kingdoms, together with gaseous substances; these latter being generally
derived from the atmosphere.

By far the purest water which occurs in nature is rain-water, and if
this be collected in a secluded district, and after the air has been
well washed by previous rain, its purity is remarkable; the extraneous
matter consisting of little else than a trace of carbonic acid and other
gases dissolved from the air. In fact, such water is far purer than any
distilled water to be obtained in commerce. The case is very different
when the rain-water is collected in a town or densely populated
district, more especially if the water has been allowed to flow over
dirty roofs. The black and foully-smelling liquid popularly known as
soft water is so rich in carbonaceous and organic constituents as to be
of very limited use to the photographer; but by taking the precaution of
fitting up a simple automatic shunt for diverting the stream until the
roofs have been thoroughly washed, it becomes possible to insure a good
supply of clean and serviceable soft water, even in London. Several
forms of shunt have been devised, some of these being so complex as
to offer every prospect of speedy disorganization; but a simple and
efficient apparatus is figured in _Engineering_ by a correspondent who
signs himself "Millwright," and as we have thoroughly proved the value
of an apparatus which is practically identical, we reproduce the
substance of his communication.

A gentleman of Newcastle, a retired banker, having tried various filters
to purify the rain-water collected on the roof of his house, at length
had the idea to allow no water to run into the cistern until the roof
had been well washed. After first putting up a hard-worked valve, the
arrangement as sketched below has been hit upon. Now Newcastle is a very
smoky place, and yet my friend gets water as pure as gin, and almost