[ 4 85 ] 
point, terminate in a ridge formed by two inclined 
planes, which arife from the oppofite and parallel 
lides of the column. 
Thefe cryftals are perfectly clear, and of a fine 
deep water, and may be preferved in this ftate many 
years, by wrapping them up loofely in paper fo 
as to keep them from being fullied by duft and 
other impurities. In this manner the -cryftals 
now prefented to the Royal Society have been kept 
twenty years ; and in all that time have not parted 
with any of the water of their cryftallization fo as to 
turn white and powdery; neither have they deliquefced 
by attracting the moifture of the air. Indeed none 
of the falts, formed by germination, that I have feen 
are fubjedt to difiolve in a moift air ; on the contrary, 
the falts, fo produced, lhoot moft vigoroufly in a 
clofe and moift air; a certain portion of moifture 
being required in their formation. Some part how- 
ever of this moifture feveral of thofe falts readily 
loofe when expofed for a confiderable time to the 
open air. The Aphronitrum or fal murarim (which is 
a fixed alcali) afforded me an inftance of this kind. 
For, having depurated a, confiderable quantity of 
this fait, and reduced it into rhomboidal cryftals of a 
very regular form ; on examining thefe cryftals, after 
I had kept them ten or twelve years in a phial that 
was corked, though not with great exadtnefs, I then 
found them dry, and in part reduced to powder fo as 
to have loft their tranfparency, and in a good mea- 
fure their proper figure. The green vitriol is alfo 
apt to grow rufty, and to lofe fome of its water in 
the open air, though its acid, when pure, attracts 
moifture more greedily than, perhaps, any other 
faline body. 
The 
