In subsequently repeating these experiments, and examining under polarized light the film thus produced by compression and traction, I was surprised to observe that the streaks and separate lines of the film, as well as the film itself, had regular axes of double refraction, as if they were regularly crystallized portions of the substance under examination. These streaks and capillary lines, which were often of extreme minuteness, did not appear to consist of insulated particles merely dragged into a line; but when the substance possessed the new property in perfection, the lines of polarized light were continuous, and the crystallographic as well as the optical axes of the particles were placed in that line. In other cases, where the experiment was less successful, the insulation of the particles was easily recognised, though the general mass of them was crystallographically arranged.

In making these experiments, the natural crystalline powder, or the particles of the crushed crystal, may be placed either upon a polished glass surface or upon a piece of glass ground on one side. In those cases where the substance is soft, the polished surface is preferable; but when the powder is hard, and considerable pressure necessary, it is better to place it upon the ground surface of a piece of glass, as the particles are detained between its minute elevations, and submit more readily to the combined force of pressure and traction. When the powder is thus placed, I take a polished and elastic knife, and with its broad point I compress and drag the powder in a given direction till there is the appearance of a polished surface on the compressed substance. In general, I have used both the smooth and the rough glass, and have frequently obtained results with the one which were not given by the other.

If we now place the plate of glass in a polarizing microscope with the field dark, we shall find that the streaks and lines produced by traction have, in certain substances, regular neutral and depolarizing axes, as if they were prismatic crystals of the substance under examination. With the chrysammate of magnesia, a red powder with specks of yellow reflected light, the phenomena are peculiarly splendid; the natural colors of the substance, which vary greatly with the thickness of the streaks and films, being combined with the different tints which they polarize. As the crystals of this substance possess unusual reflexion, this property is displayed in the crystallized streaks produced by traction; and the superficial colors which they reflect vary with the azimuth which the plane of incidence forms with the plane passing through the axis of the prism.

The remarkable property which I have now described I have found, in a greater or a less degree, in the following crystals:—

In submitting other crystals to the influence of compression and traction, I have found great numbers which do not exhibit the least trace of transparent streaks and lines, the separate particles being merely dragged into lines, and exhibiting only a quaquaversus polarization. On the other hand, there is another class of crystals whose powders or particles are forced into distinct and transparent streaks and lines, in which the individual particles have a quaquaversus polarization and no trace of a prismatic arrangement. As these crystals have a peculiar relation to those in the preceding list, I shall enumerate the most important of them in the following table; that is, those in which the powder has been dragged into transparent and continnous streaks and lines, resembling externally portions of a solid body; for it is only by a comparison of the physical, or perhaps the chemical qualities of the two classes of bodies, that we can expect to explain the new property which is possessed only by one of them.

As both compression and traction are necessary in producing the transparent streaks and lines in both classes of the substances I have

enumerated, it became interesting to ascertain what effect was produced by each of these forces acting separately, and which of them was chiefly influential in developing the doubly refracting arrangement exhibited by the substances that possessed it.

The force of compression was undoubtedly the agent in forcing the separate particles into optical contact, while that of traction drew them into a line, and tended to dilate the film in the direction of that line, and to draw its particles from each other; or overcome their attraction of aggregation in that direction. It is quite possible, too, that these forces may have exercised some influence in modifying the doubly refracting structure of the substance under examination; but as such a question has no bearing upon our present subject, I have not attempted its solution.

Without expecting any very interesting result, I submitted to examination several of the soft solids which possess double refraction, such as bees' wax, oil of mace, tallow, and almond soap. The last of these substances, though in common use, is a very remarkable one. Owing to its particles not being in optical contact, it has a fine pearly lustre, and may be drawn out into long and slender strings. Upon laying a portion of it on glass, it has a quaquaversus polarizing structure, with a tendency to form circular crystals; but when it is drawn out into strings, and laid upon glass, these strings have neutral and depolarizing axes, like the streaks formed by compression and traction. In the present case it is by traction alone that this crystalline arrangement of the particles is produced.

In oil of mace and tallow a similar effect is produced by compression and traction. With bees' wax the depolarizing lines are still better displayed, and the effect is considerably increased by mixing the bees' wax with a small quantity of rosin.

As the preceding experiments place it beyond a doubt that the optical or crystallographic axes of a number of minute particles are dragged by pressure and traction into the same direction, so as to act upon light like regular crystals, it became interesting to discover the cause of phenomena which certainly could not have been anticipated from any theoretical principle with which we are acquainted. The primary force, and indeed the only apparent one exerted in these experiments, is a mechanical force; but it is not improbable that a secondary force, namely, that of electricity, may be generated by the friction which accompanies the forces of pressure and traction. That such a force is excited with certain crystals will not admit of a doubt; but even if it were developed in every case, this would not prove that electricity was the agent in producing the phenomena under consideration. In subjecting asparagine to compression and traction, I observed, upon placing it in the polarizing microscope, that its particles were moving about under an electrical influence, but in no other case did the same phenomenon present itself to me.

The experiments with soft solids, but especially those made with the almond soap, exclude the supposition that the electricity of friction is the cause of the crystalline arrangement of its particles; though it is not improbable that the sliding of the particles upon one another, as produced by traction, and their mutual separation, as in the case of

tearing asunder mica or paper, may produce enough of electricity to have some share in giving the same direction to the axes of the particles.

