F WOOL AND FUR; AND OPTICAL PROPERTIES OF COLOURS

You have no doubt a tolerably vivid recollection of the illustrations
given in Lecture I., showing the structure of the fibre of wool and fur.
We saw that the wool fibre, of which fur might be considered a coarser
quality, possesses a peculiar, complex, scaly structure, the joints
reminding one of the appearance of plants of the _Equisetum_ family,
whilst the scaled structure resembles that of the skin of the serpent.
Now you may easily understand that a structure like this, if it is to be
completely and uniformly permeated by a dye liquor or any other aqueous
solution, must have those scales not only well opened, but well
cleansed, because if choked with greasy or other foreign matter
impervious to or resisting water, there can be no chance of the
mordanting or dye liquids penetrating uniformly; the resulting dye must
be of a patchy nature. All wool, in its natural state, contains a
certain amount of a peculiar compound almost like a potash soap, a kind
of soft soap, but it also contains besides, a kind of fatty substance
united with lime, and of a more insoluble nature than the first. This
natural greasy matter is termed "yolk" or "suint"; and it ought never to
be thrown away, as it sometimes is by the wool-scourers in this country,
for it contains a substance resembling a fat named _cholesterin_ or
_cholesterol_, which is of great therapeutical value. Water alone will
wash out a considerable amount of this greasy matter, forming a kind of
lather with it, but not all. As is almost invariably the case, after
death, the matters and secretions which in life favour the growth and
development of the parts, then commence to do the opposite. It is as if
the timepiece not merely comes to a standstill, but commences to run
backwards. This natural grease, if it be allowed to stand in contact
with the wool for some time after shearing, instead of nourishing and
preserving the fibres as it does on the living animal, commences to
ferment, and injures them by making them hard and brittle. We see, then,
the importance of "scouring" wool for the removal of "yolk," as it is
called, dirt, oil, etc. If this important operation were omitted, or
incompletely carried out, each fibre would be more or less covered or
varnished with greasy matter, resisting the absorption and fixing of
mordant and dye. As scouring agents, ammonia, carbonate of ammonia,
carbonate of soda completely free from