force diagrams, it may be as well in this place to make some remarks upon their production.

1. Very striking, but somewhat coarse pictures are obtained by using fine iron filings; they are especially suited for demonstration before a large audience.

That the filings may be as black as possible against the white paper, they should be once heated to redness before being used. Part of the iron will in this way be changed into ferro-ferric oxide, which (§ 1) is also susceptible to magnetic influences; at the same time any oil or grease adhering to the filings will be burnt off. The sieve used should be free from iron; for example it may be of porcelain.

2. If the finer features of the diagram are to be studied, ferrum pulveratum should be employed, or the finest iron powder, ferrum redactum. A small linen bag should be filled with this and shaken, when a fine grey powder will be distributed over the paper.

If kept for long the finely powdered iron tends to absorb moisture. It becomes caked together and produces disturbing spots on the diagram. To avoid this it should be heated red hot (for example on a copper plate before the blowpipe). When ground in a mortar it then yields a very fine black powder, which can be quite easily sifted through linen. In no case should it be strewn too thickly over the paper.

The best paper to use is smooth white cartridge paper. If the magnetic forces are very strong the filings rush over the surface of the paper, and stand up in high projecting tufts. In this case it is better to use rough writing-paper, or in some instances even coarse-grained drawing-paper.

For purposes of projection, line-of-force diagrams on glass plates may be used, though they are not to be recommended. They must be brought in a horizontal position into the beam proceeding from the lantern.' If only the picture itself is to be shown, it must be fixed by one of the methods described in the following paragraph.

'The beam, which ordinarily leaves the lantern horizontally, must thus be made to travel vertically through a short part of its course, which is best effected by means of two large totally-reflecting prisms.

To make the diagrams sharp and clear, the sheet of paper should be tapped with a little hammer (made by sticking a cork on to a piece of fish-bone), by which means the particles of iron will be for an instant raised clear of the paper.

If it is desired to tap the paper at particular points, or to hold other points at rest to keep them from being further shaken, a pointed glass rod will be found useful. By tapping in this way the diagram has to be, as it were, hammered into shape.

Other methods which serve to give an idea of the special distribution of magnetic lines of force will be described in connection with the more powerful magnetic forces which are essential to their employment.

7. Fixing the line-of-force diagrams.-For rendering permanent the figures obtained under the influence of a magnet several methods may be employed.

1. Fixing by means of a spray of shellac.-As soon as a clear diagram of lines of force has been obtained, let the paper be sprayed over with a thin solution of white shellac or mastic in alcohol; as soon as the solution has dried, the particles of iron will be held firmly in position. No large drops should be allowed to issue from the spray-diffuser, nor should the spray be turned so as to play directly upon the paper.

2. Transference to gummed paper.-Upon the diagram lay a sheet of paper, thinly and uniformly coated with starch-paste or gum-arabic, and press with a handkerchief or with the ball of the hand; then remove the gummed sheet (PFAUNDLER). The gum should not be too thin, or it will adhere to the lower sheet. It is better, therefore, to allow the surface to dry partially before applying it. If the transferred diagram is intended for purposes of projection, a glass plate must be used, upon which the gum is spread and allowed to become nearly dry; after the transfer has been effected, streaks and bubbles may be easily removed by means of a camel's hair pencil. Figs 1, 3, 5, 12, 13.

3. Fixing by softening the underlay.-The sheets of paper or glass plates on which the diagrams are to be prepared are coated with a solution of shellac and allowed to dry; the so-called negative varnish, which is commonly used for coating gelatine plates, will be found very suitable for the purpose. When the line of

force diagrams have been obtained, a little of the same solution, or even of alcohol, is sprinkled on the plate, and the particles of iron are thus held fast in position. Figs. 10, 11, 15 are photographic reductions of diagrams fixed in this way. On gently warming, the layer of shellac becomes sufficiently transparent for the diagram to be copied photographically by transmitted light (daylight).

Another way is to cover a sheet of paper or glass with a thin layer of paraffin-wax, and after forming the line-of-force diagram to fix by warming.

