Next, pour a drop or two of the water from the writing into a test tube from off the letter, add a little distilled water and one or two drops of the nitrate of silver solution.
If muriatic acid has been used to colour the ink, a thick white precipitate will be seen in the tube immediately.
If not, pour a few more drops of the water which has been washed over the writing into a second test tube, add a little distilled water and a few drops of lime water. A white precipitate will be seen in the tube if either oxalic acid or binoxalate of potash has been employed.
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.