for twenty-four to thirty-six hours, filtered on a tared filter, washed with water containing 12 per cent. of alcohol, dried, ignited, and weighed. One part of sulphate is equivalent to 0·68319 part of metallic lead and to 1·25 parts of acetate of lead. The estimation of the lead, especially if the amount be small, may be more accurately made by the volumetric colorimetric method. The metallic lead deposited on the platinum-foil is dissolved in nitric acid, and distilled water added, and an aliquot portion placed in a Nessler glass. A few drops of freshly-prepared H₂S water, or H₂S gas itself, may be added to or bubbled through the contents of the glass so as to form lead sulphide. The colour formed is now matched in a similar glass, using a standard solution of lead nitrate, and forming the lead sulphide as before. Some have advocated the magnesium wire deposition test, originally devised by von Jaksch, and modified by Hill Abram, for the detection of lead in the urine of persons who are suspected to be suffering from chronic lead poisoning (see _ante_). The colorimetric method of estimating lead has been made use of with very great success by Duckering in the estimation of lead in the air of potteries. The method is that devised by Mr. Vernon Harcourt[8], with a few modifications. The whole of the method is given, as observation on the quantity of lead present in the air of lead factories helps greatly in suggesting rational methods of precaution. “After drying and weighing the filters, the following is the method. Solutions required: “_Nitric Acid._--One part of pure concentrated nitric acid to three parts of water. “_Caustic Soda._--One hundred grammes of pure caustic soda dissolved in 250 c.c. of water. “_Sugar._--A saturated solution of sugar in water. “_Sulphuretted Hydrogen._--A saturated solution of sulphuretted hydrogen in water. “_Coloured Solution._--Cotton-wool dissolved in concentrated nitric acid and evaporated to dryness, and the residue dissolved in a little water and filtered; the solution is deep yellow in colour. “_Standard Lead._--A solution of lead acetate or lead nitrate made up to contain exactly 0·0001 gramme of lead per c.c. of solution.