alts in water, adding potassium chromate, and titrating with N/10
silver nitrate solution.

_Copper_, _lead_, _iron_, _magnesium_, and _calcium_ may also be tested
for in the salts, by ordinary reactions.

_Arsenic_ is best tested for by the Gutzeit method. About 5 c.c. is
placed in a test-tube, a few fragments of granulated zinc free from
arsenic, and 10 c.c. dilute hydrochloric acid added, and the mouth of
the tube covered with a small filter paper, moistened three successive
times with an alcoholic solution of mercury bichloride and dried. After
thirty minutes the filter paper is examined, when a yellow stain will be
observed if arsenic is present.

_Sulphates._--These may be precipitated with barium chloride in acid
solution, in the usual way, dried, ignited, and weighed.

_Sulphites_ give with barium chloride a precipitate soluble in
hydrochloric acid. If the precipitate is well washed with hot water, and
a few drops of iodine solution together with starch paste added, the
presence of sulphites is proved by the gradual disappearance of the blue
starch-iodine compound first formed.

_Thiosulphates_ are detected by precipitating any sulphite and sulphate
with barium chloride, filtering, acidifying, and adding a few drops of
potassium permanganate solution, when in the presence of a mere trace of
thiosulphate, the solution becomes cloudy.

_Sulphides._--Lewkowitsch recommends testing for these by replacing the
mercury bichloride with lead acetate paper in the Gutzeit arsenic test.
Any sulphide causes a blackening of the lead acetate paper.

_Sugars_ may be tested for both before and after inversion, by boiling
with Fehlings' solution, when no reduction should take place, if pure.

_Fatty acids_ are detected by the turbidity they produce when the
diluted glycerine is acidified.

CHAPTER XI.

STATISTICS OF THE SOAP INDUSTRY.

Until the year 1853 the amount of soap produced annually in this country
was readily obtainable from the official returns collected for the
purpose of levying the duty, and the following figures, taken at
intervals of ten years for the half century prior to that date, show the
steady development of the industry during that period:--

_______________________________________________________________
| | | | | |
| Year. | Manufactured. | Consumed. | Exported. | Duty per Ton. |
|_______|_______________|___________|___________|_