_Test for Lead in Drinking Water._--I will, lastly, give you a test that
will be useful in your own homes to detect minute quantities of lead in
water running through lead pipes. Place a large quantity of the water in
a glass on a piece of white paper, and add a solution of sulphuretted
hydrogen and let stand for some time. A brown colour denotes lead. Of
course copper would also yield a brown coloration, but I am supposing
that the circumstances preclude the presence of copper.

I have already said that rain water is the purest of natural waters; it
is so soft, and free from dissolved mineral matters because it is a
distilled water. In distilling water to purify it, we must be very
careful what material we use for condensing the steam in, since it is a
fact probably not sufficiently well known, that the softer and purer a
water is, the more liable it is to attack lead pipes. Hence a coil of
lead pipe to serve as condensing worm would be inadmissible. Such water
as Manchester water, and Glasgow water from Loch Katrine still more so,
are more liable to attack lead pipes than the hard London waters. To
illustrate this fact, we will distil some water and condense in a leaden
worm, then, on testing the water with our reagent, the sulphuretted
hydrogen water, a brown colour is produced, showing the presence of
lead. On condensing in a block tin worm, however, no tin is dissolved,
so tin is safer and better as the material for such a purpose than lead.

_Filtration._--We hear a great deal about filtration or filters as
universal means of purifying water. Filtration, we must remember, will,
as a rule, only remove solid or suspended impurities in water. For
example, if we take some ivory black or bone black, and mix it with
water and afterwards filter the black liquid through blotting-paper, the
bone black remains on the paper, and clear, pure water comes through.
Filtering is effective here. If we take some indigo solution, however,