344 
ACTION OF WATER ON LEADEN PIPES. 
(g.) Hydraled peroxide of iron (iron rust) in water is not 
reduced by lead. Hence may be inferred the freedom from 
corrosion of leaden pipes connected with iron mains, so far 
as the reduction of the pulverulent peroxide of iron may 
influence it. 
(h.) Alkaline chlorides in natural waters deprived of air 
do not corrode lead. (/.) Salts generally impair the action 
of waters upon lead, by lessening their solvent power for 
air, and by lessening their solvent power for other salts. 
A coat of greater or less permeability forms in all natural 
waters to which lead is exposed. The first coat (j.) is a 
simple suboxide absolutely insoluble in water and solutions 
of salts generally. This becomes converted in some waters 
into a higher oxide, and this higher^>xide, uniting with 
water and carbonic acid, forms a coat (k.) soluble in from 
7000 to 10,000 times its weight of pure water. The above 
oxide unites with sulphuric and other acids, which some- 
times enter into the constitution of the coat (&.); uniting 
with organic matter and iron rust, it forms another coat (/.) 
which is in the highest degree protective. The perfection 
of this coat, and of the first above mentioned, may be in- 
ferred from the small quantity of lead found in Croton water 
(New York) after an exposure in pipes of from twelve to 
thirty-six honrs, and from the absence of an appreciable 
quantity in Fairmount water (Philadelphia) after an expo- 
sure of thirty six hours, when coucentated to s^ths of its 
bulk. 
Method of determining small Quantities of Lead. — The 
recognition and quantitative determination of very minute 
quantities are not always without difficulty; where many 
and rapid determinations are required, the processes of ga- 
thering upon a filler, washing, drying, igniting and weigh- 
ing consume far too much time, and are sometimes less ac- 
curate than other and more indirect methods. 
That which I have employed is based upon the mode of 
analysing silver coin proposed by Gay-Lussac, and general- 
