774 POLARIZATION-OBJECTS. MOLECULAR COALESCENCE, 



It not unfrequently happens that a remarkably-beautiful spe- 

 cimen of Crystallization developes itself, which the observer desires 

 to keep for display. In order to do this successfully, it is neces- 

 sary 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 removed from the 

 margin, a coat of Gold-size or other varnish is to be applied. — 

 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. Polyzoaries (§ 445) 



r1 . . , tt . , Q , t Spicules of GorgonijB (§ 426) 



Cuticles, Hairs, and Scales, from Tongues (Palates) of Gasteropoda, 



Leaves (§§ 281, 311) mounted in Balsam (§ 474) 



Fibres of Cotton and Flax Cuttle-fish bone (§ 469) 



Raphides (§ 293) ScaIes of Fishes (§§ 547) 548) 



Spiral cells and vessels (§§ 291, 295) Sections of Egg-shells (§ 599) 



Starch-grains (§292) of Hairs (§§ 551, 552) 



Wood, longitudinal sections of, of Q u iu s (§ 553) 



mounted in balsam (§ 302) , of Horns (§ 554) 



Animal of Shells < §§ 458 ' 467 > 



Ammal - of Skin (§ 560) 



Fibres and Spicules of Sponges of Teeth (§§ 545, 546) 



(§ 406) of Tendon, longitudinal, 



Polypidoms of Hydrozoa (§ 420) (§ 558) 



599. Molecular Coalescence. — Remarkable modifications are 

 shown in the ordinary forms of Crystallizable substances, when 

 the aggregation of the Inorganic particles takes place in the pre- 

 sence of certain kinds of Organic 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, has been 

 brought to light by the ingenious researches of Mr. Rainey, * whose 

 method of experimenting essentially consists 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 for- 

 mation of spheroidal concretions of Carbonate of Lime, which 

 progressively increase in diameter at the expense of an amorphous 

 deposit which at first intervenes between them ; two such spherules 

 sometimes coalescing to produce ' dumb-bells, ' whilst the coales- 

 cence of a larger number gives rise to the mulberry-like body 

 shown in Fig. 412, b. The particles of such composite spherules ap- 

 pear subsequently to undergo re-arrangement according to a definite 

 plan, of which the stages are shown at c and d ; and it is upon 



* 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 Coa- 

 lescence, demonstrable in certain artificially -formed products " (1858) ; 

 and his 'Further Experiments and Observations' in "Quart. Journ. of 

 Microsc. Science," N.S., Vol. i. (1861), p. 23. 



