The phenols of naphthalene are called naphthols--they bear the same relationship to naphthalene that carbolic acid bears to benzene. Owing to the structure of the naphthalene molecule there are two isomeric naphthols, whereas there is only one phenol. The naphthols--known as alpha- and beta-naphthol, respectively--are now made on a large scale from naphthalene, by heating the latter with sulphuric acid; at a low temperature the alpha sulpho-acid is produced, and at a higher temperature the beta sulpho-acid, and these acids on fusion with caustic soda furnish the corresponding naphthols. Similarly there are two amidonaphthalenes, known as alpha- and beta-naphthylamine respectively. As aniline is to benzene, so are the naphthylamines to naphthalene. The alpha-compound is made in precisely the same way as aniline, viz. by acting upon naphthalene with nitric acid so as to form nitronaphthalene, and then reducing the latter with iron dust and acid. Beta-napthylamine cannot be made in this way; it is prepared from beta-naphthol by heating the latter in presence of ammonia, when the hydroxyl becomes replaced by the amido-group in accordance with a process patented in 1880 by the Baden Aniline Company. The principle thus utilized is the outcome of the scientific work of two Austrian chemists, Merz and Weith. Setting out from the naphthols and naphthylamines we shall be led into industrial developments of the greatest importance. The first naphthalene colour was a yellow dye, discovered by Martius in 1864, and manufactured under the name of "Manchester yellow." It is, chemically speaking, dinitro-alpha-naphthol; but it was not at first made from naphthol, as the latter was not at the time a technical product. It was made from alpha-naphthylamine by the action of nitrous and nitric acids. When a good method for making the naphthol was discovered in 1869, the dye was made from this. The process is just the same as that employed in making picric acid; the naphthol is converted into a sulpho-acid, and this when acted upon by nitric acid, gives the colouring-matter. Manchester yellow is now largely used for colouring soap, but as a dye-stuff it has been improved upon in a manner that will be readily understood. The original colouring-matter being somewhat fugitive, it was found that its sulpho-acid was much faster. This sulpho-acid cannot be made by the direct action of sulphuric acid upon the colouring-matter--as in the case of acid yellow or acid magenta--but by acting upon the naphthol with very strong sulphuric acid, three sulphuric acid residues or sulpho-groups enter the molecule, and then on nitration only two of these are replaced by nitro-groups, and there results a sulpho-acid of dinitro-alpha-naphthol. This was discovered in 1879 by Caro, and introduced as "acid naphthol yellow." It is now one of the standard yellow dyes.