206 Mr. H. Medlock on the Reciprocal Action of Metals 
ficially softened to 3 degrees of hardness, had no solvent effect 
on lead. Thus, then, the notion that waters containing a small 
proportion of mineral matter necessarily act upon lead, seems 
to have no foundation in fact. 
The conclusions resulting from many experiments arrived at 
by these able investigators may be briefly mentioned. They are 
as follows :— 
Ist. That certain salts, particularly sulphates, to which a 
protecting effect is usually ascribed, appear not uniformly to 
exercise this useful property. 
2ndly. That some salts, such as chlorides, and more particu- 
larly nitrates, may increase the solvent action of water. 
3rdly. That of all protecting agents, that of carbonate of lime 
dissolved by carbonic acid is the most considerable and surest. 
The most important observation on this subject is in reference 
to the extraordinary influence of a small quantity of carbonic 
acid upon the lead actually in solution. This acid seizes upon 
the soluble oxide and converts it into carbonate of lead, of which 
not more than ;4,th of a grain is soluble in a gallon of pure 
water. Even water per se free from carbonic acid, and contain- 
ing as much as six grains of oxide of lead in a gallon, by mere 
exposure to the air for twenty-four hours was found to deposit 
all its lead as insoluble carbonate, except ;,th of a grain which 
still remained in solution. From this it will be seen, that 
although carbonic acid does not prevent certain waters acting 
upon lead, it nevertheless has the effect of precipitating the lead 
dissolved by other agencies, and so rendering it comparatively 
mnocuous. The authors conclude their observations on this 
subject in the following words :—“‘ The properties of water which 
enable it to act at times with unusual vigour upon lead are little 
understood, and seem often to arise from the accidental action of 
local and very limited causes, such as the presence of decaying 
leaves and other impurities.” That the conclusions of these and 
other investigators are very unsatisfactory cannot be denied ; 
and the question, Why do some waters act on lead and others 
not? still remains unanswered. That this is a question of 
paramount importance, and one attended with unusual difficul- 
ties, is universally admitted; nevertheless I trust that the expe- 
riments detailed in the followmg pages will throw additional 
light upon the subject. 
In looking at the analyses of waters which are known to act 
strongly on lead, we invariably find either a large proportion of 
organic matter or of its products of oxidation. ‘To these, as will 
be presently shown, is entirely due the action of such waters on 
lead. 
The organic matters held in solution by water, and which 
