1880.] 



MICROSCOPICAL JOURNAL. 



127 



face of the paper of an impercept- 

 ible thickness, and no matter how 

 maiiy marks cross at any one place 

 the surface is not raised ; it is im- 

 possible, therefore, to tell which of 

 these marks was made iirst.* 



It is often of importance to de- 

 temiine the age of writing. This 

 can often be done with approxima- 

 tion to accuracy by observing the 

 changes in coloi', which writing un- 

 dergoes by time and exposure to 

 the air. Every one who has seen 

 old writings knows that by the lapse 

 of time they change from black to 

 brown, and gradually become fainter 

 and fainter until they finally be- 

 come quite illegible. This is due 

 to the action of the oxygen of the 

 air on the ink. Most inks, as every 

 one knows, are made by adding to 

 a solution of gall-nuts, sulphate of 

 iron, together with a little gum. 

 The iron unites with the galHc and 

 tannic acids, forming a tannate of 

 iron, or gallo-tannate of iron, which 

 is black. In time these substances 

 separate from the iron, which then 

 unites with oxygen, forming an iron 

 rust, which is much paler than the 

 ink. This action takes place with 

 much greater rapidity in the finer 

 lines of the writing than in the 

 coarser ones, on account of the re- 

 latively greater extent of surface 

 exposed to the action of the air. 

 From these lines it gradually spreads 

 to the coarser lines. The process 



* Professor Babcock has given an instance 

 in which such a thing was determined with 

 the microscope. It became important at one 

 time to ascertain which of two pencil marks 

 which crossed, was made first. Without ex- 

 pecting to learn anything by the examination, 

 he put them under the microscope, when, to 

 his surprise, he saw four or five clean cut 

 grooves in the course of the lines, which 

 showed a clear white in contrast with the 

 dark lines. The grooves in one line ploughed 

 through those of the other line and showed 

 clearly which was made first. These lines 

 were probably made by crystals of carbon in 

 the graphite of the pencil. 



{Proceedings of State Mic. Soc. of Ills.) 



does not take place with equal ra- 

 pidity in all kinds of ink, but if 

 care is used in the examination of 

 two signatures made at different 

 times, it is usually possible to de- 

 termine with a fair degree of cer- 

 tainty which was made first, by 

 noting the comparative degree in 

 which this change has taken place 

 in the two samples. 



The browning of ink is some- 

 times imitated by the use of coffee 

 and sugar ; these means, if used 

 carefully, may possibly produce 

 what seems to the naked eye to be 

 a fair copy of the original, but it 

 will not bear examination with the 

 microscope. The color is too uni- 

 form and the surface is glossy : 

 chemical tests will settle the point 

 at once, showing in the one case 

 the presence, and in the other the 

 absence, of iron. 



There are a very large number 

 of ■ inks containing iron, but there 

 are some inks which do not contain 

 this substance, the most important 

 of which is called chrome ink. It 

 consists of chromate of potash 1 

 part, and solution of logwood 1000 

 parts. This is much more per- 

 manent than ordinary iron inks. 



Pure vegetable substances are 

 also sometimes used as in the case 

 of the Cariaria thymofolia, or ink 

 plant, of New Grenada, the pure 

 juice of which becomes black by 

 exposm-e to the air. It is said that 

 all the old Spanish documents of 

 Central America are written with 

 this substance, and that they keep 

 their color well. The ink does not 

 corrode the pen. 



Other vegetable substances have 

 been used as ink with more or less 

 success. The subject is worth ex- 

 periment, for our ordinary writing 

 ink is anything but perfect. 



Of colored inks red ink is made 

 of carmine dissolved in ammonia 

 and diluted, or of a decoction of 



