.

In many cases it will be sufficient to place the tip of the tongue to a
thick stroke. An unmistakable acid taste will be noticed.

Further and fuller particulars of the methods resorted to by forgers to
simulate ancient documents will be given in the chapter on Autographs.

It is sometimes important to know whether a stroke has been made over
another, as in the famous case in which the real issue turned on the
question whether an apparent alteration in a signature was really a
pen-mark made to indicate where the signatory should sign. It was
obvious that if the mark was made first the signature would be over it;
if, as was suggested, the mark was added in an attempt to alter or touch
up the signature, it must have been written over the signature.

In cases of this kind an enlarged photograph leaves no room for doubt.
The ink is seen lying over the lower stroke as plainly as a layer of
paint in a picture can be seen overlying the stroke beneath.

This is one of those apparently difficult points which become
marvellously simple when dealt with in a practical manner.

Pages might be needed to explain what a very simple experiment will
reveal at a glance.

Take a word which has been written long enough for the ink to have
become dry, and make a stroke across it. For example, make a letter _t_
without the bar, then, after a lapse of an hour or two, add the cross
bar. When this is quite dry and has become as dark as the first mark,
examine it with a good glass. The ink of the added bar will be seen
plainly overlaying the vertical stroke, but any doubt can be promptly
removed by taking an enlarged photograph.

Even when the second stroke is added while the ink on the first is still
wet the upper stroke can be distinguished, though not so clearly as if
the first stroke had been allowed to dry first.

By practising and examining such strokes, the student will soon learn to
distinguish important signs which leave no doubt as to which stroke was
first made.

CHAPTER XI.

ERASURES.

The alteration of the figures and amount written on a cheque is
generally effected by erasure. At one time chemicals were used for this
purpose, but fortunately the modern cheque is forgery-proof in this
respect. No means are known to chemists by which ordinary writing can be
removed from a cheque without leaving a sign too pronounced to escape
detection.

But even erasure on a cheque is extremely difficult, and the experience