washer or scourer of wool, when he uses soaps, should first ascertain if
they are free from excess of alkali, _i.e._ that they contain no free
alkali; and when he uses soda ash (sodium carbonate), that it contains
no caustic alkali. Lime, in water or otherwise, acts injuriously,
rendering the fibre brittle.

_Reactions and tests proving chemical differences and illustrating modes
of discriminating and separating vegetable fibres, silk and wool, fur,
etc._--You will remember I stated that the vegetable fibre differs
chemically from those of silk, and silk from wool, fur, and hair, in
that with the first we have as constituents only carbon, hydrogen, and
oxygen; in silk we have carbon, hydrogen, oxygen, and nitrogen; whilst
in wool, fur, and hair we have carbon, hydrogen, oxygen, nitrogen, and
sulphur. I have already shown you that if we can liberate by any means
ammonia from a substance, we have practically proved the presence of
nitrogen in that substance, for ammonia is a nitrogen compound. As
regards sulphur and its compounds, that ill-smelling gas, sulphuretted
hydrogen, which occurs in rotten eggs, in organic effluvia from
cesspools and the like, and which in the case of bad eggs, and to some
extent with good eggs, turns the silver spoons black, and in the case of
white lead paints turns these brown or black, I can show you some still
more convincing proofs that sulphur is contained in wool, fur, and hair,
and not in silk nor in vegetable fibres. First, I will heat strongly
some cotton with a little soda-lime in a tube, and hold a piece of
moistened red litmus paper over the mouth of the tube. If nitrogen is
present it will take up hydrogen in the decomposition ensuing, and
escape as ammonia, which will turn the red litmus paper blue. With the
cotton, however, no ammonia escapes, no turning of the piece of red
litmus paper blue is observed, and so no nitrogen can be present in the
cotton fibre. Secondly, I will similarly treat some silk. Ammonia