358 ESSAYS ann OBSERVATIONS 
to the vitriolum Martis, (e. g. if to a folution 
of two, three or four grains of vitriolum Mar- 
tis in eight ounces of water, be dropt one 
gutt. of o/. vitriol) this, with powder of galls, 
tho’ indeed it changes but flowly, will give 
a blue, not a redifh tin@ure; and again, if 
the acid is weakened, by dropping into this 
blue tincture o/. zartari, the blue will change 
toaredifh. And this conjecture feems to be 
confirmed by the following experiment, viz. 
if vitriol. Mart. be burnt on a hot iron, till it 
becomes quite dry, and of a yellowifh brown, 
and diffolved in water, it gives much the 
fame redifh purple with galls, as before it 
was burnt; if this burnt /a/ Martis be pow- 
dered, and the dry powder wrapt up in a fin- 
gle paper, and left in a room without a fire, 
for three or four weeks, it will become moift, 
and, when diffolved in water, ftrikes with 
galls no more the former colour, but a blue; 
which, I prefume, is owing to the aczdum va- 
gum which it has imbibed. 
SEEING therefore the falt of Hartfell Spaw 
has all the properties of witriolum Martis, 1 
cannot fee why it may not be allowed, that 
this Spaw contains a native, fixed vitriol of i- 
ron: and it isthe only water (I know of) 
which 
