576 Scientific Proceedings, Royal Dublin Society. 
if a fluid assemblage in this critical condition is brought into con- 
tact with a solidified portion of the same assemblage of practically 
the same density which displays one of the two dimorphous forms, 
it will adapt itself to, and become continuous with this form, to 
the exclusion of the other. Indeed a fluid assemblage capable of 
dimorphism, if it is anywhere near the critical condition referred to 
will show this readiness to adopt the arrangement of whichever of 
the two dimorphous forms comes in contact with it in a solid state. 
For in so doing it will bring about a closer-packing at the place 
of junction with this solid portion, than it would do if it were not 
congruent with the latter, even although the result of the con- 
gruence is that the fluid assemblage has not, when taken alone, 
the absolutely elosest-packed arrangement possible to it.’ 
An essential difference in the broad features of the various 
dimorphous changes of which assemblages of balls undergoing 
alteration in size are capable enables us to classify these changes 
under two distinct heads. Thus we have :— 
1. Dimorphous change which consists in the uniform shrinkage 
or expansion or shearing in one or two directions of an assemblage 
taken as a whole, and which is wnaccompanied by any further re- 
arrangement or redistribution of the parts than this involves. 
2. Dimorphous change which consists in a rearrangement or 
redistribution of the parts beyond what can be effected by any mere 
orthogonal projection, or successive projections, or simple shearing of 
the original assemblage.” 
An instance of a change of the nature included under the first 
of these heads would be presented if an assemblage consisting of 
two kinds of balls, or groups of balls, placed respectively at the 
angles and centres of a number of similar parallelopipeda fitted 
together to fill space without interstices in the most symmetrical 
manner possible, became so changed that the two kinds came re- 
spectively to occupy the angles and centres of similar cubes 
1 Aneffect of the kind above described is also capable of being produced by an altera- 
tion in the ratio between the shortest distances separating linked and unlinked centres 
respectively, and the latter kind of dimorphism may be brought about in assemblages 
consisting of a single kind of ball as well as in those composed of more kinds than one, 
if an adequate change of conditions takes place. 
2 Compare p. 590. 
