ur of wool and woollen goods already dyed, in order
that they may be re-dyed. Listen, however, to the important precautions
followed: A nitric acid not stronger than from 3 deg. to 4 deg. Twaddell is
used, and care is taken not to prolong the action more than three or
four minutes.

_Action of Alkalis._--Alkalis have a very considerable action on fur and
wool, but the effects vary a good deal according to the kind of alkali
used, the strength and the temperature of the solution, as also, of
course, the length of period of contact. The caustic alkalis, potash and
soda, under all conditions affect wool and fur injuriously. In fact, we
have a method of recovering indigo from indigo-dyed woollen rags, based
on the solubility of the wool in hot caustic soda. The wool dissolves,
and the indigo, being insoluble, remains, and can be recovered. Alkaline
carbonates and soap in solution have little or no injurious action if
not too strong, and if the temperature be not over 50 deg. C. (106 deg. F.).
Soap and carbonate of ammonium have the least injurious action. Every
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