fraction. It is shown best with Iceland spar or transparent calcite. If a crystal of this mineral be placed over a page containing printed characters, each letter will appear double, and if the crystal be revolved, one set of letters will revolve around the other. The two rays which emerge from the doubly refracting crystal are known as the ordinary and the extraordinary ray, and are found to be polarized or set in a plane at right angles to each other. One of these rays may be entirely suppressed and a single polarized ray transmitted by making from a crystal of Iceland spar what is known as a Nicol's prism. A rhombohedron of calcite is sawed through from one obtuse angle to the other, and the surfaces, after being polished, are cemented together with Canada balsam. After refraction one of the rays strikes the balsam surface at an angle greater than the critical angle, and is, therefore, totally reflected, passing out at the side and leaving only the one ray to pass through the prism. Two Nicol's prisms are used together, one as a polarizer for light and the other as an analyzer to examine it. Light polarized by reflection from an inclined mirror may also be examined with a Nicol's prism as an analyzer. Plates or sections cut from the mineral tourmaline also serve for polarizing light, and a pair of these plates is often used to examine objects by polarized light.

3. Applications of Polarized Light.-By the aid of polarized light we are able to distinguish between minerals or salts of different crystallographic systems, and determine to which a given fragment of a crystal may belong. Thin slices of the crystal in question, if examined between two Nicol's prisms or with the tourmaline plates, will show rings of color in case the crystal belong to any other than the regular system.

Those belonging to the tetragonal or hexagonal systems show circular rings on which appears a cross, which is black or white according to the position of the analyzing prism. Such crystals are called uniaxial. Those belonging to the orthorhombic, monoclinic, or triclinic systems show elliptical rings on which appear black or white bands or curves. Such crystals are called biaxial.

Beautiful colors are produced by the action of polarized light, even when the object is not definitely crystallized. Microscopes are frequently provided with a set of two Nicol's prisms, one under the stage and the other in the eye-piece, in order that the delicate structure of objects can be studied under polarized light. Many organic substances, such as sugars, essential oils, and

alkaloids, when in solution show characteristic action upon the ray of polarized light, rotating it through a circle to the right or left. The polariscope is an instrument based on this principle, for examining sugar solutions, and by its aid the strength or purity of all classes of saccharine products may be determined.

It is shown in Fig. 27. The glass tube containing the sugar solution is shown lying in the axis of the telescope and the polarizing prisms. To the right below is shown the polarizing prism; to the left are the analyzing prism, a quartz plate, quartz wedges of opposite rotatory power, and the lenses of the telescope.

CHAPTER V.

MAGNETIC ENERGY.

I. MAGNETS, NATURAL AND ARTIFICIAL.

A CERTAIN black mineral (ferroso-ferric oxide, Fe,O,) was early known to possess the property of attracting iron or steel. It was known as magnes, from the city of Magnesia, in Asia Minor, near which it was first discovered. It was later called lode-stone, because of the property of leading or pointing north and south when freely suspended. Fragments of this mineral are now known as native magnets, and the property thus manifested is called magnetism. Native magnets, however, as a rule, have been replaced in experimental work by the smaller and more convenient artificial magnets of steel. Soft iron can also be magnetized, but, as we will see later, is not adapted for permanent artificial magnets, as it does not retain the power as well as steel. These magnets are commonly known by the designations bar magnets, magnetic needles, and horseshoe magnets. The latter have the convenience that because of their shape the two ends of the magnet are near each other, and can be readily covered by a piece of soft iron, known as an armature, which tends to strengthen and preserve the full magnetic power of the magnet. It is found, moreover, that in a steel magnet the magnetic effect does not penetrate very far, so that several bar or horseshoe magnets, separately magnetized and then riveted together, are more powerful than a single magnet of the same size as the compound one.

II. PROPERTIES OF MAGNETS.

The most important property possessed by the magnet, either natural or artificial, is the power of drawing to it or lifting up small pieces of iron, such as nails or tacks. Iron is not the only metal thus drawn to the magnet. Nickel and cobalt are also attracted, although in a lesser degree. This influence of the magnet is not dependent upon the presence of air, nor is it hindered by the interposition of solids, like wood or glass. The

attraction takes place in vacuo and through non-magnetic solids as readily as under normal conditions.

