tallic oxyds only when the nascent potassium or sodium is at hand to combine with a part of the oxyd not yet reduced. Pure potash and soda, the properties of which are almost unknown to us, do not appear to form at that moment. And as alumina can very readily combine with alkalies to form an aluminate, it should be inferred, that the reduction of alumina through the alkaline metals might succeed in the end.

But even if it should become possible to obtain aluminium directly from alumina, still cryolite may for a long time be employed for the purpose, unless its price should rise immoderately. Nature furnishes this substance in a state of rare purity, the aluminium is combined in it only with fluorine and sodium-two substances which cannot act injuriously during the production of the metal Clay or aluminous earth is however seldom found in a pure state, and always of great density. To reduce aluminous earth in large quantities from its compounds and to purify it from substances which could act injuriously during the production of aluminium-would be attended with great difficulties.

The globules of aluminium obtained by me are for the most part so extensible, that they may be flattened very considerably and rolled into the thinnest sheets without their showing fissures on the sides. They have at the same time a strong metallic lustre. On the contrary some few masses, found on the bottom of the crucible and sometimes adhering to it and which had no globular form, showed fissures when rolled, and differed somewhat in color as well as in lustre. They evidently were not as pure as the great majority of the globules, and probably contain some iron.

A larger globule of aluminium of 38 grm. weight having been sawn in two, it could be plainly seen that the metal was brittle for about half a line from the outside; but inside it was soft and pliable. Sometimes excavations are found in the interior of the globules.

According to the observations of Déville I happened also to obtain the aluminium in crystalline form. One of the greater globules became in cooling radiated with crystals on its under surface. Déville believes he has obtained regular octahedral crystals, but does not however assert this positively. According to the investigations of my brother, the crystalline structure does not appear to belong to the regular system.

Trying to melt a larger globule accidentally contaminated after being rolled without flux, before the heat rose so as to melt the whole, small globules went floating on the surface. The impure aluminium is less fusible than the pure which is mixed with it, expands in melting and rises out of the mass not yet fused. It is a phenomenon similar to that observed by Mr. Schneider with impure bismuth.

I have stated that the cryolite has been employed here in Berlin under the name of mineral soda for the preparation of caustic soda-ley, which owing to its aluminous earth appears eminently adapted to the manufacture of soap. In fact the pulverized cryolite is decomposed entirely if boiled in this condition with caustic lime and water. The fluorid of calcium thus generated contains no aluminous earth, this being entirely dissolved in the hydrate of soda, which again is free from fluorine or shows but a trace of it.

ART. XXI.-On a new Species of Unio; by T. A. CONRAD.

Unio diversus.

Trapezoidal, ventricose, inflated posteriorly, substance of shell generally thin, thicker anteriorly and thickest or somewhat callous towards the base; valves contracted from beak to base, posterior margin obliquely truncated, rectilinear, ligament and basal margins parallel; posterior extremity obtusely rounded; basal margin contracted; umbonal slope rounded; beaks decorticated; lines of growth profound'; epidermis yellowish olive

clouded with dark brown; rays obsolete or wanting; within greenish or wax-colored; dirty white towards the anterior base; cardinal tooth in right valve compressed, oblique, crested, prominent; in the opposite valve 3-lobed, the posterior lobe opposite the apex, middle lobe small or obsolete; lateral teeth straight, reversed.

Inhabits Shoal Creek, N. Alabama. Prof. Thomas P. Hatch. Remarkable for its resemblance to U. heterodon, Lea, and like that species having the double lateral tooth in the right valve. The cardinal tooth of the left valve has the same extended character as the heterodon, and I think these two species will be found to constitute a distinct subgenus when the animals have been compared.

U. viridis, Raf. Perhaps it may interest conchologists to learn that this species inhabits the Schuylkill river, at Phoenixville, where I found three specimens in a very limited time; and some years since I procured a number of living specimens in the Delaware, opposite Trenton, just below the falls.

ART. XXII.-Observations on Binocular Vision; by Professor WILLIAM B. ROGERS.

(Concluded from page 95.)

29. Of the form of the curve resulting from the binocular union of a straight line with a circular arc or of two equal circular arcs with one another.

A. Binocular resultant of a straight line and a circular arc. Assuming the optical centres of the two eyes L and R, figs.

74 and 75, as fixed during the act of combination, it is evident that the centre of the eye directed to the circular arc ab or A B may be regarded as the vertex of a cone whose surface includes all the positions of the optical axis of that eye as successively directed to the different points of the arc. This cone will of course be right or oblique according to the direction in relation to the plane of the paper, of the line joining the optical centre with the centre of the circle of which the arc is a part. The axis of the other eye in ranging from end to end of the vertical line cd or CD vibrates in a plane RCD which during the binocular combination intersects the conical sur

h

74.

m

N

D

R

face, in an attitude depending on the distance between the optical centres, the place of the diagram, and the position of the component lines, a b, cd... or A B, CD.

