Respecting the primitive rhomboid of carbonate of lime, he has
already communicated to the Society an obsa‘vation, that its angle
is greater by full half a degree than that assigned to it by crystallographers; and he now adds two corresponding observations respecting those substances which are so nearly allied to it.
By employment of the same improved method of measurement by means of the reflective goniometer, he has found that the obtuse angle of the primitive rhomboid of bitter-spar, exceeds that of carbonate of lime by full 1° 10'; and that the corresponding angle of iron—spar exceeds the same angle by nearly 2°, and accordingly is, in fact, 2? greater than former measures had given it.
The angle of carbonate of lime is here said to be 105°, and nearly 5'. That of bitter-spar 106?; that of iron-spar 107°. And since in the last instance the author found the substance under examination to be wholly free from lime, he infers that when the same form occurs in other specimens that do contain carbonate of lime, it does not depend on the presence of that ingredient, but depends on the carbonate of lime alone.
He thinks it, however, possible, that in certain mixtures each of these substances may exert their crystalline powers; and in consequence of the near agreement of their primitive angle, may occasion that degree of curvature of the surfaces which gives the peculiar lustre of what is called pearl-spar.
Among the varieties of these minerals which contain manganese, the author has thought it not improbable that the form of some of them might be altered or modified by its presence; but he has not hitherto succeeded in detecting any other form which could be ascribed to that ingredient.
In the cobra di capello, Mr. Home formerly observed to the Society, that the power which it possesses of elevating its hood, depends on the motion of the ribs of the neck, which have a peculiar form adapted to that purpose. He has lately found that this motion is not, as he then supposed, confined to those ribs alone of that snake, but appears to be common to all the ribs of the whole tribe of snakes.
Mr. Home acknowledges himself indebted to the President, who first remarked an apparent motion of the ribs in succession, like the feet of a caterpillar, in a large coluber, brought for his inspection into his library. And Mr. Home, by placing his hand. underneath the belly of the snake, distinctly felt the ends of the ribs press in succession on the palm of the hand as the animal passed over it.
By examining the skeleton of a large boa, formerly sent from India by Sir William Jones, and now deposited in the Hunterian collection. the structure of the ribs which adapts them for such motion was very evident, and is described by the author with figures, which show a
