Thus it happens that when characters written in blue-black ink are kept, the blue pigment will gradually fade out, leaving the black pigment; and when this stage is reached the ink in old writing is readily distinguished from ink that has been freshly put upon paper. Prior to this, however, the blue provisional colouring matter appears to become enveloped in the particles of iron tannate so that it no longer reacts rapidly with chemical reagents. Thus, if writing done within the last year or two be treated with acetic acid there is an immediate diffusion of the blue pigment, whereas in the older writing, diffusion, if it occurs at all, is very slow and limited in extent. A still more useful reagent for this purpose is a saturated solution of oxalic acid, which causes the pigment of relatively fresh writing to give an immediate smudge, but has very little, if any, effect on writing six or eight years old. The differences in the behaviour of old and relatively recent writing are seen in the tests here illustrated, in which the old writing of 1898 was hardly affected by the reagents, whereas the writing done in 1908 gave the results shown. Both writings were in the same kind of ink and the tests were applied simultaneously. Speaking generally, a writing done with blue-black ink ceases to show such diffusion after five to six years. When slight diffusion occurs in an older ink it is seen under the microscope to differ in character and only to affect the surface of the letters, whereas the diffusion in an ink written within the last two or three years affects the whole of the pigment in the letters. The first occasion on which chemical evidence as to the age of blue-black ink has been given in the law courts was in the recent forgery case, in which Colonel Pilcher was accused of forging his cousin's will. This will was alleged to have been written in 1898; and assuming this to have been the case, the ink should only have reacted very slowly with the different reagents; there should have been little or no diffusion with oxalic acid: and if any slight diffusion occurred it should only have been upon the surface of the letters. The ink upon the will, however, gave an immediate reaction with the different reagents, the blue pigment diffused at once with oxalic acid, and the diffusion extended throughout the whole of the letters. There was thus no doubt but that the ink upon the will had been written within the last year or two--certainly within the last six years. Cheques written by the deceased lady during the last thirteen years were also subjected simultaneously to the same tests, and while those written quite recently gave an immediate diffusion, the ink upon those written in 1903 showed only the slightest diffusion in the heaviest writing, and no diffusion at all was obtained upon the cheques written in 1901.