which is represented in the annexed Plate under its two principal appearances; that is, having the primitive faces, the predominant ones of the prism; and having the secondary ones such, and which will be fully sufficient to make it known. In the first infancy of the study of crystals, it might be necessary to attend to every, the most trifling, variation of them, to trace each of their changes, step by step, to, as it were, spell the subject; but in the state to which the science has now attained, to continue to do so would be not only superfluous, but most truly puerile.

I have a very small, but very regular, crystal of the form of Fig. 1.

By mensuration the faces a and m appear to form together an angle of about 135°, and the faces c and b an angle of about 125°.

It is said in the account above quoted, that the primitive form of this matter is a rectangular tetraedral prism, but no proofs of this have been offered; nor have the dimensions of this prism been given, a circumstance of the first moment to the determination of true or primitive form, nor have any quantities been assigned to the decrements supposed. I will, therefore, supply these very important omissions.

That the atom of this substance is a rectangular tetraedral prism, is inferable, not from the striæ on the crystals, for striæ are by no means invariably indicative of a decrement in the direction of them; but from the angles which the faces a and c make with the faces m and b, and these angles also prove, that the height of this prism is equal to the side of its base, that is, that it is a cube.

Hence the face a is produced by a decrease of one row of

atoms along the edge of the cube, and the angle it forms with the face m is really of 135°.

The face c is produced by a decrease of two rows of atoms at the corners of the cube, and the angle it forms with the face bis = 125° 15′52′′.

The face b being produced like the face a, forms the same angle with the face m.

No crystal I possess, has enabled me to measure the inclinations of the faces g, d, or f; should the face g, as is presumable, result from a decrease of one row of atoms at the corners of the cube, it will form with the face b, an angle of 144° 44′ 8", and if the faces d and ƒ are, as is also probable, produced by a decrease of two rows of atoms along the edges of the cube, the first will form an angle of 116° 33′ 54′′, and the latter one of 153° 26′ 6′′, with the face m.

The angles assigned here differ considerably from those given in the former account of these crystals; but the angles there given have not only appeared to me to be contradicted by observation, but, crystallographically considered, are inconsistent with each other, as the tetraedral prism of dimensions to produce an angle of 135° by a decrement along its edge, would not afford angles of 140° and 120° by decrements at its corners.

The sum of the faces of these crystals is 50.

IV. On Oxalic Acid. By Thomas Thomson, M. D. F.R. S. Ed. Communicated by Charles Hatchett, Esq. F. R.- S.

OXALIC

Read January 14th, 1808.

XALIC acid, from the united testimony of EHRHART, HERMBSTADT, and WESTRUMB, appears to have been discovered by SCHEELE; but it is to BERGMAN that we are indebted for the first account of its properties. He published his dissertation on it in 1776, and since that time very little has been added to the facts contained in his valuable treatise. Chemists have chiefly directed their attention to the formation of that acid, and much curious and important information has resulted from the experiments of HERMBStadt, Westrumb, BertholLET, FOURCROY, and VAUQUELIN, &c. but the properties of the acid itself, have been rather neglected. My object in the following pages is not to give a complete history of the properties of oxalic acid, but merely to state the result of a set of experiments, undertaken with the view of ascertaining different particulars respecting it, which I conceived to be of importance.

I. Water of Crystallization.

Oxalic acid is usually obtained in transparent prismatic crystals more or less regular; these crystals contain a portion of water, for when moderately heated they effloresce