Col. Yorke on the Solubility of the Oxide of Lead. 19 



remaining six hours the liquid was poured off and tested: it 

 gave a pale brown, and it required that the liquid which had 

 not been in contact with the paper should be diluted with ten 

 times its volume of water to produce the same tint. This ex- 

 periment was repeated with a stronger solution of oxide of 

 lead in water, the water was poured off at the end of four 

 hours; it then gave a pale brown, and it required that the 

 original liquid should be diluted with four times its bulk of 

 water to produce the same tint. A fresh portion of the same 

 solution was then poured on the same paper and left for a 

 night; then, on testing, the liquid gave a brown tint, barely 

 perceptible, and it required that the original liquid should be 

 diluted with from fifteen to twenty times its volume of water 

 to produce the same. 



From these experiments it is clear that the effect in ques- 

 tion is dependent on a power possessed by the paper in com- 

 mon with several other porous bodies and organised fibres, 

 of separating certain substances from their solutions, a power 

 sufficiently well known, though little understood*. In consi- 

 dering this view of the subject in the present instance, there 

 is a circumstance of some practical importance which it would 

 appear ought to follow, viz. that after the fibres of the paper 

 had been saturated with the oxide of lead, then this substance 

 should pass through in solution. To ascertain whether this 

 was the case I made the following experiments. 



I obtained a strong aqueous solution of oxide of lead by 

 immersing slips of clean lead in about three quarts of distilled 

 water, contained in a two-necked bottle, through which oxygen 

 gas was passed and maintained in contact with, under a slight 

 pressure. In this manner I procured a solution which when 

 quite clear yielded 7 joo tn or * ignited oxide of lead. A filter 

 of paper rather less than 2 o~o tn °f an mcn thick and four 

 inches in diameter was prepared and washed ; then, by fitting 

 into one of the two necks of the bottle a siphon with equal legs, 

 so as- to resemble Gay-Lussac's apparatus for washing filters 

 (except that I used a contrivance to prevent the necessity of the 

 air supplied to the bottle from bubbling through the solution), 

 I was enabled to allow the filtration to go on with consider- 

 able regularity for many hours. The first portion of liquid 

 which passed through gave a pale brown when tested ; when 

 nine fluid ounces had passed through the effect was the same 

 as at first, and a portion [a) was reserved for future com- 

 parison. When forty fluid ounces had passed through, the 



* The effective filter mentioned by Dr. Clark is formed of well-washed 

 sand, and has been in use during twelve months without any apparent 

 diminution of power. 



C2 



