MR. J. B. DANCER ON CRYSTALS CONTAINING FLUID. 183 



13. Transverse section in the plane of the verticillate medullary radii : a, 



pith ; c, c, woody wedges ; /, /, /, medullary radii. 



14. Portion of fig. 13, more highly magnified : a pith; c, extremities of 



reticulated vessels ; d, d, a. few rows of prosenchymatous cells ; /, /, 

 two medullary radii. 



15. Tangential section of part of a node, close to the pith : c, vascular 



tract below the node ; c', c', two similar tracts above the node ; d, 

 lower extremity of a prosenchymatous tract above the node, with 

 section of the vessels supplying a lateral branch ; d, d, uppermost 

 portions of two prosenchymatous tracts below the node ; /, /, two 

 transversely divided medullary radii. 



16. Diverging vessels of fig. 15 c, more highly magnified: g, g, reticulated 



vessels ; k, k', transverse sections of intercalated prosenchymatous 

 cells. 



17. Diagram illustrating the apparent direction of the vessels in fig. 10: 



a, pith ; b, node ; c, vessels of vascular tract deflected into outward 

 curves on crossing the node ; d, prosenchymatous cells ; e, vessels 

 passing to the surface to supply a branch. 



XI. Some Remarks on Crystals containing Fluid. 

 By J. B. Dancer, F.R.A.S. 



Read before the Microscopical and Natural-History Section, Dec. 2nd, 1867. 



Mineralogists and most microscopists are aware that 

 many years ago it was noticed that certain natural crystals 

 contained fluid pent up in cavities in their interior ; and in 

 the year 1818 Sir David Brewster had his attention acci- 

 dentally directed to this subject by the explosion of a 

 crystal of topaz, which he had exposed to a red heat for 

 the purpose of expelling its colouring-matter. The sudden 

 expansion of the fluid imprisoned in the cavities of this 

 specimen caused it to split into numerous fragments ; and 

 Sir David was then induced to investigate the nature of 

 the fluid, the form of the cavities which contained it, and 

 the arrangement of these cavities in reference to the crys- 

 talline form of the mineral. 



It appears that Sir Humphry Davy was the first to 



