blue-black ink--is placed in the ink, to make the writing visible
at first, and gradually fades, giving place to the black of the
tannate which is formed. The dyestuffs employed in the commercial
inks of to-day vary in colour from pale greenish blue to indigo
and deep violet. No two give identical reactions--at all events
not when mixed with the iron tannate to form the pigment in
writing.

"It is owing to the difference in these provisional colouring
matters that it is possible to distinguish between writing
written with different kinds of ink. I was able easily to obtain
samples of the inks used by the Vandams, by Mrs. Popper, by Mr.
Farrington, and by the druggist. I have compared the writing of
the original prescription with a colour scale of my own
construction, and I have made chemical tests. The druggist's ink
conforms exactly to the writing on the two pill-boxes, but not to
the prescription. One of the other three inks conforms by test
absolutely to the ink in that prescription signed 'Dr. C. W. H.'
as a blind. In a moment my chain of evidence against the owner of
that bottle of ink will be complete."

I could not help but think of the two pendulums on the shelf
behind the curtain, but Craig said nothing for a moment to
indicate that he referred to that apparatus. We sat dazed.
Farrington seemed nervous and ill at ease. Mrs. Popper, who had
not recovered from the hysterical condition of her exposure, with
difficulty controlled her emotion. Vandam was crushed.

"I have not only arranged this laboratory so as to reproduce Mrs.
Popper's seance-room," began Craig afresh, "but I have had the