SOILS, MANURES, &c. 245 
but all the acid falts change it to a red: 
in order to prove it by experiment, take 
a narrow flip of white paper, dip it into 
the tinfture, and, when dry, apply it to 
the folution, it will immediately difcover, 
by its appearance, whether an acid or 
alkali prevails; if neither the one nor 
the other predominates, the colour of 
the ftained paper will undergo no altera¬ 
tion ; therefore, if the folution contains 
any falts at all, they are neutrals, i. e. 
eonfifting of equal proportions of alkalis 
and acids. 
Of neutral falts there are many, but 
nitre is, perhaps, the moft perfe6t, and 
is fuppofed to be the- belt adapted to all 
the purpofes of vegetation. 
The nitrous acid, according to the 
opinion of chymifts, is always produced 
by the laft ftage of putrefaftion; the 
tobacco plant, however, fee ms to exhibit 
feme 
