thick liquid on to the
tissue-paper, where it quickly solidified. A second and third
application resulted in a broad ring of solid plaster an inch thick,
forming a perfect mould of the inside of the hat, and in a few minutes
the slight contraction of the plaster in setting rendered the mould
sufficiently loose to allow of its being slipped out on to a board to
dry.

We were none too soon, for even as Polton was removing the mould, the
electric bell, which I had switched on to the laboratory, announced a
visitor, and when I went down I found a police-sergeant waiting with a
note from Superintendent Miller, requesting the immediate transfer of
the hat.

"The next thing to be done," said Thorndyke, when the sergeant had
departed with the bandbox, "is to measure the thickness of the hairs,
and make a transverse section of one, and examine the dust. The section
we will leave to Polton--as time is an object, Polton, you had better
imbed the hair in thick gum and freeze it hard on the microtome, and be
very careful to cut the section at right angles to the length of the
hair--meanwhile, we will get to work with the microscope."

The hairs proved on measurement to have the surprisingly large diameter
of {~FRACTION NUMERATOR ONE~}{~SUBSCRIPT ONE~}{~SUBSCRIPT THREE~}{~SUBSCRIPT FIVE~} of an inch--fully double that of ordinary hairs, although they
were unquestionably human. As to the white dust, it presented a problem
that even Thorndyke was unable to solve. The application of reagents
showed it to be carbonate of lime, but its source for a time remained a
mystery.

"The larger particles," said Thorndyke, with his eye applied to the
microscope, "appear to be transparent, crystalline, and distinctly
laminated in structure. It is not chalk, it is not whiting, it is not
any kind of cement. What can it be?"

"Could it be any kind of shell?" I suggested. "For instance--"

"Of course!" he exclaimed, starting up; "you have hit it, Jervis, as you
always do. It must be mother-of-pearl. Polton, give me a pearl
shirt-button out of your oddments box."

The button was duly produced by the thrifty Polton, dropped into an
agate mortar, and speedily reduced to powder, a tiny pinch of which
Thorndyke placed under the microscope.

"This powder," said he, "is, naturally, much coarser than our specimen,
but the identity of character is unmistakable. Jervis, you are a
treasure. Just look at it."

I glanced down the microscope, and then pull