“The bulk of the dust in the filter was removed to a beaker (No. 1), by gently tapping the inverted funnel. The cotton-wool was then removed from the funnel, and the upper one-third, containing the remainder of the dust, was cut off and added to the dust in beaker No. 1. The remainder of the cotton-wool was placed in a second beaker (No. 2); 2¹⁄₂ c.c. of hot nitric acid was dropped on the dust in beaker No. 1 from a pipette, a little water added, and the whole heated. The solution was filtered into a 50 c.c. Nessler glass, and the liquid remaining in the cotton-wool also removed by squeezing the wool with a glass rod against the side of the beaker. The cotton-wool in beaker No. 2 was similarly extracted with 2 c.c. of nitric acid, and the solution added to that remaining in beaker No. 1. The liquid was heated, the cotton-wool macerated in it and filtered as before. The wool was then washed with hot water about three or four times, and the liquid filtered into the Nessler glass. A number of standards were then made up by running into Nessler glasses, from a burette, varying amounts of standard lead solution covering a fair range. Usually five standards were made up, containing 0·5, 0·8, 1·0, 1·2, and 1·4 c.c. lead solution, depending on the volume of the air aspirated and the quantity of lead expected in the known weight of dust found. To each standard 4¹⁄₂ c.c. nitric acid was added, and 5 c.c. of the caustic soda solution and 4 c.c. of the sugar solution were run into all the six solutions--_i.e._, one test and five standards--from pipettes. It was invariably found that the test was coloured faintly yellow, and if this is not allowed for in the standards high results are obtained. Hence a drop or two of the coloured solution (see solutions required) was added to the standards placed on white paper till they matched the test. Lastly, to the contents of each of the six glasses was added 4 c.c. of sulphuretted hydrogen solution, and the liquid in each made up to the 50 c.c. mark, and the whole well stirred. Usually it was found that the colour of the test came somewhat deeper than that of one standard, and a drop or two of lead solution was added to the standard till its colour matched that of the test. The elaborate method of making up a number of standards was adopted because it was found that any other way gave high results. In the way described many trial experiments were made, and they were invariably correct within half a drop of the standard solution.”