IN THE CURE OF THE STONE. m
In which lime-water, thus expofed, lofes its virtue, will be more or
lefs, according to the proportion which the furface bears to the
quantity of the fluid.
The fcum which lime-water, expofed to the air, throws up, is at
firft an extremely thin pellicle, exhibiting various colours like a
rainbow, or foap-bubble ; thefe colours, however, gradually change,
till, by the confiant appofition of new particles, the fcum becomes
thick enough to reflect all the rays of light equally, and fo appears
white.
Th is cruft which lime-water affords, being well beat, and mixed
with fyrup of violets, and then fome common water added to it,
the mixture, after Handing a little, acquires a green colour. The
parts of this fcum are fo minutely divided, and intimately mixed
with the water when it is firft poured off the lime, as to be abfo-
lutely invifible, and to remain infeparable from it, as long as it is
kept in a clofe veflel. Why they fhould immediately begin to fepa-
rate from the water, and .unite together when expofed to the
open air, may not be eafy to account for. But is not lime-water
rendered weaker by boiling, becaufe its parts are thus more ex¬
pofed to the air, and not by the heat’s expelling any thing out
of it? And does not lime-water, when diftilled in clofe veffels,
lofe fome of its virtue, becaufe its parts, when elevated in the
form of a vapour, are more expofed to the aftion of the air
in the retort and receiver? Is not this rendered probable by Dr
Black’s obfervation, viz. That though magnefia alba which a-
bounds with air, deftroys the virtues of quick-lime and its water,
yet, after it is deprived of its latent air by calcination, it has no
fuch effetft * ?
58. The calcarious matter which feparates from lime-water upon
Its being expofed to the air, can by no art be again diffolved in wa¬
ter, or intimately united with its particles, but perpetually falls to
the bottom, in the form of a white powder, which is altogether
infipid, and void of tafte.
ßbb (1.) This
f DiiTertatio inauguralis, Dc magnefia alba,p. 31. 32. 33, and 37.
