The addition of neutral salts to the tan liquor diminishes the effect of the acids on pelt (dehydrates the pelt) and prevents "drawing" of the grain. If, for instance, common salt be added to a solution of crude Neradol, the original quantity of sulphonic acid present would remain constant, but the presence of salt would diminish the degree of dissociation and consequently the swelling. This effect is still more pronounced when the absolute amount of free sulphonic acid is diminished. Hence, if crude Neradol is treated with increasing amounts of caustic soda, a series of products containing increasing quantities of salt and decreasing concentrations of sulphonic acid is obtained. The acidity of the Neradols may be determined by titration with N/10 caustic soda; this procedure hence establishes a means of determining the (unknown) acidities which may be expressed in terms of c.c. N/10 NaOH. The acidity of crude Neradol was found to be-- 1 gm. = 50 c.c. N/10 NaOH _i.e._, 1 gm. of crude Neradol requires 50 c.c. N/10 NaOH for complete neutralisation; the decrease in acidity causes a decrease in contents of tanning matters and the quantities of salts increase. The following table gives the figures obtained by differently neutralised neradols:-- Acidity. Tanning Matters. Na_2SO_4. Per Cent. Per Cent. 1 gm. = 50 c.c. 68 ... 1 gm. = 40 c.c. 59 4 1 gm. = 30 c.c. 50 8 1 gm. = 20 c.c. 41 12 1 gm. = 10 c.c. 33 17 1 gm. = 5 c.c. 28 20 1 gm. = 0 c.c. 20 ... Tanning experiments with these different neradols (employing solutions of 1° Bé. strength) demonstrated that neradols of acidity 50°, 40°, and 30° exerted strong swelling and gave comparatively hard leathers; neradols of acidity 20°, 10°, and 5° exert no swelling, yield quick tannage and soft leather. The swelling (hardening) effect of the acid and the dehydrating (softening) effect of the salts in this case, therefore, are well balanced, and this fact affords an explanation of the rapid change from hardening to softening effects exhibited by partly neutralised Neradol where less acid and a greater quantity of salts respectively are present.