What compound did Edison find to chemically mark paper for telegraph messages?

Question

Please list a link so that I may verify and research the source.

Thanks

Answer 1

He used three different formulations of compounds to mark paper.
See U.S. Patents 160,402/403/404
Scroll sites /Select patent numbers.

Answer 2

Thomas Edison’s quadruplex telegraph of 1874 is the most famous of a series of inventions of the late 19th century intended to increase the efficiency and profits of telegraph companies. A central goal of inventors in the telegraph field during and after the 1870s was to learn how to transmit and receive multiple messages simultaneously on a single telegraph line. Running additional wires, which would have accomplished the same thing, was becoming too expensive.