568 Abstract Report of Agricultural Discussions. 
matter and the absence of carbonic acid, which is a protection against 
the action of water on lead. In very soft waters we often find very 
little, sometimes almost a total absence of carbonic acid, and whenever 
this is the case the water acts upon lead. If you boil water and bring 
it in contact with clean lead, the latter is acted upon by it. Bright 
sparkling water, provided it is free from carbonate of soda, does not 
act upon lead. We have, then, three conditions which will account 
for the great action that some waters exercise upon lead. The first is, 
the presence of organic matters ; secondly, the presence of alkaline 
salts or alkalies ; and thirdly, the absence of carbonic acid. I would 
particularly impress upon you that there are soft waters which do not 
act upon lead, and that there are, on the other hand, hard waters that 
rapidly act upon lead. 
In order to exhibit correctly the action of alkaline substances, I 
have here a piece of leaden pipe which has been brought into contact 
simply with mortar, and you will notice how the lime contained in the 
mortar acts upon the lead in the same way as the carbonate of soda 
which occurs sometimes in water, causing it to be converted into 
oxide of lead. The pipe itself has been converted almost entirely into 
litharge ; indeed, it is worn into a thin film or sheet. It appears that 
all alkaline earths, like lime, have this effect, causing the lead to change 
rapidly into oxide of lead, which you will observe is an incrustation. 
I have analysed the sample I have in my hand, and I find it to be 
nearly pure litharge. 
In laying down water-supply pipes, particular care should be taken 
not to put them in cement, for the lime acts upon the lead, but to 
surround them with gravel. Pipes are frequently eaten away in con- 
sequence of inattention to this point. Leaden tanks and water-cisterns 
are sometimes rapidly eaten away by the corrosive action of some 
waters. A very good protection is to throw in a few pieces of zinc, 
which is more readily attacked than lead : and as long as there is any 
zinc present the lead is saved from attack. When zinc is so employed 
the water may be used for feeding steam boilers, but is not to be 
recommended for drinking purposes. It is fortunate that, even when 
lead finds its way into the water it is removed by exposure to the air. 
Air contains carbonic acid, and in contact with carbonic acid the 
lead which has passed into the water becomes again precipitated. 
Carbonate of lead is insoluble, and filtration through an ordinary 
water-filter entirely deprives the water of its lead. The danger, then, 
of poisoning by lead, or by water containing lead, is very much less 
than is usually believed. Indeed, I do not think there is a single 
instance on record in which the presence of lead in water has proved 
injurious ; for at the most only small quantitcs of lead can find their 
way into the water, and these again are rapidly and certainly removed 
by standing and by filtration. 
Time does not admit of my making any observations on the qualities 
of water for irrigation ; and there are so many points of practical 
importance connected with the subject, that perhaps it is as well I 
should defer that matter for a future occasion. 
In the course of a discussion which ensued, 
Sir J. Johnstone drew attention to the remark of Dr. Voelcker, 
