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DUST AND DISEASE. 3 1 9

rounded by hydrogen, but the water was not free from
floating matter. It is so hard to be clean in the midst of
dirt. Here, however, is an approach to pure water. It is
from the Lake Of Geneva, and the bottle was carefully
filled for me by my distinguished friend Soret. The . track
of the beam through it is of a delicate sky blue; there is
scarcely a trace of grosser matter.

The purest water that I have seen—probably the purest
which has been seen hitherto—has been obtained from the
fusion of selected specimens of ice. But extraordinary
precautions are required to obtain this degree of purity.
An apparatus was devised and constructed by my assistant
for this purpose. Through the plate of an air-pump passes
the shank of a large funnel, attached to which below the
plate is a glass bulb. In the funnel is placed a block of the
most transparent ice, and over the funnel is a glass receiver.
This is first exhausted and refilled several times with air,
which has been filtered byits passage throughcotton-wool,
the ice being thus surrounded by pure -moteless air. But
the ice has previously been in contact with mote-filled air;
it is, therefore, necessary to let it wash its own face, and
wash the bulb which is to receive the water of liquefaction.
The ice is permitted to melt, the bulb is filled and emptied
several times, until finally the large block dwindles to a
small one. We may be sure that all impurity has been
thus removed from the surface of the ice. These two bulbs
contain water obtained in' this way, the purity of which is
the maximum hitherto attained. Still I should hesitate to
call the water absolutely pure. When the concentrated
beam is sent through it the track of the beam is not invis-
ible, but of the most exquisitely delicate blue. This blue
is purer than that of the sky, so that the matter which pro-
duces it must be finer than that of the sky. It may be,
and indeed has been, contended that this blue is scattered
by the very molecules of the water, and not by matter sus-

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