The two optical axes directed each moment to corresponding points of the vertical line and the arc, as m, n.... a, c... b, d or M, N .... A, C .... B, D, &c., meet in the conical surface, forming optically a series of resultant points, v, s, r, &c., which together constitute the binocular resultant curve. This curve must therefore be a conic section, the nature of which will depend on the direction of the cutting plane in reference to the conical surface. The effects of the several conditions of the experiment will be seen more clearly by considering separately each of the following cases which taken together include all the variations that can occur.

First. When the arc is convex towards the right line and the two are combined by directing the optic axes beyond the plane of the diagram.

These conditions are represented in the upper part of fig. 74. Here the arc a b and right line cd have for their binocular resultant the curve r vs. Since the points m and n unite optically at a less distance behind the diagram than any other pair of corresponding points in ab and cd, it follows that the vertex v in which they combine must be the point of the resultant curve, nearest to the observer, and as the curve lies wholly in the plane RCD it must therefore present its convexity obliquely forwards. According to the proportions assumed in the figure, the line RvN is more steeply inclined than the line Lh to the base of the cone, and in these conditions therefore the curve rvs is an hyperbola. But by placing a b and c d a little nearer one another we may cause RN to become parallel to Lh, in which arrangement the resultant will be a parabola; and if we bring a band cd still nearer together so as to make RN converge downwards towards Lh, we transform the curve rvs, into an arc of an ellipse. In the conditions included in the first case therefore the binocular resultant may have the form of either of the curves just mentioned.

Second. When the circular arc is concave towards the right line, and the two are united in front of the plane of the diagram.

This case is represented in the lower part of fig. 74. Here the component lines are the circular arc A B and the right line CD, which by cross-vision are made to unite in front of the plane in which they are placed in the experiment. The resultant curve rvs will evidently vary in form according to the distance between A B and C D. As shown in the figure this curve is an hyperbola, but by increasing the interval between A B and CD it may be converted into a parabola or into the arc of an ellipse. Thus in the conditions of the second case also the binocular resultant may have the form of either of these curves.

Third. When the circular arc is concave towards the right line and the two are binocularly combined behind the plane of the drawing.

The combination here specified is shewn in the upper part of fig. 75. In this case the vertex of the resultant curve rvs being formed by the optical union of the two points m and n, of the component lines which are farthest apart, must be at a greater distance behind the plane of these lines than any other point of the resultant; and since the curve rvs lies entirely in the plane of RCD it must always turn its convexity obliquely away from the observer. As the optical conditions here supposed require that R n produced shall intersect Lm produced, it follows that the plane Red when extended will pass entirely through the cone. In this case the resultant curve can never be either a hyperbola or parabola but must be an elliptic arc, varying in form according to the interval between a b and cd. Where the visual cone is

oblique as is most likely to happen, the curve rvs will of course become an arc of a circle whenever the cutting plane takes the position of the sub-contrary section.

Fourth. When the circular arc is convex towards the right line and the two are combined in front of the plane of the dia

gram.

The conditions here referred to are exhibited in the lower part of fig. 75. In this case

A B and C D are the L component arc and right line, and rvs is their binocular resultant formed by cross-vision in front of the plane in which they are presented to the observer. Since in this mode of combination the optic axes are required, for all points of the resultant, to intersect somewhere between the plane of AB CD and the eyes, it is c evident that the plane RCD must pass entirely through the cone. Hence the resultant curve rvs

must be an arc of an

M

B

75.

ellipse. As in the preceding case the form of the ellipse will vary with the distance between A B and C D, and it will become circular in the position of the sub-contrary section.

These various effects of the binocular union of a right line with a circular arc may be thus summed up.

(a.) When the arc is convex to the right line and the union is effected beyond the plane of the diagram, or when the arc is concave to the line and they are combined in front of the diagram, the binocular resultant may be either an ellipse, a parabola, or an hyperbola, but in either case it will turn its convexity obliquely towards the observer.

(b.) When the arc is concave to the right line, and they are united beyond the plane of the diagram, or where it is convex to the line and they are combined in front of the diagram the binocular resultant is always an arc of an ellipse turning its convexity obliquely away from the observer.

B. Binocular resultant of two circular arcs.

In this as in the preceding combinations the optical centres are to be regarded as immoveable during the experiment. Each eye