~Effect of Foreign Salts.~--As already stated, sulphates interfere. Twenty c.c. of the lead nitrate solution were taken, precipitated with sulphate of soda, and the precipitate dissolved in 10 grams of sodium acetate and titrated as before. Duplicate experiments required 18.6 c.c. and 18.7 c.c. of the chromate solution. A similar experiment with 40 c.c. of lead nitrate required 37.4 c.c. of chromate. If the sulphate had not been present, the results would have been 19.7 c.c. and 39.4 c.c. respectively. ~Effect of Varying Lead.~--In these experiments the conditions were as before, but with varying amounts of lead. Lead nitrate solution present 10.0 c.c. 20.0 c.c. 50.0 c.c. 100.0 c.c. Chromate solution required. 9.4 " 19.7 " 48.8 " 98.2 " ~Determination of Lead in Galena.~--Weigh up 1 gram of the powdered and dried ore, and boil in an evaporating dish with 10 c.c. of dilute hydrochloric acid. When the action becomes sluggish, dilute with an equal bulk of water, and add a weighed piece of zinc rod about 1 inch long and quarter-inch across. Keep up a moderate action by warming till the ore is seen to be completely attacked and the lead precipitated. Decant off the solution, wash once, strip off the lead, wash and weigh the remaining zinc. Dissolve the lead in 5 c.c. of dilute nitric acid, and 5 c.c. of water with the aid of heat. Dilute and transfer to a pint flask; add a slight excess of dilute ammonia, and render faintly acid with acetic acid. Dilute to 150 c.c., heat to boiling, and run in the standard chromate in slight excess, noting the amount required, and make up to 250 c.c. with water. Boil the solution, allow to settle for a minute or so, filter off 50 c.c., and determine the excess of chromate colorimetrically. As an example, 1 gram of an impure galena was precipitated with 75 c.c. of standard chromate (100 c.c. = 1.020 gram lead). The excess found in 50 c.c. was 0.3 c.c., which, multiplied by 5, gives 1.5 c.c. as the excess in the whole solution. The remaining 73.5 c.c. of "chromate" required by the assay, are equivalent to 0.7497 gram of lead. The zinc used up weighed 1.5 grams, and contained 0.0165 gram of lead. Thus we get-- Lead in the assay 0.7497 gram Lead from the zinc 0.0165 " ------ .'. Lead in the galena 0.7332 " Equivalent to 73.3 per cent. Another sample, in which the galena was accompanied with a large quantity of pyrites, gave the following results:--Three grams were treated with 30 c.c. of dilute hydrochloric acid and a rod of zinc. The zinc and lead were carefully transferred to another vessel, the zinc cleaned, and the lead (dissolved in 5 c.c. of dilute nitric acid and 20 c.c. of water) treated as before. 14.5 c.c. of the chromate were required = 0.1479 gram lead Lead in 2 grams of zinc = 0.0220 " ------ .'. Lead in 3 grams of the ore = 0.1259 "