In four years the accidental observation of Natanson and Hofmann, made, be it remembered, in the course of abstract scientific investigation, had thus developed into an important branch of manufacture. A demand for aniline on an increased scale sprung up, and the light oils of coal-tar became of still greater importance. The operations of the tar-distiller had to undergo a corresponding increase in magnitude and refinement; the production of nitrobenzene and necessarily of nitric acid had to be increased, and a new branch of manufacture, that of arsenic acid from arsenious acid and nitric acid, was called into existence. Perkin's mauve prepared the way for the manufacture of aniline, and the discovery of a good process for the production of magenta increased this branch of manufacture to a remarkable extent. Still later in the history of the magenta manufacture, attempts were made, with more or less success, to use nitrobenzene itself as an oxidizing agent, and a process was perfected in 1869 by Coupier, which is now in use in many factories. The introduction of magenta into commerce marks an epoch in the history of the coal-tar colour industry--pure chemistry and chemical technology both profited by the discovery. The brilliant red of this colouring-matter is objected to by modern aestheticism, but the dye is still made in large quantities, its value having been greatly increased by a discovery made about the same time by John Holliday and the Baden Aniline and Soda Company, and patented by the latter in 1877. Magenta is the salt of a base now known as rosaniline, and it belongs therefore to the class of basic colouring-matters. The dyes of this kind are as a group less fast, and have a more limited application than those colouring-matters which possess an acid character, so that the discovery above referred to--that magenta could be converted into an acid without destroying its colouring power by acting upon it with very strong sulphuric acid--opened up a new field for the employment of the dye, and greatly extended its usefulness. In this form the colouring-matter is met with under the name of "acid magenta."