It may finally be noted that the acidity of Neradol D, 1 gm. = 10 c.c. N/10 NaOH, has been found to be the most suitable one for practical purposes. The author has, however, successfully employed some neradols of considerably higher acidities. The acidity above mentioned is possessed by a Neradol D containing 17 per cent. ash and 30 per cent. sodium sulphonates and Glauber's salts crystals respectively. This large quantity of salts present on the one hand effects the rapid pickle and tanning effect exhibited by Neradol D, on the other hand it also effects the softness in the leather resulting from its use either alone or in admixture with natural tannins. 4. Phlobaphene Solubilising Action of Neradols A special feature of Neradol D is its property of solubilising phlobaphenes, which may be ascribed to its contents of sulphonic acids or their salts. In order to demonstrate whether the sulphonic acids and their salts are capable of solubilising the insoluble or sparingly soluble anhydrides of the tannins (the phlobaphenes) before and after condensation, the following experiments were carried out:-- Crude Argentine solid quebracho extract was converted into a highly viscous liquid by treating it for several hours with water at 100° C., and the anhydrides rendered insoluble by diluting the liquid with a large volume of cold water. The precipitate formed, consisting of quebracho phlobaphenes, was separated from the liquid by decantation, and purified by washing it several times with water. Each 10 gm. of this moist paste were treated in the cold with (_a_) free phenolsulphonic acid; (_b_) sodium phenolsulphonate; (_c_) crude Neradol and (_d_) Neradol D, 20 c.c. of water at 45° C. added, and the mixture allowed to cool slowly; the following solutions resulted:-- (_a_) Opalescent solution, much deposit, (_b_) Clear solution, no deposit. (_c_) Nearly clear solution, very little deposit. (_d_) Clear solution, no deposit. This clearly proves that free and condensed phenolsulphonic acids as such are not capable of completely solubilising phlobaphenes, whereas the sodium salts of free and condensed phenolsulphonic acids possess this property. The salt contents of Neradol D, therefore, constitute an advantage in this respect, that not only may Neradol D be mixed with solutions of any natural tannin without insolubles being thereby deposited, but it may also be added in large quantities to a tannin solution with the result that the sparingly soluble and wholly insoluble constituents (phlobaphenes) are completely brought into solution. The practical importance of the solubilising effect of Neradol D relating to solid Argentine quebracho extract is demonstrated in the following series of investigations carried out by the author:-- [Footnote: _Collegium_, 1913, 478; Austr. Pat., 68, 796.]