354 ESSAYS anp OBSERVATIONS 
ters, yet J am of opinion, that no fuch thing 
exifts in them in a found, natural ftate; and 
that it is never to be found, till the acid, 
or whatever it is, that diffolves the ifon- 
principle, leaves the water, or, at leaft, its u- 
nion with the iron-principle, then it ap- 
pears in the form of an okry fubftance ; but, 
while the diffolvent of the chalybeate princi- 
ple remains united toit, that is, as long as the 
water continues in a found ftate, okry earth 
is never to be found init. The earth, there- 
fore, of this kind, which is left in the filter 
after evaporation and filtration, is owing to 
its diffolvent principle being loft in that pro- 
cefs. AndI have reafon to believe, from ex- 
periments which I have made, that, by re- 
peated folutions, evaporations, and filtrations, 
the pureft and mof genuine cryftals of wi- 
triolum Martis may be all reduced to an okry, 
or bolar earth; efpecially if the folutions be 
made in common water. 
7. THE quantity of mineral principles in 
this water varies ; and is lefs after a dry fea- 
fon, than a wet one (28); an unvfual circum- 
{tance, which probably proceeds from this 
caufe, that the water in a wet feafon, rifes 
higher in the veins, which contain the mine- 
ral 
