15G On the Composition and Properties of Drinldng- Water, 
dangerous composition. The facility with which nitrogenous 
organic matters are oxidised in porous soils and converted into 
nitrates, adds to the danger of water becoming impregnated with 
poisonous soluble compounds of lead, for, according to the 
uniform experience of all chemists who have studied the action 
of the different constituents of natural waters, no saline matter 
corrodes lead so readily as nitrates. 
Most soft waters act more or less energetically upon lead when 
they are well aerated and impregnated with atmospheric oxygen, 
which appears to be a primary cause of the action of soft water 
upon lead, for pure, distilled, or rain water, purposely deprived 
of air, does not attack lead in any appreciable degree. 
This explains why some soft waters in contact with lead 
become impregnated with this poisonous metal, whilst others 
scarcely attack lead, and may with safety be conveyed through 
leaden delivering-pipes. Hard waters, as a rule, do not act 
upon lead so readily as soft, especially if they contain carbonate 
of lime dissolved in carbonic acid gas. The effect of this com- 
pound is fortunately to neutralise to an extraordinary degree 
the usual solvent action on lead which water exercises through 
the agency of the oxygen dissolved in it. The soluble oxide of 
lead is converted into carbonate, which, although not absolutely 
insoluble, appears to be the least soluble of all the salts of lead. 
Carbonic acid is usually present in moderately hard spring, 
river, and well-waters, and also in most soft natural waters, in 
sufficient quantity to prevent the solution of a dangerous amount 
of lead. 
On the other hand, certain salts, especially sulphates to which 
a protecting effect is usually ascribed, do not appear to exercise 
uniformly that useful property. Hard waters containing an 
abundance of so-called protecting salts, sometimes corrode lead 
with remarkable rapidity, but, fortunately, no lead passes into 
solution, for the carbonate of lead resulting from this corrosive 
action is wholly insoluble, even in water highly charged with 
carbonic acid gas. Even excessively hard waters sometimes 
rapidly corrode leaden pipes, especially if they have an alkaline 
reaction. I have in my collection pieces of originally stout 
leaden pipes, which in the course of less than twelve months 
were eaten away to a layer as thin as writing paper, surrounded 
by a thick hard coating of carbonate of lead. Although car- 
bonate of lead cannot pass into solution, this dangerous lead- 
compound may be present in water in a suspended state, and in 
that condition may be mechanically introduced to the system. 
The practice of filtering water kept in leaden cisterns, and 
intended for drinking purposes, cannot, therefore, be too stroYigly 
recommended. 
