Royal Society. 373 



is, that an excess of magnesia is hurtful to vegetable life, and 

 we find but little magnesia present either in fossil or recent 

 organisms. 



We know, however, that some plants, in which potash is one 

 of the principal inorganic constituents, may, when potash fails, 

 and soda is present or supplied as a manure, take up that alkali 

 which otherwise is not assimilated by them to any degree ; in 

 like manner it may be possible that corals living in water 

 containing both lime and magnesia may, after having exhausted 

 all the lime, supply the deficiency by magnesia. 



LII. Proceedings of Learned Societies. 



ROYAL SOCIETY. 



[Continued from p. 293.] 



June 19, 1856. — The Lord Wrottesley, President, in the Chair. 



n^HE following communications were read : — 

 ■^ "On the Geometrical Isomorphism of Crystals." By Henry 

 James Brooke, Esq., F.R.S., Hon. M.C.P.S. &c. 



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 

 angles, as shown in each system, and the general disagreement in 

 the symbols hitherto assigned to the faces which make with some 

 other face those nearly agreeing angles. 



