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 vel 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 

 tractahis explicit Jeliciter." 



And now a word with M. Libri. When he says, " Si 

 1' 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.I.A. 



TOURING the crystallization of a large quantity of iodide 

 ■'-^ of potassium, in the manufacturing laboratorj' of Apo- 

 thecaries' Hall, I observed a large group of long prisms to 

 be formed, of great lustre and regulai-ity. 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 was 

 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 



