Mr . keir. on the 
538 
confiderably more intenfe than is fufficient merely for its 
fulion, and afterwards haftily cooled, lofes all its acquired 
properties, and is again reduced to the ft ate of tranfparent 
brittle glafs, which, however, by means of the evapora- 
tion it has fuftained of its lighter and more volatile parts, 
is rendered conliderably harder, denier, and lefs fufible, 
than it was before the cryftallizatiom 
Many analogous inftances might be adduced to fhew 
how much the properties of bodies depend merely on 
the different arrangements of their integrant parts, or on 
their modes of cryftallization. Thus, for inftance, caft- 
iron and fteel, when cooled fuddenly, acquire a much 
finer grain or texture than when annealed, or flowly 
cooled, and are alfo more hard, elaftic, brittle, and fonor- 
ous. From the above defcription of vitreous cryftals 
we learn, that very different cryftallizations occur in the 
fame kind of fubftance expofed to different circum- 
ftances; and even that fometimes differently-fhaped 
cryftals are found in the fame piece of glafs; in which 
cafe, the circumftances muft have been the fame. Per- 
haps, indeed, the difference, obfervable in the fhape of 
the cryftals in the fame piece of glafs, may only mark 
the different periods in the progrefs of cryftallization; 
for the cryftals reprefented by fig. 6. 7. and 8. which 
are found in the fame piece of glafs a t hofc reprefented 
by fig. 5. do chiefly differ from thef m their altitude; 
and perhaps the latter kind may have oeen compofed of 
a number of the former uniting by leir bales. The 
wheel-like cryftals alfo, fig. 10. feem t j coniift of leveral 
of 
