80 PROCEEDINGS OF THE AMERICAN ACADE»IT 



first blackened by the formation of sulphide of lead, becomes clear. 

 Test-tube No, 1 contains one tenth of a gramme of lead in the form of 

 sulphide, — a black powder at the bottom. Test-tube No. 2 contains 

 one hundredth of a gramme. No. 3, one thousandth. No. 4, one ten- 

 thousandth. No. 5, one hundred-thousandth. No. 6 yielded no pre- 

 cipitate without concentration. Each succeeding precipitate in the se- 

 ries, setting aside a slight allowance to be made on account of solubil- 

 ity, was one tenth as voluminous as the one above. 



" Having prepared this scale of quantities, it is required to determine 

 the amount of lead in a given diluted solution. An experiment is made 

 to ascertain if the quantity be large enough to give a direct precipitate 

 with sulphide of ammonium. This being decided in the negative, fifty 

 cubic centimetres or grammes of water (corresponding with fifty parts 

 of the scale of solutions) are carefully evaporated to dryness and ignit- 

 ed in a small porcellain capsule (to expel any organic matter that may 

 have been present), moistened with nitric acid, and then warmed, with 

 the addition of acetic acid and water, till the volume becomes ten cubic 

 centimetres. A drop of acetate of potassa is then added, and then hy- 

 drosulphuric acid gas transmitted through the solution. A precipitate 

 results, or it does not. If it does, to know its value or the amount of 

 lead it contains, the scale is resorted to. Though it might rarely be 

 possible to identify it with either one of two precipitates in the scale, 

 there could be no difficulty in deciding between which two it should 

 fall, or nearest to which one of two it should be placed. If fifty cubic 

 centimetres thus treated yielded no precipitate, one hundred cubic cen- 

 timetres were evaporated to dryness, and the residue similarly treated. 

 If this failed, five hundred cubic centimetres were taken, and in some 

 instances more, and the®!same course pursued. 



" It was natural to suppose that the presence of foreign bodies, such 

 as occur in natural waters, might embarrass the precipitation. This 

 led to the preparation of a series of graduated solutions of lead, with 

 all the common salts occurring in waters, from the reagents in my lab- 

 oratory. They were similarly treated with acetate of potassa, free 

 acetic acid, and a stream of hydrosulphuric acid, and though it was 

 possible to see differences in the amounts of the precipitates, they fell 

 very greatly within the differences between the successive members of 

 the graduated series. 



" The precipitates in the experiments with bars of lead, the results of 

 which are given in the preceding tables, were estimated from this scale. 



