18 
ON COPPER-PYRITES.. 
The individuals do not invaiiably join in the aggregation 
according to only one of the laws expressed before ; some- 
times more than one exercise their influence at the same time. 
Thus the variety, fig. 41, is a compound of two parts, ac- 
cording to the law I. 1. ; and each of those parts, again, 
according to the law II. 1., acting simultaneously on all the 
terminal edges of P. There are, in the same manner, crys- 
tals to be found, in which I. and II. 1., others, in which 
II. 1. and III. 1., take place at the same time. In such 
cases, the resultant crystallisations commonly lose more or 
less of their regularity from the different relative size of the 
individuals entering into the composition; and it would, 
indeed, be sometimes difficult to point out the law it fol- 
lows, had Nature not, since the streaking of the faces in 
every case is aii effect of combination, and directed parallel 
to its edges, thus provided us with a means, which will al- 
ways lead us the right way, and which, if duly attended to, 
is also of great moment for similar considerations in other 
species. 
