I 100 



MICROCRYSTALLISATION, ETC. 



in the ordinary forms of crystallisable substances, when the aggre- 

 gation of the inorganic particles takes place in the presence 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 struc- 

 tures in the bodies of animals was brought to light by the ingenious 

 researches of Mr. R-ainey, 1 whose method of experimenting essentially 

 consisted in bringing about a slow decomposition of the calcium salts 

 contained in gum-arabic by the agency of potassium hydrogen car- 

 bonate. The result is the formation of spheroidal concretions of calcium 

 carbonate, 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 coalescence of a larger number gives rise to the mulberry-like 

 body shown in fig. 817, b. The particles of such composite spherules 

 appear subsequently to undergo rearrangement 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 larger con- 

 cretions as are shown at a, a> 

 are gradually produced . The 

 structure of these, especially 

 when examined by polarised 

 light, is found to correspond 

 very closely with that of the 

 small calculous concretions 

 which are common in the 

 urine of the horse, and 

 which were at one time 

 supposed to have a matrix 

 of cellular structure. The 

 small calcareous concretions 

 termed otoliths, or ear- 

 stones, found in the audi- 

 tory sacs of fishes, present an 

 arrangement of their par- 

 ticles essentially the same. 



Similar concretionary spheroids have already been mentioned as 

 occurring in the skin of the shrimp and other imperfectly calcified 

 shells of Crustacea ; they occur also in certain imperfect layers of 

 the shells of Mollusca ; and we have a very good example of them 

 in the outer layer of the envelope of what is commonly known as a 

 ' soft egg,' or an ' egg without shell,' the calcareous deposit in the 

 fibrous matting already described being here insufficient to solidify 

 it. In the external layer of an ordinary egg-shell, on the other 

 hand, the concretions have enlarged themselves by the progres- 

 sive accretion of calcareous particles, so as to form a continuous 

 layer, which consists of a series of polygonal plates resembling those 

 of a tessellated pavement. In the solid ' shells ' of the eggs of the 



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. Microsc. Sci. n.s. vol. i. 1801, p. 23. 



FIG. 817. Artificial concretions of 

 carbonate of lime. 



