If, on the other hand, completely neutralised Neradol is acidified with an organic acid, such as acetic acid, till the acidity, (1 gm.= 10 c.c. N/10 NaOH) is reached, the resulting product is in all respects similar to Neradol D and yields a corresponding leather. It is permissible to assume that the irregularity exhibited by Neradol D as regards the analytical estimation of its tannin contents is connected with the low molecular weight of the tanning principle. Whereas all tannins so far isolated from the natural tanning materials possess rather high molecular weights, that of Neradol D deviates considerably from this rule, as is shown by the following table:-- Neradol D tannin Cl_5H_16S_2O_8 358 Mangrove " C_24H_40O_2l 670 Oak bark " C_28H_28O_23 840 Myrabolam " C_54H_48O_35 1256 Dividivi " C_54H_46O_35 1270 Malletto " (C_4lH_50O_20)_2 1724 This low molecular weight may mainly account for the figures obtained by the incorrect oxymetric estimation with permanganate; the apparent tannoid property of the tannoid-inactive neutral salt of dicresylmethanedisulphonic acid may be explained by assuming that though it is, probably, in the colloidal state, and as such adsorbed by hide powder, it is still devoid of astringent properties. G. Different Methods of Condensation as Applied to Phenolsulphonic Acid In addition to formaldehyde, many other substances may, theoretically, induce condensation of phenolsulphonic acid; condensation takes place either with the elimination of water or, in addition to this, with the introduction of methane group. So far, the following condensing agents have been investigated:-- (1) Heating _in vacuo_. (2) Sulphur chloride. (3) Phosphorus compounds. (4) Aldehydes. (5) Glycerol. 1. Condensation Induced by Heat If phenolsulphonic acid is heated _in vacuo_ at 130° C. for twenty hours, condensation takes place [Footnote: Austr. Pat., 64,479.] without the addition of any condensing agent, and an anhydride of the ^ __O__ ^ | | | | | | | | v v HSO_3 HSO_3 composition is formed. This product is a viscous liquid, possessing a very corrosive action. Added to a solution of gelatine, a light, fine flocculent precipitate is thrown down. Analysed by the shake method of analysis, the tannin content of the product equals about 46 per cent. Its strongly acidic and hence swelling character does not express qualities consistent with the conception of suitability for tanning purposes: a sample of the product was therefore partly neutralised to the acidity of Neradol D, when the shake method of analysis yielded the following figures:-- Tanning matters 21.5 per cent. Soluble non-tannins 48.3 " Water 30.2 " -------------- 100.0 per cent.