baric sulphate is then filtered off, washed, dried, scraped in a
weighed crucible, filtered, ignited separately, residue added to
main portion, ignited at' a dull red heat for ten minutes, cooled
and weighed. BaS04 x '412 X 70 = grains combined SO4
(sulphion) per gallon.
Estimation of Nitrates. — A very neat method of estimating
the proportion of nitrates and nitrites in waters, is the adoption
of Cram's method for the refraction of nitre, in which process
the determination is deduced from the volume of nitric oxide
gas given off when a concentrated so-
lution of the sample is shaken up with
36 E H3S04 and mercury, in a gradu-
ated tube termed a nitrometer. The
following is the manner in which the
process is carried out : — 500 cc, or more
if necessary, of the sample, are evapo-
rated to dryness in a porcelain basin,
and the residue drenched repeatedly
with small quantities of hot water, the
solution of the nitrates being assisted by
rubbing the residue with the clean finger.
