different. Ink
is composed of iron tannate, which on exposure to air gives the black of
writing. The original pigment--say blue or 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 cabinet
placed in relatively the same position a similar cabinet occupies in Mr.
Vandam's private seance-room in the Vandam mansion.

"One night, Mr. Jameson and myself were visiting Mr. Vandam. At
precisely twelve-thirty we heard most unaccountable rappings from that
cabinet. I particularly noted the position of the cabinet. Back of
it ran a hallway. That is duplicated here. Back of this cabinet is a
hallway. I had heard of these rappings before we went, but was afraid
that it would be impossible for me to catch the ghost red handed. There
is a limit to what you can do the first time you enter a man's house,
and, besides, that was no time to arouse suspicion in the mind of
anyone. But science h