7º. If the paper possesses a friable appearance, it has possibly been washed with sulphuric acid. This property may however originate from other causes, and the presence of the acid should be confirmed by washing the document with distilled water, and adding a solution of chloride of barium to the washings. The precipitate should form in a considerable quantity, as a slight cloudiness could be due to sulphates contained in the water used in the preparation of the pulp.
If much sulphuric acid be present, it may be so concentrated by heating as to cause the carbonization of the paper.
8º. It is also well, should washing with sulphuric acid be suspected, to ascertain, by aid of a lens, if the filaments on the surface of the manuscript possess an inflated appearance. This would be caused by the escape of carbonic acid, originating from the action of sulphuric acid upon the carbonates contained in the water used in the manufacture of the paper.
9º. Old ink is more difficult to remove than new, and it is therefore sometimes possible to cause the reappearance of old writings, over which words have been subsequently written. For this purpose, a solution containing 50 per cent. of oxalic acid is applied with a fine brush over the suspected points. As soon as the ink disappears, the acid is immediately removed by washing with water, and the paper dried. Upon now repeating the operation, the presence of a former writing may be detected after the complete disappearance of the words last written.
10º. According to M. Lassaigne, when the same ink has not been used throughout a document, washing with dilute hydrochloric acid will demonstrate the fact. This acid, while causing the gradual obliteration of characters written with ordinary ink—the shade of the paper not being altered—produces a red color, if ink containing log-wood has been employed, and a green coloration, in case the ink used contained Prussian blue.
The expert may possibly be called upon to give evidence as to the existence of a "trompe-l'oeil;" as was the case in the trial of M. de Preigne, which took place at Montpelier in 1852. A "trompe-l'oeil" consists of two sheets of paper, glued together at the edges, but having the upper sheet shorter than the other which therefore extends below it. This species of fraud is executed by writing unimportant matter on the uppermost sheet, and then obtaining the desired signature, care being taken that it is written on the portion of the paper projecting below. The signature having been procured, it is only necessary to detach the two sheets in order to obtain a blank paper containing the signature, over which whatever is desired can be inserted. The trial referred to above, was in reference to a receipt for 3,000 francs. The expert, upon placing pieces of moistened paper upon the suspected document, noticed that they adhered to certain points, and that these formed a border around the paper but passing above the signature. The fraudulency of the act was thus established, and so recognized by the court, although the accused was acquitted by the jury.
Numerous means have been proposed, in order to render the falsification of documents a matter of difficulty. The most reliable of these is the use of "Grimpe's safety-paper," containing microscopic figures, the reproduction of which is impossible. Unfortunately, up to the present, the government has adopted methods less sure.
EXAMINATION OF WRITINGS IN CASES WHERE A SYMPATHETIC INK HAS BEEN USED.
Sympathetic inks are those which, although invisible at the time of writing, become apparent by the application of certain agents. They are of two classes: those which are rendered visible by the mere application of heat, such as chloride of cobalt, or the juice of onions; and those which are brought out only by the action of a reagent. The inks of the second class most frequently used are solutions of acetates of lead, and other metals which give a colored sulphide when treated with sulphuretted hydrogen. Characters written with a solution of ferrocyanide of potassium acquire a blue color, if washed with a solution of perchloride of iron. It is scarcely necessary to add that the latter solution can be used as the ink, and the ferrocyanide as the developer.
When the presence of characters written with a sympathetic ink is suspected, the document is examined as follows: