If you put a piece of potassium in contact
with the water, the latter will at once decompose, the potassium
absorbing the oxygen, and setting free the hydrogen as gas, which you
could collect and ignite with a match, when you would find it would
burn. That hydrogen was the hydrogen forming part of your cotton, silk,
or wool, as the case might be. We must now attack the question of
sulphur. First, we prepare a little alkaline lead solution (sodium
plumbate) by adding caustic soda to a solution of lead acetate or sugar
of lead, until the white precipitate first formed is just dissolved.
That is one of our reagents; the other is a solution of a red-coloured
salt called nitroprusside of sodium, made by the action of nitric acid
on sodium ferrocyanide (yellow prussiate). The first-named is very
sensitive to sulphur, and turns black directly. To show this, we take a
quantity of flowers of sulphur, dissolve in caustic soda, and add to the
lead solution. It turns black at once, because the sulphur unites with
the lead to form black sulphide of lead.
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