agreeable with objects of equal lustre and a greater proportion of orange and reddish tints.

Looking at slides of these kinds can, however, scarcely be termed making experiments, unless their appearance is varied by employing tinted lights for their illumination. Monochromatic lights of course are inadmissible; but when a good beam of white light is mingled with a little of some coloured light, the effect is analogous to putting a transparent wash or glaze over a picture.

It is, however, by the use of polarized light and appropriate crystals that colour studies are best made with the microscope. It is necessary to have, in addition to the usual polarizing and analyzing prisms, a set of selenite films of different thicknesses, to vary the tints. Darker's films well mounted are the best, but another arrangement, devised by Mr. Ackland (Horne and Thornthwaite) will do exceedingly well. This consists in an ebony frame carrying a selenite plate, revolving easily by applying the finger to a toothed wheel. Above this can be placed one of three separately mounted films, marked respectively,,, according to their wave retarding power. Each of these is readily rotated. The microscope should have a rotating stage, and the analyser mounted so as to rotate with facility.

A series of crystals of double refracting substances should be prepared either with colloid silica or a little sugar, or by such manipulation as does not allow the crystals to form until the mother fluid is in a viscid state. The silica is used by dissolving a pure water-glass in distilled water, gently adding hydrochloric acid to unite with the alkali, and allowing the resulting salt to dialize out of a parchment-paper drum, leaving 2 or 3 per cent. of pure colloid silica in solution. Those who are accustomed to chemical operations will know how to do this; others will find it best to get it done for them. Having obtained the silica solution, dissolve the salt to be crystallized in it, and prepare a slide in the usual way over a spirit-lamp, not being satisfied unless complicated patterns are obtained, and a good many tertiary tints obtainable with the polarising apparatus. These slides should be mounted with Canada Balsam.*

Let the experimenter take for a beginning an ordinary shop slide of salicine which, when the polariser and analyser are crossed so as to give a dark field, exhibits the well known circles with strong black and coloured crosses. The effect is not altogether pleasant; though it could not be called discordant,

The sort of patterns to be obtained are shown in the plates to the author's paper on Crystalline Forms modified by Colloid Silica, "Monthly Microscopical Journal," March, 1871.

it is crude. No turning the polariser or analyser gets rid of this crudity, though some improvements may be made. Now introduce selenite on the stage: say Mr. Ackland's construction, using the bottom plate only, so as to get, with the right positions of polariser and analyser, a pale yellowish green tint. Two beams of the cross in the slide before the writer now appear pink, shaded; two others pale blue, and four others green, shaded and varied in tint. This is a great improvement upon the former trials, but the pattern is not a good one, and the colours look too separate to blend. A little pleasant variety is obtained by placing Ackland's selenite over the foundation one of his apparatus. With this combination some flowery patterns adjacent to the big circles and crosses come out with more variety and harmony. We find now contrasts of green or yellow, and red purple, no colour standing too sharply or in too large masses. A salicine slide prepared with colloid silica, and crystallized in elegant little rosettes, being placed in the field shows blue, yellow, orange and red, all looking crude. Revolving the analyser works no satisfactory improvement, but several positions of the polariser introduce harmony, and in some we notice elegant citron greens relieved by purples, pale tints, light yellow, and reddish brown, well disposed. This by another position of the polariser is all made ugly, through the introduction of inharmonious orange red. Beauty is again obtained by a slight left rotation of the bottom selenite on the stage, and we have sage and other greens with purples, violet, and something like primrose, well arranged.

Another richly foliated salicine slide affords amongst other tints fine bluish greys, well harmonized by their surroundings. Cupric sulphate with silica, or by other means made to give spiral forms, rosettes, and patterns something like loosely twisted hanks of wool, affords very fine effects, abounding in tertiary tints. Cupric and magnesic sulphate crystallized together in somewhat similar patterns are also very useful. One before the writer gives, in one position of the polarising apparatus, an ugly contrast of blue, orange and golden brown badly assorted, but which can be instantly changed to sage green and other tints of great beauty by slight motion of the polariser to the left.

With the selenites arranged to give a pale neutral tint ground not differing much from white, a slide of hippuric acid in floral rosettes, affords bluish greys, dark and light; pale greeny greys, and fawns, all subdued tints, exquisitely combined.

Cadmic chloride in detached patterns, like small nosegays of various flowers and leaves, presents an admirable pattern when the apparatus gives a neutral tint ground. By varying the selenites and their position, several fine changes are obtainable.

A nearly black ground, a gros bleu one, and a pale blue, being among the best.

