EXAMPLES OP PSEUDO-CRYSTALLIZATION. 



By Professor Liverstdge. 



[Bead before the Boyal Society, 1st December, 1875.] 



The accompanying illustrations represent some peculiar and 

 interesting examples of fracture, which at first sight might be 

 taken for groups of crystals similar to the common dendritic 

 crystallizations of impure oxide of manganese so often met with in 

 the crevices of rocks, upon fossils, ancient stone implements, and 

 other objects, and to the similar crystallizations of impure copper 

 compounds, often found on paper,* both of which are well known 

 and usually spoken of as dendrites, dendritic spots, or by other 

 well-known terms. 



The markings here represented, however, have a widely different 

 origin. They were met with upon the lenses of a field-glass, or, 

 to speak more precisely, between the surfaces of the achromatic 

 combinations of the two object-glasses of a field-glass, which 

 had been lost upon the Liverpool Plains, and there left exposed 

 to the sun and weather for a period of five or six years. The 

 long continued exposure to alternate heat and cold had evidently 

 caused the Canada balsam, or other material used for cementing 

 the crown and flint glass portions of the lenses together, to con- 

 tract and crack along certain lines ; the contraction and conse- 

 quent fractures being due to the loss of turpentine from the 

 balsam by gradual volatilization. 



It will be noticed that the groups of cracks in both object- 

 glasses are somewhat similar — the ramifications start from central 

 spots and radiate outwards, and, omitting the minor branches, it 

 will be seen, in most cases that they assume the form of irregular 

 six-rayed stars, somewhat like the hexagonal crystals presented 

 by snow-flakes and ice crystals. 



In many of the larger branches, and in some of the larger 

 central spots, there will be noticed curved dark lines, which are 

 probably caused by variations in the thickness of the thin film 

 left between the two surfaces of glass, just as in Newton's 

 experiment. This may be due to the balsam having fractured 

 with a conchoidal surface. 



I am indebted to Mr. F. Barton, B.A.., for the opportunity to 

 examine this interesting example of what may be termed pseudo- 

 crystallization ; and to Mr. J. M. Smith, a student in the 

 University Laboratory, for the very fine photographs which I am 

 able to lay before you. Great difficulty was experienced in 



* Dendritic Spots on Paper. — A. Liversidge, Jour. Chem. Soc, London, 

 1872. 



