by cleaving crystals in the direction of their natural joints, which almost always agree; when they do not, the cause may always be discovered by the observer.
Diopside.
The crystals of this substance in my collection are not brilliant enough on the natural planes to give perfect rejections. On applying the pincers to one of them parallel with the planes of its prism, I found that it did not yield in that direction, but in that of its diagonals. The only three fragments submitted to the reflecting goniometer agree in affording the measurement of M on M, 87° 5′; being 3′' less than that obtained by Haüy on the natural planes. Two of these fragments also yield the complementary incidence of 92° 55′; being the angle of M on the adjacent plane over the edge A.
The diopside is considered by Haüy to be a variety of the pyroxene. In the attempt to cleave the latter substance, I have not been able to overcome the difficulties it presents. One crystal yielded to the equal pressure of the edges of the pincers, but did not present brilliant surfaces in more than one direction. It may therefore be true of the pyroxene as of many other minerals, that its cleavage is more difficult in one direction than another; but the circumstance just mentioned may perhaps in this instance be attributable to the heat which this crystal had the appearance of having undergone. Two others, of considerable external lustre, fell into powder under the pressure of the pincers. Two crystals presenting clear reflections on the natural planes, gave the incidence of M on M, one 86° 55′, the other 87° 5′. Two others, also brilliant, gave the value of M on the adjacent plane over the edge A, each 93°.
