424 



Respecting the primitive rhomboid of carbonate of lime, he has 

 already communicated to the Society an observation, that its angle 

 is greater by full half a degree than that assigned to it by crystallo- 

 graphers ; and he now adds two corresponding observations respect- 

 ing 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 de- 

 pend on the presence of that ingredient, but depends on the carbo- 

 nate of lime alone. 



He thinks it, however, possible, that in certain mixtures each of 

 these substances may exert their crystalline powers ; and in conse- 

 quence 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 as- 

 cribed to that ingredient. 



Observations intended to show that the progressive Motion of Snakes is 

 partly performed by means of the Ribs. By Everard Home, Esq. 

 F.R.S. Read February 27, 1812. [Phil Trans. 1812, p. 163.] 



In the cobra di capello, Mr. Home formerly observed to the So- 

 ciety, that the power which it possesses of elevating its hood, de- 

 pends 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 



