Extract

In a communication read before the Society about the middle of last year, I detailed some reasons for concluding that the angles of some primitive crystals included in the terms parallelopiped, as well as some varieties of the octohedron, had not been accurately ascertained. Since that time, further attention to the subject has confirmed those observations. I proceed to lay before the Society the results of investigations in regard to ten other substances, two or three of which have been measured by the assistance of the reflecting goniometer only upon their natural planes, on account either of their not yielding to mechanical division with sufficient freedom, or not yielding to it at all. The rest have been fractured with exactness enough to allow the use of that instrument; and for that reason, the results allow of more complete confidence, than if there had been a necessity for relying on their natural planes.

It would have spared me much time and difficulty, if to the other labours of the Abbé Haüy and the Count de Bournon, they had added some account of the means by which the mechanical division of each substance may be most readily attained. Concluding that the same difficulties are felt by others, I shall add some remarks on that subject, in regard to such of the substances as I have been able to cleave with regularity, presuming that it may tend to render the way more easy for those who may desire to attain the same object.