The fibers of dyed black silk may be viewed with interest under the
microscope. If a few threads of its warp are placed on a glass slide, and
one or two drops of concentrated nitric acid placed in contact with them,
the black color changes first to green, then to blue; a life-like motion
is observed in all the fibers; they appear marked crosswise like the
rings of an earthworm; the surface of each fiber appears loaded with
particles of dyestuff; finally the fibers wholly dissolve in the acid. If
we now treat a few threads of the weft in the same manner, a similar
change of color takes place. When the fibers assume the blue color, a
dark line is observed in the center of each, running longitudinally the
whole length; this dark line is doubtless the dividing line of the two
original normal threads formed directly by the two spinnerets; the dark
air line or shadow finally breaks up, and in the course of a few minutes
the silk is wholly dissolved. Were ramie, cotton, flax, or hemp present,
they would be observed, as all their fibers remain unchanged under this
treatment. If wool be present, rapid decomposition will follow, giving
off copious fumes of nitrous acid, allowing, however, sufficient time to
observe the separation of the scales of the fibers and to demonstrate by
observation under the microscope that the fibers are those of wool.
In making these experiments it is not necessary to use a glass disk over
the treated fibers. If a disk or cover is pressed on them while
undergoing this treatment, the life-like motion of the silk will not be
so apparent.
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