intrinsic evidence that the
thumb-print on it is a forgery." "We will consider that evidence
presently, Dr. Thorndyke. To return to the possibility of forging a
finger-print, can you explain to us, without being too technical, by
what methods it would be possible to produce such a stamp as you have
referred to?"

"There are two principal methods that suggest themselves to me. The
first, which is rather crude though easy to carry out, consists in
taking an actual cast of the end of the finger. A mould would be made by
pressing the finger into some plastic material, such as fine modelling
clay or hot sealing wax, and then, by pouring a warm solution of
gelatine into the mould, and allowing it to cool and solidify, a cast
would be produced which would yield very perfect finger-prints. But this
method would, as a rule, be useless for the purpose of the forger, as it
could not, ordinarily, be carried out without the knowledge of the
victim; though in the case of dead bodies and persons asleep or
unconscious or under an anaesthetic, it could be practised with success,
and would offer the advantage of requiring practically no technical
skill or knowledge and no special appliances. The second method, which
is much more efficient, and is the one, I have no doubt, that has been
used in the present instance, requires more knowledge and skill.

"In the first place it is necessary to obtain possession of, or access
to, a genuine finger-print. Of this finger-print a photograph is taken,
or rather, a photographic negative, which for this purpose requires to
be taken on a reversed plate, and the negative is put into a special
printing frame, with a plate of gelatine which has been treated with
potassium bichromate, and the frame is exposed to light.

"Now gelatine treated in this way--chromicized gelatine, as it is
called--has a very peculiar property. Ordinary gelatine, as is well
known, is easily dissolved in hot water, and chromicized gelatine is
also soluble in hot water as long as it is not exposed to light; but on
being exposed to light, it undergoes a change and is no longer capable
of being dissolved in hot water. Now the plate of chromicized gelatine
under the negative is protected from the light by the opaque parts of
the negative, whereas the light passes freely through the transparent
parts; but the transparent parts of the negative correspond to the black
marks on the finger-print, and these correspond to the ridges on