701 
of Edinburgh, Session 1871 - 72 . 
exist in very small proportion, is to act as preventives. The 
author sometimes obtained the same result, and found the action 
such as might prove dangerous. But its limit requires to be de- 
fined ; and there is reason to suppose that the proportion required 
to permit action will be found so great as never occurs in the instance 
of waters applicable to household use. 
It has been also stated, but in general terms, without experi- 
mental proof, that the presence of carbonate of soda, even in a 
hard water, takes away the preventive influence of other salts, and 
enables water to dissolve lead. There appears to be some founda- 
tion for this statement. But here, too, it is necessary to fix what 
is the limit to such influence before its importance can be valued. 
Moreover, as bicarbonate of soda appeared to the author to have 
no such effect, and this is the usual form of the carbonate in 
natural waters, the practical importance of the fact is inconsiderable. 
The author called attention to some observers not having under- 
stood the nature of the corrosive action of water on lead, and having 
confounded it with other causes of corrosion. Thus the true action 
has been confounded with the corrosive action of potent agents 
accidentally coming in contact with the metal in presence of water, 
— as, for example, when a lead pipe has been led through fresh 
mortar which is frequently or permanently kept moist, or when 
lumps of fresh mortar have been allowed to fall upon the bottom 
of a lead cistern. Several remarkable examples of rapid corrosion 
of this local kind were exhibited. The true or simple action of 
water had been not infrequently confounded also with the effects 
of galvanic action. Thus, if a lead pipe or cistern be soldered 
with pewter-solder, and not with lead, erosion takes place near 
the line of junction of the solder with the lead, of which character- 
istic examples were shown. The presence of bars of other metals 
crossing lead, or bits of them lying on it, will also develope the 
same action ; and some facts seem to point to the same property 
being possessed in a minor degree by some stony and earthy sub- 
stances. This observation may explain the local erosion sometime 
observed in cisterns containing hard water ; since, if galvanic action 
be excited, it will be increased by saline matter existing more 
largely in these waters than in soft, or comparatively pure, water. 
Lastly, some observers have contradicted former statements, 
5 A 
VOL. VII. 