Polarity. A piece of iron is attracted with unequal intensity by the different parts of a magnet. The two ends possess this power in the highest degree, and the middle of the bar or horseshoe is apparently almost destitute of the power. This is seen to advantage if a bar magnet or needle is dipped into iron filings. Thick bunches of the filings will adhere at either end, while the central part of the bar is practically bare and free from filings. The ends of the magnets, or points in which the magnetic power is concentrated, are called the poles, and the part of the magnet which seems to be destitute of power is called the neutral line. Again, if a magnetic needle or bar magnet be suspended freely, we shall find that one pole will always point to the north and the other to the south. Hence we designate one as the north pole and the other as the south pole of the magnet. On bar magnets they are usually marked N and s, while on the magnetic needle the north pole is usually arrow-pointed.

III. LAWS OF MAGNETISM.

1. Attraction and Repulsion.-If the two poles marked N of two bar magnets be brought close to each other, we find no evidence of any attraction, and if, instead of two bar magnets, we take a freely suspended or oscillating magnetic needle and approach its pointed end with the north pole of a bar magnet, we find an actual repulsion,-the north pole of the needle is driven violently away while the two are yet some distance apart. If, on the other hand, the pole of a magnet marked N be approached by the pole of another magnet marked s, the two are attracted, and on touching hold together strongly. In the case of the oscillating magnetic needle, a pole of a magnet will cause the end of the needle of opposite name to swing violently toward the approaching magnet. These observations are summarized in the law of magnetic attraction: Poles of the same name repel and poles of contrary name attract one another.

2. Location of the Magnetic Power.-When a steel bar or needle is magnetized, even though the magnetism be effected by rubbing it with one pole of a magnet only, it acquires both north and south polarity, and if this bar be broken in two and the division continued until the pieces are quite small, the same condition exists in each of the fragments. These results are very important as giving us some idea of the nature of the magnetic

influence. It undoubtedly resides in the molecules themselves, each of which must be supposed to have been given magnetic polarity when the bar or needle was magnetized. Therefore no mechanical subdivision can rob them of this property.

same time the strong manifestation of the magnetic power at the two ends, and the existence of a neutral line for the time being in the middle of the bar, shows that the force is cumulative in either direction, and it is wanting in the middle of the bar because the forces temporarily neutralize each other there.

3. Magnetic Induction.—A piece of soft iron brought close to a strong magnet is affected by it more strongly than appears in the simple attraction. While under the influence of the original magnet it becomes a magnet itself, and is capable of attracting and repelling another piece of iron, according to the laws of magnetic attraction. This is due to what is termed magnetic induction. The end of the piece of iron next to the inducing magnet is given a polarity the opposite of that possessed by the pole exerting the influence, while the end of the iron farthest from the inducing magnet shows a polarity the same as that of the pole with which the iron is in contact. This induction takes place through glass or paper or other non-magnetic substance, without appreciable loss.

The influence in the case of iron is only temporary, and when detached or removed from the inducing magnet it loses its power at once. In the case of steel, on the other hand, this power is not immediately or wholly lost on detaching it from the inducing magnet. This is due to the fact that while steel has considerable magnetic retentivity, or coercive force, soft iron has very little. Hence permanent magnets are made of steel, while, as we will see later (see p. 97), electro-magnets are made of soft iron.

4. Lines of Magnetic Force.-If a bar magnet be laid upon a horizontal surface like a table and covered by a glass plate or a sheet of card-board, upon sifting over the glass or card-board fine iron filings we will find that they arrange themselves in peculiar curves, as illustrated in Fig. 28. These are known as the lines of magnetic force, and indicate the lines in which the influence of the magnet is felt. It will be noticed that they seem to radiate out from the neighborhood of the two poles, and that the curved lines from the two poles join and make a series of circles enclosing the neutral line of the magnet. These curves thus formed by the iron filings, however, do not indicate that the magnetic force is felt in narrow lines only, but they mark its direction. The entire