Chromate of Ammonia.

The primary form is an oblique rhombic prism. This may be cleaved in the direction of its two diagonals, but there is not any distinct cleavage parallel to the primary planes.

The planes P are frequently rounded, and

the crystals are very thin.

P on M, or M'.

114° 00'

98

139 4

135 47

Chromate of Soda.

I have not perceived any distinct cleavage in these crystals, which effloresce so rapidly that the surfaces cease to reflect the images of objects almost before the planes can be measured. The primary form is an oblique rhombic prism, the crystals being sometimes considerably lengthened in the direction of the horizontal diagonal.

The primary form of this salt was given by Rome De L'Isle as an octahedron with a rhombic base, which form has been adopted by Haüy, and other writers, evidently without examining the crystals; for even with the common goniometer, the difference of more than 3° between the inclinations of M on M', and e on e', of the annexed fig. 1, might have been readily detected.

On examining some large and bright crystals received from Mr. R. Howard, I observed that two of the four edges of the supposed base of the octahedron were replaced by narrow planes; and on looking through the crystals, I perceived indications of cleavages parallel to the edges that were not replaced. In this direction, they may be very easily cleaved, but I do not find that they yield to cleavage parallel to the replaced edge, or to any of the other

M

Fig. 1.

M

planes. On dissolving and recrystallising this salt, I obtained crystals resembling fig. 2, and others much more reduced in height; some of these are so thin as to leave scarcely a vestige of the planes M and h, and several are hemitropes, the plane of imaginary section being parallel to P. I have, therefore, been induced to consider the primary form an oblique rhombic prism.

Fig. 1 represents the ordinary shape of the crystals.

ARTICLE X.

Description of the Galvanoscope. By the Rev. J. Cumming, MA. FRS. and Professor of Chemistry in the University of Cambridge.

(To the Editor of the Annals of Philosophy.)

MY DEAR SIR, Cambridge, Sept. 17, 1823. I HAVE found the galvanoscope, mentioned in the note of my last communication, so useful in detecting minute electromagnetic action, that I wish it to be more generally known than it seems to be at present; you will, therefore, oblige me by inserting an account of it in the next number of your Annals.

The drawing and description (Plate XXIII), are taken from the first volume of our Cambridge Transactions, with the addition of the mode of neutralizing the needle, which I find preferable to what I then proposed.

Its delicacy is such as to show a deviation of from 20° to 30° by the galvanic action of zinc and copper surfaces not exceeding 1-1600th of an inch. Disks of one inch diameter moistened with spring water, alcohol, or sulphuric ether, give nearly the same deviation. Two wires of silver and platina, each 1-100th inch diameter, and 3 inches long, twisted together at one end, and heated by a spirit lamp, gave a deviation of 90°,

Fig. 1, the galvanoscope.

A K, tubes filled with mercury, to be connected with the galvanic plates.

ABCDEFGHK, a wire placed in a spiral form, round the compass needle n s.

abc, def, brass wires inserted in the galvanoscope, and carrying the sliding wires b g and e h.

ik, lm, the neutralizing magnets attached to the wires b and e h.

g

opqr, a brass wire inserted in the galvanoscope at o, and carrying a small magnet moveable round q r.

The galvanoscope is placed east and west; the compass needle is then brought nearly into the plane of the spiral by the large magnets i k, lm, and the adjustment is completed by the small magnet t v.

It is desirable that the spiral wire should not be less than 1-25th of an inch, and that there should be as little space as possible between the spiral parallelogram and the compass needle.

There should be at least four or five revolutions in the spiral, of which the vertical form, fig. 2, seems preferable to the horizontal, fig. 3, as permitting a better view of the needle.

ARTICLE XI.

Remarks on M. Longchamp's Memoir on the Uncertainty of Chemical Analysis. By Richard Phillips, FRS. L. and E. &c.

In this paper* M. Longchamp has detailed a great number of experiments performed with the intention of ascertaining the cause of the uncertain results which he obtained in analysing sulphuric acid and sulphates, by means of barytic salts. The subject is one of unquestionable importance, and if the experiments detailed by M. Longchamp are accurate, his inferences are just, and chemical analysis is at an end. As, however, all statements which tend to envelope the sciences in uncertainty are productive of mischief, by discouraging their cultivation, I shall endeavour to show that the evils to be apprehended from M. Longchamp's experiments are merely imaginary; and without minutely examining all the details into which M. Longchamp has entered, I think it will appear from his statements respecting the action of the barytic salts upon sulphuric acid, that but little confidence can be placed in his results.

One hundred parts of sulphuric acid of specific gravity 1.812

* Annales de Chimie et de Physique, tom. xxiii, p. 155.

New Series, VOL. VI.

U