4. Photographic fixing.-The following method gives results richest in detail, and at the same time best suited for purposes of projection. In the dark-room the gelatine film of a bromide of silver dry-plate is sprinkled over as evenly as possible with the finest iron dust; then the body exerting magnetic influence is laid upon the plate, or placed underneath it, and the edges of the plate are tapped with a glass rod. On lighting the lamp of the dark-room, or by means of the light which comes through the red glass of the window, it is easy to ascertain whether the iron dust has separated into fine and regular lines. The magnet is then removed from the dark-room and illuminated for a short time along with the iron-sprinkled negative. (The light of a lucifer match at a distance of a metre answers perfectly with ordinary plates.) After this exposure the iron dust is blown off the plate, which is then brushed over with a soft dry brush, and developed and fixed in the usual way. The black chains of opaque iron particles appear bright against a dark ground. From the negatives, positives can be printed on paper or on glass, the glass positives giving very satisfactory pictures when projected by means of the lantern. They may be finally intensified' by means of mercuric chloride, so as to increase the contrast. Negatives which are to serve for purposes of projection should be developed so as to have less density than those intended for printing out. Figs. 2, 4, 6, 16, were obtained in this way; while figs. 73, 75, 76, 77, 78 of Section II. were reproduced from photographs, directly taken from figures on paper. In the latter case the camera was supported at the proper height above the paper, with the objective pointing downwards.

Finally, the ordinary method of exposure may be advantageously used, especially for large figures, and with coarser particles of iron dust. The figures are formed on sensitised paper, and after exposure the picture is fixed in the usual way.

8. The line-of-force diagram of a natural magnet.-Fig. 1, which has already been mentioned in § 5, must now be considered more closely.

a. At corners and edges, where experiments 1 and 2 showed the magnetic force to be strongest, the particles of iron adhere closely to the lodestone, having left more distant points to rush to these places. When the magnet is taken from the diagram, these particles are removed along with it, and this explains the light patches a, b, c, d.

B. From these places of greater activity lines of force are seen to proceed. Those proceeding (for example) from the position a do not pass in straight lines from a to c following the shortest course, but are bowed out into arcs, more or less strongly curved, and concentrate again towards c, where they terminate. The same is true of b and d.

7. The lines of force are crowded together in the neighbourhood of a and b, and on their way from a to bare spread further apart. They are therefore densest in the neighbourhood of these places, and are more thinly scattered as we proceed to greater distances; that is, they crowd most closely together in those places where experiment 1 showed the magnetic influence to be strongest.

Accordingly the line-of-force diagrams furnish an indication of the direction and magnitude of the magnetic force.

9. Polarity and pole. From a certain contrariety between the properties of different points at the surface of the magnet, it may be concluded that the influences which proceed from some places re-enter the magnet at other places. This does not apply to the simple attachment of iron filings to the magnet, but to the outgoing and returning of lines of force.

Both kinds of places are called poles (from óλos=axis, Téλ=I turn), like the opposite ends of the axis of rotation of our planet; and the contrariety of properties which they exhibit is called polarity.'

We shall refer to all those parts of a magnet where the issuing (or re-entering) lines of force are most numerous as polar regions, or, more shortly, as poles.

We shall afterwards have to consider more closely the analogous property of the polar regions of our earth; from one of these, magnetic lines of force proceed outwards into space, while at the other the lines pass back into the interior of the earth.

10. Localisation of the poles of a natural magnet.Experiment 4'.-Take several sheets of paper, and in each cut an opening of such shape and size that some particular cross-section of the magnet will just fix within it. Disposing the sheets in different places, but keeping them all horizontal, obtain on each one a figure of lines of force. By comparing these figures one with another, we shall be able to discover how the polar regions are distributed over the magnet.

11. The magnetic field. The entire space surrounding a magnet, as far as the force exerted has any perceptible magnitude, is referred to as the field' of the magnet.

Since for the present at least we are unable to trace the course of the magnetic influence within the substance of the magnet itself, we may consider the field to be confined to the external region. We may also consider it to be limited by a spherical surface enclosing the magnet, and of so great a radius that the magnetic force at all points external to the surface is not perceptible to the tests which we have so far employed. The field occupies the hollow region which lies between this spherical sur. face and the external surface of the magnet, a remark which is of importance in view of subsequent developments. As our means of detecting magnetic influences become more refined, the outer limits of the field become more extended; and strictly speaking there are no limits at all, the field extending indefinitely into space.

12. Course of the lines of force in the field of a natural magnet.—To obtain a picture of the field of a natural magnet, we must combine together the line-of-force diagrams obtained in § 10. Each of them evidently gives no more than a plane sectional view of the field. We remark, then, how bundles of lines of force start from the boundary of the field (surface of the magnet) in certain polar regions, and after becoming more or less widely spread out as they run their course, are once more closely crowded together as