by the methods proposed, the construction of the correcting lens would certainly demand that great care which Signor Santini considers necessary; for it would require, as in the ordinary construction of an achromatic, a very nice adaptation of the foci and curvatures of the lenses to the refractive and dispersive powers of the glass. But the changes of position, by changing the apertures, and, consequently, the aberrations of the lenses of the correcting glass, give us so great a command over these aberrations, that this construction neither requires a very accurate knowledge of the refraction and dispersion of the glass employed, nor a great nicety in the foci and curvatures of the lenses; and, therefore, it is much easier in practice than the common construction.

This I have verified by actual experiment; having had a telescope made upon this principle, calculating from the average values of refraction, assuming a dispersive ratio somewhat lower than is met with, and making the focus of the concave lens superabundantly short. Having, by these means, given to the correcting lens a surplus of concave spherical aberration, and a surplus of dispersive power, I was enabled, by varying the positions of the lenses, to reduce these surplus aberrations, so as to render the performance of the telescope not inferior to that of an achromatic of the ordinary construction. The object lens is of crown glass, 61 inches focus, and 5 inches aperture; the convex of the correcting lens is also of crown, the radius of each surface 9 inches, and the aperture 3 inches; the concave is of flint glass, the radius of the first surface 9 inches, that of the second surface 10.4 inches; the distance of the correcting lens from the object-glass is 25.1 inches; and the separation of the lenses 0.16 inch. The correcting lens acts as a concave, extending the focus to 64.7 inches. The two convex lenses were originally of plate glass; but they were so veiny and imperfect in other respects, that I had them replaced by two others of crown glass, made by Mr. Dollond, in whose hands the telescope was a short time ago.

The danger of deranging the centering of the lenses, by the minute separation required, does not appear insurmountable by very simple mechanism; two tubes, sliding steadily upon one another by the action of a screw, seem sufficient for the

purpose. One advantage, however, might be obtained by giving up this facility of adjustment; for by making the interior surfaces of the correcting lens to coincide, and by cementing them together, we could prevent all loss of light by reflection from these surfaces. Nor does the construction appear, even with this limitation, so difficult as the ordinary one; since the alteration of one surface only, namely, the second of the concave lens, would be necessary. If the curvature of this surface were too great, the surplus of concave aberration would have to be removed by flattening it, instead of separating the lenses.

Leith, 28th October, 1829.

ALEXANDER ROGERS.

Chemical Examination of a Native Arseniuret of Manganese. By Robert John Kane, Member of the Meath Hospital Medical Society, Dublin.

SOMETIME since I obtained from a person, who had a collection of minerals for sale, a number of specimens of various ores of manganese, amongst which was one ticketed "Manganese Ore, Saxony." It was sold me as a specimen of the native peroxide. It was rather a small piece, about two and a half ounces weight, and being based on a mass of foliated galena, was pierced, in every direction, by small veins of ferruginous quartz. I did not take much notice of it at the time, but some months afterwards, being desirous of comparatively examining the different specimens of the native oxide which I possessed, I exposed, in a tube retort, to a red heat, a few grains of this mineral. No oxygen came over, but I was much surprised to see that a substance rose in vapour, and condensed under the form of brilliant acicular crystals on the inside of the cool portion of the tube; I immediately removed the retort from the fire, and in so doing, it cracked. The external air got admission, and the mineral inflamed, burning with a blue flame, throwing off copious white vapours, and emitting a strongly alliaceous odour.

I was surprised at this phenomenon, as there had been no

native arseniuret of manganese described by mineralogists, therefore I resolved to more thoroughly investigate the subject; and after the most careful study of the nature of this substance which I was competent to effect, I can reconcile the appearances which I observed, to no other supposition than that which I before mentioned.

EXTERNAL CHARACTERS.

The specific gravity of a homogeneous fragment was found

to be 5.55.

