187 



The principles of the paper are applied to the determination of 

 the pressure of earth against walls, and the power of earth to sustain 

 huildings. The weight of the building which a horizontal bed of 

 earth will sustain, exceeds the weight of the earth displaced by the 

 foundation, in a ratio which is a function of the angle of repose. 



V. " On the Geometrical Isomorphism of Crystals." By HENRY 

 JAMES BROOKE, Esq., F.R.S., Hon. M.C.P.S. &c. Re- 

 ceived June 11, 1856. 



(Abstract.) 



The author commences by remarking that all the crystals at pre- 

 sent known have been divided into the six following groups or 

 systems: the cubic, pyramidal, rhombohedral, prismatic, oblique, 

 anorthic. 



He then states that he has constructed tables which accompany 

 this paper of the minerals comprised in each of these systems, except 

 the cubic, in a manner new, as he believes, to crystallography ; and 

 that the unexpected facts exhibited by the tables present that science 

 under a new aspect. 



The author explains briefly the language and notation he employs 

 in discussing the results of the new tables. 



It appears that the crystals in each system, except the cubic, are 

 distinguished from each other by what are termed their elementary 

 angles, that is by angles between particular faces of what may be 

 termed elementary forms. 



It is next observed that there is not in crystals any natural cha- 

 racter which indicates an elementary or primary form, and it is 

 shown that cleavage which Hauy regarded as such an indication, is 

 only a physical character depending upon the degree of force with 

 which the crystalline particles cohere at the surfaces of particular 

 faces. 



The question of high indices is also considered with reference to 

 their influence on the choice of an elementary or primary form, and 

 a general explanation is given of the nature of such indices. 



The author then states that the most important of the facts pre- 

 sented by these tables, are the horizontal ranges of nearly equal 



