CRYSTALLIZATION. - POLARIZATION. 323 



Sulphate of Copper and Magnesia Snlphate of Soda 



-- of Copper and Potass -- of Zinc 



--- of Iron Tartaric Acid 



-- of Iron and Cobalt Tartrate of Soda 



-- of Magnesia Uric Acid 



- of Nickel Urate of Ammonia 



- of Potassa -- of Soda 



It not unfrequently happens that a remarkably-beautiful specimen of 

 Crystallization develops itself, which the observer desires to keep for dis- 

 play. In order to do this successsully, it is necessary to exclude the air; 

 and Mr. "Warrington recommends Castor-oil as the best preservative. A 

 small quantity of this should be poured on the crystallized surface, a 

 gentle warmth applied, and a thin glass cover then laid upon the drop 

 and gradually pressed down; and after the superfluous oil has been re- 

 moved from the margin, a coat of Gold-size or other varnish is to be ap- 

 plied. Although most of the objects furnished by Vegetable and Animal 

 structures, which are advantageously shown by Polarized light, have 

 been already noticed in their appropriate places, it will be useful here to 

 recapitulate the principal, with some additions. 



Vegetable. Spicules of Gorgoniae ( 529) 



Wood, longitudinal sections of, mount- ' Jj * 9 } 



ed in balsam ( 368) Z f 



Animal -- f Shells ( ^ 563 ~ 574 ) 



Animal - -- of Skin ( 670) 



Fibres and Spicules of Sponges ( 510) - of Teeth ( 655, 656) 

 Polypidoms of Hydrozoa ( 521) -- of Tendon, longitudinal ( 668) 



712. Molecular Coalescence. Remarkable modifications are shown in 

 the ordinary forms of crystallizable substances, when the aggregation of 

 the inorganic particles takes place in the presence of certain kinds of or- 

 ganic matter; and a class of facts of great interest in their bearing upon 

 the mode of formation of various calcified structures in the bodies of 

 Animals, was brought to light by the ingenious researches of Mr. Rainey, 1 

 whose method of experimenting essentially consisted in bringing-about a 

 slow decomposition of the salts of Lime contained in Gum-arabic, by the 

 agency of Subcarbonate of Potash. The result is the formation of spher- 

 oidal concretions of Carbonate of Lime, which progressively increase in 

 diameter at the expense of an amorphous deposit which at first inter- 

 venes between them; two such spherules sometimes coalescing to produce 

 ' dumb-bells,' whilst the coalescence of a larger number gives rise to the 

 mulberry-like body shown in Fig. 499, . The particles of such compo- 

 site spherules appear subsequently to undergo re-arrangement according 

 to a definite plan, of which the stages are shown at c and d; and it is 

 upon this plan that the further increase takes place, by which such 



1 See his Treatise " On the Mode of Formation of the Shells of Animals, of 

 Bone, and of several other structures, by a process of Molecular Coalescence, 

 demonstrable in certain artificially-formed products" (1858); and his 'Further 

 Experiments and Observations,' in "Quart. Journ. of Microsc. Sci.,'' N.S. , Vol. i. 

 (1861), p. 28. 



