the total quantity of the assay solution in c. cm.; the titer will be

if hydrated peroxide of iron has been used as the indicator, or

if

paper saturated with ferric chloride (p. 209) has been used.

In titrating the samples, one-third to one-half of the approximate quantity of the precipitating agent required should be added every time to the solution and this well shaken. A drop of solution of ferric chloride is added to the zinc solution from a small pipette. The mass of hydrated peroxide of iron which is formed is broken up into as equally sized flakes of 1 to 1.5 millimeters diameter (0.039 to 0.059 inch) as possible by vigorously swinging the flask to and fro. This swinging is constantly continued while solution of sodium sulphide is added by cubic centimeters, until the color of the flakes commences to change. A minute is then allowed for the reaction of the particles of liquid which have remained ineffective, and the precipitation is then finished by adding sodium sulphide drop by drop. If Vis the quantity of the precipitating agent consumed in c.cm., 7' the titer, M the total quantity of the solution in c.cm. after the assay is finished, the percentage of zinc when ferric hydrate has been used as indicator, will be

when slips of paper saturated with ferric chloride have been used. When the first plan is employed the titration must be continued, in order to ascertain the titer, until the flakes become entirely black, but, in titrating solutions of ore, only until the reddish-brown color has passed into a greenish tint. The following conditions are required for obtaining a sharp reaction: the flakes must be nearly of the same size, the flask should be carefully swung to and fro to prevent the flakes from being broken up any further; not too much of the precipitating agent must be added at one time, and it should not fall directly upon the flakes, but run down the sides of the flask; the titration should be done at the ordinary temperature, whereby the reaction appears more gradually and uniformly, but the precaution must at the same time be observed of adding the standard solution at longer intervals; uniformity of quantity and time in treating the assays; judging the tone of color by reflected light; and finally the flake reaction should be controlled by a drop test.

If it is required to determine the amount of over 0.5 per cent. of lead in zinc ore, 2 grammes (30.87 grains of the sample are dissolved in nitric acid and evaporated to dryness. Some diluted sulphuric acid is added to the dry mass, and this is again evaporated until white vapors appear. It is allowed to become cold, diluted with 20 cubic centimeters (1.22 cubic inches) of water, filtered, and washed until the wash water shows no reaction with ammonium sulphide. The precipitate (lead sulphate, calcium sulphate, gangue) is rinsed off into a beaker and digested with a mixture of ammonium tartrate and ammonia in excess. The lead solution is filtered off, the lead precipitated with sulphuric acid, and the lead sulphate dried and weighed; or, it is detached from the filter, the latter incinerated, and the precipitate ignited (see also p. 98).

INDEX.

Absorption by the cupel, correction Antimony—

Arsenical iron in nickel ores, slagging | Bischof on colorimetric process for lead,

Beilstein and Jawein on galvanic assay Braunite, 246

Belgium, treatment of lead matt in, Bromyrite, 131
89

Berlin School of Mines, furnaces used
in, 54
Berthier on the reducing power of
various agents, 78

Berthier's method of determining the
absolute heating power of fuel, 266,
267

of various forms of copper, 103
Charging the sample, 31, 32
Chemical operations, 32-49
Chili method of sampling, 21
Chlorination, Plattner's, process, 165
Chlorine, determination of percentage
of manganese in an ore by, 246, 247
Chrome iron ore, 243

ores, Calvert's, Britton's, and
Dittmar's methods for de-
composing, 245

Chromium, 243-246

direct assay of, 243-245

gravimetric assays of, 243-245
indirect assay of, 245

containing copper and nickel,
how to roast, 200

impure, effects of too slight
roasting, 200

pure, roasting of, 200
roasting of 199, 200
smalt, assay of, 199

pyrites, 198

removal of, in zinc assays, 211
separation of, in nickel ores, 198
slagging off from nickel ores, 186
speiss, 200

wet assay, 199
Cobaltous oxide, 199

Coins, gold, various composition of, 170
samples for producing, 25
silver, 147

Coke, anthracite, and graphite, substi-
tutes for charcoal, 76

Coking quality of coal, determination
of, 264

Colorado method of sampling, 21

non-precipitation of, by galvanic Colorimetric analysis, 17

action, 116

ores, 243

removal of, in zinc assay, 211

volumetric assay of, 245, 246
wet assays of, 243-246

Cinnabar, 228

assay of, 231

assays by, 48, 49
reagents for, 83

assays for copper, 110

how employed, 18
of copper, 128-130
of nickel ores, 198
processes for lead, 100

Clamond's thermo-electric battery, 116 Combined lead and silver assay, 140,