[ *2 ] 
that name ; and -will juft endeavour to illuftrate what* 
I have faid more particularly by theinftance of Briftol 
ftones. In their firft ftate, thefe are of a dirty red, or 
fome other dufky color ; but afterwards, as more falts, 
or cryftalline corpufcles, are added, by the defcent of 
moifture, or the palTage of more vapor, they begin 
to be more compact ; and then, the pores becoming 
fmailer, they approach nearer to transparency, and put 
on a yellow or whitifh color; and at la ft, receiving 
a further addition of frits, and having the com- 
ponent particles drawn ftill clofer together by their 
mutual attraction, they become ftill harder and more 
tranfparent, till they acquire, by a length of years, 
their greateft degree of perfection. 
As to the Briftol ftones being found of fuch 
different flzeg; I am induced to think they grow 
larger or continue fmall, only by the accident of the 
moifture bringing falts to them fafter or flower ; for 
had they any other regular method of growing, I 
think they ought never to be found in their moft 
perfedt ftate, till they had firft acquired their full 
bulk; vyhereas, on the contrary* they are found in 
their gi'eateft degree of perfection of all fizes. 
1 therercrelmagine, that, till the outfide furface is 
.hardened, whatever addition of -falts is made will 
increafe, the bulk of the ftone ; but that, after the 
outward furface is once hardened, the addition of falts 
then only helps to bring it to its moft perfeCt trans- 
parent ftate;. and therefore, that when the flow of 
moifture or vapor, and confequently the addition of 
.falts, is very quick, there (the outward furfaces not 
having time to harden till a confiderable quantity of 
falts are accumulated together) the ftone, will grow 
lunre; 
O ’ 
I 
