is their action modified by the presence of dilute sulphuric acid in the
wool? I would say that soda crystals and ammonia are alkalis, and if
used strong, are sure to do a certain amount of injury to the fibre of
wool, and more if used hot than cold. Of the two, the ammonia will have
the least effect, especially if dilute, but borax is better than either.
The influence of a little sulphuric acid in the wool would be in the
direction of neutralising some of the ammonia or soda, and shellac, if
dissolved in the alkalis, would be to some extent precipitated on the
fibre, unless the alkali, soda or ammonia, were present in sufficient
excess to neutralise that sulphuric acid and to leave a sufficient
balance to keep the shellac in solution. Borax, which is a borate of
soda, would be so acted on by the sulphuric acid that some boric acid
would be set free, the sulphuric acid robbing some of that borax of its
soda. This boric acid would not be nearly so injurious to wool as
carbonate of soda or ammonia would.

The best solvent for shellac, however, in the preparation of the
stiffening and proofing mixture for hats, is probably wood spirit or
methylated spirit. A solution of shellac in wood spirit is indeed used
for the spirit-proofing of silk hats, and to some extent of felt hats,
and on the whole the best work, I believe, is done with it. Moreover,
borax is not a cheap agent, and being non-volatile it is all left behind
in the proofed material, whereas wood spirit or methylated spirit is a
volatile liquid, _i.e._ a liquid easily driven off in vapour, and after
application to the felt it may be almost all recovered again for re-use.
In this way I conceive the use of wood spirit would be both more
effective and also cheaper than that of borax, besides being most
suitable in the case of any kind of dyes and colours to be subsequently
applied to the hats.