[ 217 ] 
by the Intervention of fixed air ; and thence, that it 
is very probable, many different chalybeate fprlngs 
fudain their metallic charge by this means only. 
The folution of iron in mineral waters, efpecially 
in' fuch as, by expofure, readily lofe the property of 
ftriking a purple colour with adringent vegetables, 
has ufually been attributed to fome fubtile gas, or vo- 
latile acid. Chymidry, however, docs not difcover 
any acid folvent for iron, but what has greater affi- 
nity with alcalies ; and by means of which, there-' 
fore, this metal will be precipitated. Hence if any 
water appears, with a predominant alcali, which has 
alfo the power of tinging with galls, and, on being 
expofed to the open air, lets fall the iron, and lofes 
that property ; may we not conclude the metal to 
have been fufpended by fome other medium ? 
This, for example, is plainly the cafe in German 
Spa water, which Dr. Brownrigg has proved to 
abound with fixed air. Your own very curious ex- 
periments, before cited, have clearly fhewn, that 
calcarious earths may be fufpended in water by this 
principle of fixed air. And thefe have led me to 
examine, whether iron might not be diffolved by the 
fame natural means. 
I would not, however, be fuppofed to deny, that 
iron is frequently found united with an acid. The 
fad: is fufficiently evinced in the pyrites and vi- 
triolic earths. Nor can I doubt, but that thefe fub- 
ftances do largely contribute to the primary impreg- 
nation of waters, they being fo readily foluble in 
them. But as an alcali, or abforbent earth, is often 
found more than Ijifficient to faturate the acid in mi- 
neral waters ; this would effedually difengage every 
VoL. LIXt F f particle 