Hard, brittle, perpendicular fracture, uneven, fine-grained, brilliant; colour greyish white, growing dull, and becoming covered with a fine blackish powder on exposure to the air; horizontal fracture, dull and mammillary. It is very easily broken in this direction; in fact, the ore seems entirely composed of a series of mammillary laminæ.

Before the blow-pipe, it burns with a bluish flame, and emits a smell similar to that of arsenic, when heated strongly. The greater part of the ore sublimes. Whilst the ore is burning, it throws off white fumes which condense on the cold part of the charcoal, under the form of a white powder. When a platina stand was used, the mineral fused and united with it.

This ore is totally dissolved by nitro-muriatic acid.

When boiled in a large quantity of nitric acid it is entirely dissolved; but when the quantity of acid used is small, the ore is converted into a white powder, soluble in more acid.

When re-agents were applied to a solution of this mineral in nitro-muriatic acid, the following effects were observed :— Alkalies precipitated it white, which gradually passed to brown.

Carbonated alkalies produced a precipitate; the colour of which was more permanent.

Lime-water and

Solution of acetate of lime produced white precipitates, in a portion of the solution which had been rendered neutral. An excess of acid re-dissolved these precipitates.

Tincture of galls did not affect it.

A solution of ferro-cyanite of potassa tinged the liquor of a light-blue, but did not produce any well-formed precipitate. Hydrosulphuret of ammonia precipitated it of a dirty yellowish white.

A portion of the ore having been heated in a glass tube, the sublimed metallic crystals were separated for examination.

They dissolved perfectly in nitric acid, and the solution being concentrated to expel the excess of nitric acid, was found to possess the following properties :

When a current of sulphuretted hydrogen was passed through it, there was produced a yellow precipitate.

When neutralized by potassa, and nitrate of silver added, the well-known brick-red precipitate of arseniate of silver was produced.

With nitrate of copper the neutral solution gave a greenish precipitate; and with hydrosulphuret of ammonia, one of the yellow colour of orpiment, when rendered slightly acid.

After several preliminary trials I resolved upon employing the following process for analytically determining the relative proportions of the constituents.

The ore was digested in nitric acid until it was entirely converted into arseniate of manganese; to it was then added an excess of a solution of potash, and the liquors were boiled for some time longer. The arseniate of manganese was thus decomposed, and the protoxide eliminated, rapidly absorbing oxygen, was converted into the brown deutoxide of that metal, which being dried and weighed, the quantity of manganese was calculated from it.

The liquor thus freed from the manganese was accurately neutralized by nitric acid; then a solution of binacetate of lead was added, as long as any white precipitate was produced. The arseniate of lead, thus formed, was dried, weighed, and the quantity of arsenic deduced from it.

From the result of three different analyses, I was induced to adopt the following numbers as the average :—

But this is somewhat an approximation to the numbers of manganese and of arsenic combined atom to atom; and perhaps the difference depends rather on my unskilfulness than on the real composition of the mineral. The relative equivalents might be so arranged, viz.

A Plan for Improving the Carriage Pavement of the Metropolis. By Lieutenant J. H. Brown, Royal Navy.

HAVING observed the unsafe and disgraceful state of the pavement of most of our public streets, and, even where Macadamizing has been adopted, the repairs continually required, and the constant accumulation of mud and dust, I have given the subject much attention for the last few years, and I am induced to propose a plan to remedy the defect.

I must premise by observing, that the foundations of most of our streets are, of necessity, bad; as, independent of the nature of the soil, the ground has been so often dug up and turned over, for the purpose of levelling, forming and repairing sewers, laying down gas, water-pipes, &c. &c.; that although the paving should be executed in the best manner, it is impossible that it can remain in the position placed; for the water, after a glut of rain, passes down between the joints of the stones, the foundation, composed of earth, rubbish, &c. speedily becomes a puddle, softer in some parts than in others, a great 'portion of which works and churns up between the joints, and allows the stones to sink under the first heavy carriage that