222 
Dr. R. Kane on a Pseudomorphous 
ving been employed chiefly by the geometers. This explains 
why Boetius introduced that passage into his treatise on geo- 
metry ; and in his treatise on arithmetic, which treats on the 
properties of numbers, no mention of it is made. This latter 
work is indeed only a new version of the treatise by Nicoma- 
chus on the same subject.’’ 
This ought to be compared with what has been stated in 
the number of this Magazine for December, and it will be 
seen that it is quite destructive of M. Libri’s principal argu- 
ment. I may add, in corroboration of the opinion of M. 
Chasles, that Abelard’s tract in the Leyden library is entitled 
de Doctrina Abaci x>el radii Geometrici ; the manuscript itself 
is thus described in the printed catalogue: — “Adolardus, qui 
statim in principio dicitur philosophorum assecla ultimus, de 
doctrina abaci, vel radii geometrici, ut ipse scribit quoque 
vocari. In fine legitur, Regularum abaci nobilis arithmetici 
tractatus explicit feliciterA 
And now a word with M. Libri. When he says, ‘^Si 
V opinion deM. Halliwell avait ete aussi explicite que le pense 
le savant geometre de Chartres, ilsemble qu’on n’aurait pas du 
employer plusieurs pages pour tocher de le prouver,” he had 
forgotten that the plusieurs pages were the produce of his own 
pertinacity. When I had explicitly stated that the Bodleian 
manuscripts indicated a knowledge of the value of local 
position, and that one of them actually made use of the sipos^ 
surely no one could reasonably accuse me of withholding 
my assent from the explanation given by M. Chasles. Much 
less, in that case, could there have been a necessity for occu- 
pying the attention of two meetings of the Institute on a mere 
question of opinion. 
XLI. On a Pseudomorphous variety of Iodide of Potassium 
By Robert Kane, M,D., M.R.LA. 
D uring the crystallization of a large quantity of iodide 
of potassium, in the manufacturing laboratory of Apo- 
thecaries’ Hall, I observed a large group of long prisms to 
be formed, of great lustre and regularity. These prisms were 
in many cases terminated by four-sided pyramids, formed by 
the joining of four rhomboidal' planes, by which the solid 
angles of the prism were replaced ; and I succeeded in ob- 
taining a series of specimens, in some of which the prism w^as 
simple and terminated by a single plane perpendicular to its 
axis ; in others the solid angles were replaced by very minute 
triangular planes, rendering the terminal plane octagonal, 
and finally, as the triangular planes increased in size, square, 
the diagonals of the square being parallel to the sides of the 