By comparing the effects produced when substances are crystallized so as to afford considerable masses of the same tints, with others in which no single tint occupies a large part of the field, but a considerable variety of tints affect the eye at once whichever part is looked at, will show how much a due proportionment of quantities has to do with artistic results. It will also be noticed that when a good effect is obtained, it will not always bear transposition to the corresponding complementary tints without the beauty being lost, and not unfrequently ugliness replacing it. This is analogous to music, which is very often spoilt by transposition to another key.

It is probable that most of the experiments described could be shown in a lecture-room by the new American lanterns and sufficiently large polarising apparatus, but private study with the microscope could not be dispensed with.

Among minerals which may give good colour lessons and suggest patterns, may be mentioned serpentine, agates, lapis lazuli, manganese crystals in talc, opalized woods, sulphur crystallized from its solution in bisulphate of carbon, &c. &c.

133

SAND-DUNES AND BLOWING SAND.

By W. TOPLEY, F.G.S.,

Assoc. INST. C.E., GEOLOGICAL SURVEY OF ENGLAND.

THE

HE action of the wind and waves upon our coast affords a most interesting field for study. On first looking into the subject, we cannot but be struck with the widely different effects which are produced by similar causes. We see the coast of Holderness yielding to the waves at the rate of from two to three yards per year; a few miles further south, at Spurn Point, we find that the same causes produce an oscillation of land and sea, sometimes the former and sometimes the latter gaining the mastery, but the nett result of which has been to prolong the point to the south; in other places (as at Dungeness) the gain of land is constant and rapid. We say the same causes produce the results, but it is manifest that the surrounding circumstances must vary greatly in the different cases Shingle sometimes acts as a destructive agent, being hurled by the waves against the land; elsewhere it forms a long line of defence, keeping the relative areas of sea and land unchanged; in other places the shingle accumulates, and increases the area of land.

The material with which we are now alone concerned is, at first sight, of little moment. Sand is proverbially weak and unstable, and one might well doubt if, either as a destructive or preserving agent, it could have much effect upon the advance of the sea; but plain facts show us that its effects are most important.

The space between high and low water mark varies very greatly in different parts of the coast, both as to its width and the materials of which it is composed. Sometimes it is formed of rock, sometimes of mud, shingle, or sand. When shingle is accumulating, as also in some beaches which are stationary, the whole slope is generally formed of pebble; but the more common arrangement is for the shingle to occupy only the upper part of the foreshore, generally to about mean sea level; below that

there is rock, mud, or sand. When the sand occurs only in the lower part it rarely becomes quite dry, and the wind has little or no effect upon it. But where sand occupies the whole of the foreshore, then that part at or near high water mark is dry for the greater part of the tide. The wind lifts the particles of sand and rolls them along. Where the coast is cliffy the sand is only blown against the cliff, to be again washed down by the rising tide; but where the shore is low the wind carries the sand beyond the reach of the waves, and there it accumulates: this is the origin of blown sand.

The seaward face of sand-dunes is generally steep. This is sometimes due to the base of the dune being cut away by the waves; but much the same effect is produced on the side facing the wind when the waves do not reach the hills. In no case, however, does the slope exceed 30° with fairly dry and loose sand; although the slope generally appears to the eye to be much greater than this. Dunes sometimes attain a great height. This is due to the sand being rolled by the wind along the surface and up the increasing slope, not to the wind raising the grains of sand to any height above the surface. For this reason quite a small stream of water will often suffice to stop the progress of sand-dunes.

The sand is blown into the the sand so blown is suffidam back the stream, the The fine dust of the desert,

water, but not over it; and unless cient to choke up the channel and sand cannot reach the further side. and of the Pampas, is carried to a great height by the wind; but this is an almost impalpable powder.

We will now describe the coast sand-hills of England, commencing with those on the north-east. The hills are nearly everywhere known as " dunes ;" but in various parts of England they go by other names-as bents, downs, denes, links, greens, towans, and starr-hills; they are often called "warrens," from the numbers of rabbits which inhabit them. The commonest plant of the dunes is the "bent," or "starr-grass "-Arundo (Ammophila) arenaria; the long rhidome of which binds the sand, and greatly checks its movements.

The "links" of the Northumberland coast are often continuous for miles; they are usually only about 150 yards wide, but expand in some places to 300 or 400 yards. The average height of the summits is about 40 feet above mean sea level; but occasionally they rise to 60 or 70 feet. At the mouth of the little river Lyne there is a hill 87 feet high. Where these links occur, the adjacent ground is mostly flat, and the 50-feet contour line is often half a mile or more from the shore. Although these links are of so small a height, yet, rising abruptly from the flat ground, they appear to be hills of no mean elevation. Some species of plants, which occur