or weight, of the atmosphere. Now, as you ascend above the level of the sea, there is, of course, less air above you, and less pressure or weight from it, on water, or anything else. If, therefore, you heat water at such an elevation, it has less of such weight or pressure to overcome before it can turn into vapour, and, so, less heat is required to make it do so. So great, indeed, is the part which the weight of the atmosphere plays in our producing the change of water into vapour, that is, its boiling, that if this pressure or weight be removed by an air-pump, even the heat of the hand is enough, as we have seen, to overcome the other hindrance, that is, the cohesion of the particles of which water consists. PHYSICS.-MAGNETISM. • ATTRACTION, REPULSION, AND POLARITY, AS ILLUSTRATED BY THE MAGNET. 1. The word magnet is derived from the word magnesia, which was the name given to an earth found at Magnesia, in Lydia, Asia * From Latin con, together, and hærio, to stick. Minor, thought by the ancients to have the power to attract, like a magnet, various principles from the air, when exposed to it. Magnets also got the name of Lydian stone from this locality, and, of this name, load-stone, a word still used at times instead of magnet, is thought by many to be a corruption; though others think it comes from a Saxon word, ladan, to lead. 2. All bodies are held together by what is called the mutual attraction of the particles of which they consist, that is, by a force in each which draws it to the others, and the others to it. In most things, this attraction is equally powerful in whatever position the particles be to each other, as in the case of a liquid, the particles of which show as much attraction for each other, that is, joir. together as thoroughly, however we may stir and confuse them. It does not matter which sides of them come together, the attraction is always equally perfect. 3. Besides attraction, however, there is another force of the very opposite nature, known as repulsion, from the fact that the particles of bodies, instead of being drawn and held together by it, are repelled, or driven apart, or back from each other by it. It is seen in the expansion of air by heat, which is just a tendency given to the particles of the air to fly off and keep apart from each other; or it may be seen when water is converted by heat into steam, which is just the loosening the particles of the water from each other, and a tendency given to them, in the same way, to fly apart. 4. We do not, however, find the attraction and repulsion of the particles of bodies the same in all cases. The particles of liquids attract each other equally, and those of gases repel each other equally, whatever be their position to each other; but there are other cases in which one side of each atom repels and the other attracts. We can never bring the particles of water into such positions to each other that they will not cohere, that is, cleave together, at once; but if we could take any two particles of a crystallized body, say of the crystals of alum, or of sugar-candy, or of ice, and turn them the reverse way to each other from that in which they join of their own accord, we should find that, instead of attracting, they violently repelled each other, and would not join together at all. Hence, if you stir a body while it is crystallizing, you destroy its power to do so. You cannot do with it as with water or other liquids. The one end of the atoms of a crystal, if we may so speak, has an attracting force; the other end a repulsive; and to this the name of polar force is given; because it is a force residing in the poles, or opposite ends of the crystal. 5. This will enable you to understand the characteristics of magnetism. There is a kind of iron ore, found in many countries, which has the property of attracting iron in a remarkable degree. In Sweden it exists so plentifully that mines of it are worked. If this magnetic iron be rubbed in a certain way over a piece of steel, its peculiar magnetic properties will be communicated to the steel, which will thus become an artificial magnet. When the piece of steel is long and thin, like a needle, it is called a magnetic needle, which is the form in which magnetism is made use of in the mariner's compass. 6. If you dip a magnet into iron filings, it will be found that they will cling in great numbers to the two ends, that there will be fewer and fewer towards the middle, and none at all in the very centre. Thus, a magnet has always two poles or points, in which all its power seems to lodge. It is no matter what shaps the magnet may be, these two poles are always found in it; and it is the same even with the smallest fragment of a magnet, if you break it. 7. If the south pole of a magnet be brought near the north pole of another, it will be instantly attracted to it, so that it will cling to it; but if a south pole be brought near another south pole, they will repel each other. Thus it is a law of magnetism that like poles repel or drive away each other, and that unlike poles attract each other. The meaning of the north pole and the south pole of a magnet is that—when a magnet is so balanced that it is free to turn horizontally on its centre,-as, for instance, in the case of a magnetic needle in a compass,-the one end will turn always to the north, and the other end always to the south. On this account the end that always points northward is called the north pole of the magnet, while that which always points south is called its south pole. This property is called polarity. 8. It is this law which has given us the mariner's compass, by which the direction of any place may be found out at sea, or anywhere on land, though there be nothing besides to tell it. If the north pole of the magnetic needle always point out the north, it is easy for a sailor to tell in what direction his ship is sailing, or for a traveller to guide himself through a pathless desert, or forest, by its help. If it be asked how the earth attracts what we call the north pole of the needle towards the north, and what we call the south pole towards the south, the explanation is that the earth is full of magnetic force, and acts on the needle as if it were itself a great magnet. But as the magnetic north pole of the earth really attracts the south pole of the needle, what we call its north pole should properly be called its south pole,* or else the northern geographical pole of the earth, that is, the pole marked in the north of our maps, should be called the southern magnetic pole. The whole earth is, in fact, as if a great magnetic ocean were flowing in currents of magnetic force nearly from south to north, through its solid body. But it must not be thought, though it is thus spoken of as like an ocean with currents, that magnetism is a substance; it is only a mysterious force or power, which passes through any body whatever, without any diminution of its force. Thus, thick plates of glass, shellac, metals, wood, etc., may be placed between a magnet and a suspended needle, without the least alteration that can be seen, in what would have been the magnetic force, at the same distance, if there had been nothing between them. It is, thus, a force, not a substance, but further than saying this we can hardly go, so little do we yet know about it. The streams of magnetic force which pulse through the earth do not come together at the Pole exactly, but at a point which moves in the course of years. Thus in 1660, the magnetic needle pointed due north and south, but from that date it began to pass to the west of the north pole, till in 1818, it had reached twenty-four and a half degrees west of it. Since then it has been slowly coming east again. There is much more magnetic force in the earth in some parts than in others. Thus, a magnetic needle, left free to move as it likes, will be found in places at the same distance from the North Pole as London is, to bend downwards towards the earth, very much at its north end. This is called the dip or inclination of the needle, and marks the direction in which the force of terrestrial magnetism, that is, of the magnetism of the earth, acts most strongly. At the equator, there is no dip, or very little, the needle remaining nearly or quite horizontal or level; but it increases as you go from the equator, till, at the magnetic pole, it draws the needle quite perpendicular. This, you will see, is just like the strong magnetic power of ordinary magnets, which shows itself strong at the ends or poles, and is wanting at the centre, as the experiment with the steel filings, mentioned in paragraph six, showed. 9. To the magnetism of the earth we owe not only the mariner's compass, but also the electric telegraph, which, though called * Continental writers do not apply the names north and south to the poles of the magnet as the English do, but very properly reverse the use. electric, is really electro-magnetic; that is, it is made possible only by the intimate connection between electricity and magnetism, by which an electric battery can make a wire magnetic, when joined to it, the magnetism ceasing when the wire is disconnected from it. A fuller explanation will be given in a subsequent Reader. 10. It was formerly thought that iron, and a few other bodies, such as metal and cobalt, were the only ones affected by a magnet; but Faraday has shown that every body, without exception, is subject more or less to the magnetic influence. Magnetism is a mysterious force, pervading and affecting all things, and its laws aro only being slowly revealed by the labours of scientific men. THE DEATH-BED.-Hood. WE watch'd her breathing thro' the night, So silently we seem'd to speak, So slowly moved about, As we had lent her half our powers Our very hopes belied our fears, We thought her dying when she slept, For when the morn came dim and sad, Her quiet eyelids closed—she had |