When a portion of almond soap is placed upon glass, the axes of its particles lie in every direction, and have no tendency to assume the crystalline arrangement. The forces of aggregation emanating from three rectangular axes, are not strong enough to overcome the inertia, as we may call it, arising from the natural quaquaversus adhesiveness of the substance, and from the water interposed between its particles; but when the portion of soap is drawn out into a thread, these resistances to crystalline arrangement are diminished; elementary prisms, or crystals whose length is greater than their breadth, will have a tendency to place their greatest length in the line of traction; and all lateral obstruction to the play of its natural polarities being to a great extent removed, when the substance is drawn into a capillary thread the molecules will have free scope to assume their natural crystalline arrange

ment.

The application of these views to the powders and particles of hard crystals is not so readily apprehended; but when we consider that the pressure brings the molecules of the substance within the sphere of their polarities, and that the force of traction reduces the compressed film into separate streaks and lines, like the threads of the almond soap, we have reason to conclude, that, even in hard substances, the atoms, when released from their lateral adhesions and brought into narrow lines, will assume the crystalline arrangement.

In the course of these experiments, I have observed in some cases where the crystalline arrangement was very imperfectly effected, a tendency in the atoms to quit their position, as if they were in a state of unnatural constraint, like the particles of silex and manganese in certain kinds of glass which experience a slow decomposition. If this should prove to be the case, either partially or generally, which time only can show, it will doubtless arise from the non-homologous sides of the elementary atoms having come into contact; a condition of the crystalline lines perfectly compatible with the existence of neutral and depolarizing axes, and of the colors of polarized light, provided that the non-homologous sides in contact deviate from their proper position, either 90° or 180°. If we cut a plate of mica, for example, into two pieces, and combine them by turning one of them round 90° or 180°, polarized light transmitted through them perpendicularly will exhibit the same colors as when they were in their natural position, and also the same neutral and depolarizing axes. If the polarized light is transmitted obliquely, the hemitropism of the combination, as we may call it, will be instantly discovered by the difference of color of the two plates.

II. GEOLOGY.

1. On the Gold Fields of Victoria or Port Philip; by H. G. WATHEN, Esq., Mining Engineer, (Quart. Jour. Geol. Soc., vol. ix, p. 74, communicated by P. N. JOHNSON, Esq., F.G.S.)-General Description, Geographical and Geological.-A chain of mountains, or rather a series of distinct ranges, runs round the southeastern corner of Aus

tralia, nearly parallel to the coast line, and from fifty to eighty miles from the sea, forming part of the main chain of the continent, and rising at its highest summit, Mount Kosciusko, to 6500 feet above the sea-level. This mountain chain in Victoria consists of clay-slates, mica-slates, and flinty slates, in successive steps, forming collectively, a recurring series.

The slates are nearly or quite vertical, with a north and south strike, and are intersected by numerous quartz-veins, running at an acute angle with the slates. Vast plains of trap, forming high table-lands, run up to the base of the mountains and probably cover their lower slopes. It is in the valleys and gullies of these mountains, and not very far from their junction with the trappean plains, that the rich deposits of gold are found. The auriferous districts are commonly broken by deep valleys and precipitous steeps. The hills are thickly forested; the soil poor and gravelly, and the surface strewn with angular fragments of white quartz.

Gold-fields. Gold has been found at several points remote from each other along this zone of mountains; but incomparably the richest deposits hitherto opened in the Colony of Victoria, and indeed in the entire continent, are those of Ballarat and Mount Alexander, the latter far exceeding the former in extent and richness, while even the former is said by Californian miners to surpass in richness and yield all that they have witnessed in that region of gold.

Mount Alexander gold-field.-Mount Alexander lies in latitude 37° South, longitude 144° 20′ East, and is about 75 miles north-west of Melbourne. It was named by the first explorers Mount Byng, and is thus distinguished on many maps. It is a rocky granitic mountain, with a rugged flattened outline, towering some hundreds of feet above the summits of the forested ranges of slate-rocks which surround it, and of which it is the centre and nucleus.

The enormous amount of gold which this district has yielded has chiefly been derived from two valleys with their lateral gullies and ravines. These valleys are known by the names of the streams or "creeks" that run through them. One of these, Forest Creek, takes its rise in Mount Alexander itself; the other, Fryer's Creek, has its source in the high and broken ranges of slate that environ the Mount. Both Creeks are tributaries of the River Loddon. The workings extend five or six miles along the valley of Fryer's Creek, and about ten along that of Forest Creek. At Fryer's Creek gold has been found in large quantities beneath the bed of the stream, near its source, in the upland gullies. Forest Creek, on the contrary, appears to grow barren as it approaches the higher granite country, where it originates. On the banks of the River Loddon gold is found in small quantities, lodged in the crevices of the rocks, but no large deposits have been met with on the river, and even the stream into which Forest Creek runs, though itself only a feeder of the Loddon, proves far less rich than Forest Creek and its mountain affluents. In short, it would seem that the gold had been arrested in the small mountain ravines and gullies, and was never washed down to the large streams. Auriferous sands on riverbanks or in alluvial plains are unknown in the Colony. When within 12 inches of the surface, the gold is disseminated in a quartzose