4 
244 SOILS, MANURES, &c. 
Boil the filtrated liquor, before de- 
fcribed, till its aqueous parts are nearly 
evaporated; put the remainder into a 
glafs or earthen velfel, and let it hand 
to cool; if any neutral falts are contained 
in it, they will fhoot into cryflals. 
Such foils will require long expofure 
to the atmofphere before they acquire 
fertility. 
Fixed vegetable alkaline falts, and 
calcareous earths, fhould never be placed 
at any confiderable depth from the furface 
of the foil; their falts cannot exhale, but 
have a continual tendency to fink down¬ 
wards, which inclination being promoted 
by rains, the foil would confequently 
foon lofe any benefit it might otherwife 
derive from them. 
Alkaline falts, of all defcriptions, in 
folution, change the purplifh blue colour 
of the tinUure of violets to a green ; 
but 
