170 A. Pedler — Oorrosion of Lead Lining 8 of hidian Tea Chests. [No. 3, 



only have been corroded by the agency of some vapourous body, and also 

 that the active agent of the corrosion was volatile in a current of steam 

 in almost all cases. 



Lead is a metal which, it is well known, is easily corroded by certain 

 substances, and the most important case is the action which is employed 

 on an enormous scale in the manufacture of white lead or carbonate of 

 lead by the Dutch process. In this process sheets of lead rolled up in 

 spiral form are exposed in earthenware vessels, which contain a very 

 small quantity of acetic acid or vinegar ; these vessels are then stacked 

 together and surrounded by decomposing tan or other organic material* 

 which on standing under the influence of moisture, heats and evolves 

 carbonic acid. The quantity of acetic acid used in such operations is very 

 small, less, I believe, than one per cent, of the lead operated upon, and yet 

 the lead under the influence of this minute quantity of acetic acid, and in 

 the presence of moist air and carbonic acid at a slightly elevated tempe- 

 rature, is very rapidly corroded, and finally becomes almost entirely con- 

 verted into white lead, which, as is well known, consists of carbonate mixed 

 with hydrate of lead, and which, as produced in the first instance, contains 

 minute quantities of lead acetate adhering to it, the acetate being after- 

 wards removed by washing with water before the article is sent into 

 commerce. 



The first seiies of bodies the action of which I tried on tea lead, was 

 the group of organic substances to. which acetic acid belongs. In organic 

 chemistry there is a large group of fatty acids as they are called, of which 

 acetic acid stands next to the lowest, and which acids possess a precisely 

 similar constitution, and act usually in precisely similar ways. These 

 acids are homologous, only differing in composition by a well known 

 increment of carbon and hydrogen. The lowest members only of this 

 series are volatile, and I experimented with the five lowest with the fol- 

 lowing results. The mode of experiment was simple : a drop or two of the 

 acid was placed at the bottom of a large bottle, a strip of tea lead was then 

 hung in it, but not so as to touch the acid and the bottle filled with moist 

 carbonic acid and air, the carbonic acid being renewed from day to day, 

 or at frequent intervals as seemed necessary. The action then could 

 only take place between the lead and the vapour of the acid, and the car- 

 bonic acid and moist air. 



The first or lowest member of this series is Formic acid, and under 

 its action the lead surface became dull and corroded to a small extent. 

 There was a grey coherent film adhering to the lead and little or no white 

 incrustation. The film on examination appeared to be Lead formate, and 

 it is therefore evident that formic acid does not act on lead in the same 

 way that acetic acid does. 



