William Phillips

“ Yet having sacrificed very many crystals in the pursuit of this object, I possess several fragments in each direction, having brilliant surfaces. The hardness, and at the same time, the brittleness of the substance were obstacles to the discovery of its natural joints, which it was difficult to find the means of overcoming; but, after resorting to various expedients, I found the employment of a pair of steel pincers the most certain mode of effecting it. Being sharp, with edges about an inch long, they seize on the square prism, equally throughout its whole length ”

“ But if the blade of the knife be not held perfectly level with the direction of the natural joints, it is apt to injure the brilliancy of the plane it produces. The terminations of the crystals being in general most tree from crevices, and most transparent, are therefore best adapted to the purpose I have been describing. [4]
Five very small fragments procured in this manner, yielded by the reflecting goniometer, co-incidences on the obtuse angle of 104°, and one of 76° on the acute angle of the prism; the one being 48 less, the other 48′ more than the measurements obtained by Haüy. ”

“ The perfect agreements afforded by the fragments, have so far satisfied me that the results are the true value of the angles of the primitive rhomboid, that I have not attempted to measure its angles by means of the reflections to be observed on the natural planes, which are often very brilliant; for experience has confirmed me in the opinion that as the natural planes do not often yield results agreeing amongst themselves, they cannot be relied on with the same confidence as the planes obtained by cleaving crystals in the direction of their natural joints, which almost always agree ”

“ The Count de Bournon, for reasons given in his “Catalogue, &c.” (p. 357) considers the primitive crystal to be a right rhomboidal tetrahedral prism, with rhombic bases, of about 78° 30′ and 101° 30′ (fig. 11.) which are about the measurements of P on P, and P over the elongated edge of a crystal represented by fig. 10. He adds that the sulphate of lead is among those substances in which there is no trace of natural joints, no possibility of obtaining a cleavage. ”

“ It must be obvious that very different means have been resorted to; for no one will imagine the same to be applicable to the sulphate of barytes and the sapphire: one of them soft, and yielding to mechanical division with the utmost ease ”

“ Owing to the extreme hardness of the substance, I found it requisite, after placing the specimen in the pincers, to envelope it and them in a piece of cloth, to prevent the escape of the fragments; as the force which it is requisite to use would otherwise have caused them to fly in various directions. The same mode was for similar reasons pursued in regard to the oriental ruby and the sapphire: the latter may be cleaved with the utmost beauty and regularity in one direction; in the others it is difficult.
One of the fragments of corundum yields the measurements o£ 86° 4′ and 93° 56′ ”

“ Crystals of quartz do not often present clear indications of their natural joints. By consulting Haüy, Traité Pl. XL. fig. 3. it will be observed that the primitive rhomboid is so situated in a dodecahedral crystal, that six of the twelve planes of the latter figure are alternately parallel with the primitive planes ”

“ The meeting of two adjacent planes on one pyramid, with their continuous planes on the other, will therefore be at different angles, as is the case also in respect of the primitive octahedron of the carbonate of lead. The angle formed by the meeting of P′ with P′ is according to Hairy 78° 28′, and that of P with P 109° 18′. Six or seven crystals submitted to the reflecting goniometer varied in P' on P from 76° 4′ to 76° 20′. Nine crystals affording 16 measurements of P with P varied from 101° 12′ to 101• 28′ ”

“ On submitting some brilliant crystals to the reflecting goniometer, it became evident that no reliance could be placed on their natural planes. The form of the primitive crystal is an oblique prism. The incidence of M on T, given by Haüy as 106° 6′, varied very much ”