q 
620 Scientific Proceedings, Royal Dublin Society. 
III. Symmetrical intercalation of homogeneous assemblages whose 
forms are identical or appropriately related, comprising for- 
mation of twin assemblages, including under this head the 
symmetrical fitting together of enantiomorphously related 
assemblages as well as that of identical assemblages, forma- 
tion of isomorphous assemblages and their intermixture, and 
the symmetrical interlocking of unlike assemblages. Com- 
parison to crystal-twinning, isomorphism, isogonism and 
crystalloid! structure, also to some kinds of diffusion. 
Hitherto the cases of intermixture of balls of different sizes 
resulting from the principle of closest-packing, with which we have 
dealt, have, with one or two exceptions,’ been cases of homogeneous 
intermixture, 7.e., the resulting assemblages have been made up of 
entirely similar space-units ; we have now to deal with certain more , 
or less symmetrical combinations which result from the action of 
the same principle, but which are not homogeneous, although com- 
posed of portions of assemblages which are so. 
Checked development of symmetry—Accidental twinning. 
When in an assemblage which is of practically uniform compo- 
sition soldification®? commences at two or more points independently, 
and coalescence of the growing nuclei at which solidification is taking 
place occurs before the closest-packed arrangement is fully reached, 
imperfections in the symmetry will present themselves. These 
may be considerable, so that the mass consists of a number of small 
similar homogeneous assemblages which meet quite unconformably, 
or they may be inconsiderable, so that the various fragmentary 
homogeneous assemblages which make up the mass are nearly, but 
not quite sameways orientated, the portion of the mass lying 
between the nuclei which is last to solidify having to accommodate 
its arrangement to some slight irregularities produced through the 
solidification having at some points advanced rather faster than has 
1 Unfortunately the word crystalloid is used in two distinct senses. In these pages 
it always denotes those regular crystalline bodies which are capable of imbibition. 
*See p. 568 and p. 548. 3 See note 1, p. 580. 
