The Liquor that is separated from these Glands is a Mucilage, not
unlike the White of an Egg, tho’ not always so clear and pellucid; when
pure it is very like it. In some Animals it is of a Colour inclining
to Yellow, and is composed of watry, saline and slimy Particles; it
is supposed that the earthy Particles may be about a two and thirtieth
Part. The Nature of this Mucilage seems nearly to approach to that of
the _Serum_ of the Blood, separated from the grumous Part upon being
exposed to the Air, and exhibit much the same Appearances upon Trials
by Mixture with other Bodies, only the _Serum_ is not so mucilaginous.
The _Serum_ is coagulated upon being mixed with Spirit or Oyl of
Vitriol, Spirit of Salt, Oyl of Sulphur, and other acid Spirits. The
_Serum_, upon being held in a Spoon over the Fire, becomes a thick
Jelly, and at length a sort of friable Glew; on the contrary, the
Mucilage grows thinner, upon the same Application, throws up a slight
thin Film at Top, and produces but a slight Coagulum. After the aqueous
Parts are evaporated, there remains scarce a thirtieth Part of the
whole Mass.