O= ^=_____ ^ OH | | | | | |== __| | =v v v C | C_6H_5 In alkalies, the hydroxylated triphenylmethanes dissolve without imparting any colour to the solution; by concentrated sulphuric acid they are taken up with intense coloration. If the hydroxyls occupy the ortho-position to methyl, they may form xanthenes by splitting off water-- O ^ ^ ^ | | | | | | | | CH_3 v v v CH_3 CH | C_6H_5 In the benzene series this reaction is difficult to establish, and has to be induced by distilling the particular dihydroxy-diphenylmethane at ordinary pressure. In the naphthalene series, on the other hand, the ring closes up by, for instance, the condensation of [Greek: b]-naphthol with benzaldehyde or paraldehyde, and yields the following compounds:-- C_10H_6 C_10H_6 C_6H_5-CH{ }O CH_3-CH{ }O C_10H_6 C_10H_6 These xanthenes are white, silk-glossy needles, which are soluble in water and in alkalies. In concentrated sulphuric acid, they are taken up with beautiful fluorescence. 6. Summary From the qualitative reactions of the different condensation products described it may be seen that their tannoid properties are not dependent on whether they precipitate gelatine or are adsorbed by hide powder or not. Hydroxynaphthylmethanesulphonic acid, for instance, precipitates gelatine but does convert pelt into leather; on the other hand, sodium dicresylmethanesulphonate does not precipitate gelatine, and neither does it tan pelt; nevertheless it is adsorbed by hide powder as "tanning matter". The author discovered that _o_-nitrophenol does not precipitate gelatine, but has some tanning action on both hide powder and pelt. Relatively to the possibilities of forming condensation products possessing tannoid properties, the following may be stated:-- All mono- and polyhydric phenols may be converted into true tanning matters by either condensing them as such, or after their conversion into the corresponding sulphonic acids, by substances capable of eliminating the elements of water. It makes no difference to the final product whether the condensation is the first step followed by sulphonation and consequent solubilisation of the intermediary insoluble product, or whether, vice versa, the sulphonic acid is subjected to condensation. Alkaline solution of phenols may also be condensed, the reaction products, when condensed, constituting tanning matters soluble in water. Among the substitution products of the phenols, the thio-, chloro-, bromo-, nitro-, and aminophenols as a rule yield tanning matters similar in character. The quinones are as such--_i.e._, without being condensed--substances possessing tannoid properties. The aromatic dihydric alcohols are easily condensed with the different sulphonic acids and yield valuable tanning matters.