Of aromatic acids all those which yield water-soluble sulphonation products seem suitable for the industrial production of tanning matters. If the acids themselves do not yield water-soluble sulphonation products, the alkali salts of the latter may be condensed with formaldehyde, and the resulting products then constitute tanning matters provided their solutions can be neutralised or faintly acidified without the solute being thrown out of solution in insoluble form. The diphenyl derivatives of the above groups often possess tannoid properties. The same holds good of those compounds with condensed nuclei (naphthalene, anthracene, etc.), and all their derivatives which satisfy the above conditions. The choice of condensing agent is, as a rule, of little significance. Elimination of the elements of water by the mere application of heat succeeds in few cases only, since the high temperature required to induce reaction in many cases causes decomposition of the substances. This difficulty is overcome by heating _in vacuo_. Condensation with formaldehyde always succeeds, with acetaldehyde and benzaldehyde only partly. The action on hide powder, pelt, and gelatine by these characteristic substances is tabulated below:-- Relative Behaviour towards Substance. Gelatine. Hide Powder Pelt Formaldehyde ... ... Tanning Phenol Ppte. ... ... Chlorophenol " ... ... Surface Tribromophenol Slight ppte Tanning tanning _o_ Nitrophenol No ppte " " Bromonitrophenol Slight ppte " " Trinitrophenol Ppte " Tanning Bromotrinitrophenol Slight ppte " " _p_ Aminophenol Ppte ... ... _m_ Dihydroxybenzene " ... ... Orcinol " ... ... _p_ Dihydroxybenzene " Tanning Tanning Monochloro _p_ dihydroxybenzene " ... ... _o_ Dihydroxybenzene " ... ... Pyrogallic acid " ... ... Surface Tribromopyrogallic acid " Tanning Tanning Gallic acid No ppte Not tanning Not tanning Bromophloroglucinol Ppte Tanning " Gallotannic acid " " Tanning Galloflavine Slight ppte " Not tanning Quinone " " Tanning Bromosalicylic acid " " Not tanning