The potential at the neutral point, using an auxiliary electrode filled with N/1 potassium chloride, was 0·69 volts. Potentials below this point, therefore, indicate an acid reaction, and, conversely, potentials above 0·69 volts indicate an alkaline reaction. The apparatus is thus very useful in following the course of the bating process. The initial potential also enables the hydrion concentration of the bate liquor to be determined directly (see Chapter III.). It was found that during the bating of a pack of skins the hydrion concentration of the liquor diminished from 10^{-5·2} to 10^{-7·4} normal. Such acidities as are found by this method, e.g. 7–8 c.c. N/10 acid per 100 c.c., are too great if made up by taking a solution of the free acids. The ionization must be almost entirely repressed by the presence of sufficient neutral salts of the same acids in the bating liquid, in order to obtain a similar result to the puer. The table opposite gives the values found for a series of puer liquors, made with dog manure from hunting-kennels. The reaction of puer liquors (expressed in c.c. of N/1 alkali or acid per 1000 c.c. of bate), in a series in which the washed goods were run for one hour in old puer, out of which goods had been taken, and then transferred to a freshly-made puer, was as follows:-- ----------+----------------+-------------- Bate | Before Goods | After Goods ----------+----------------+-------------- Old puer | Neutral | 28 c.c. acid Fresh | 10 c.c. alkali | Neutral Old puer | Neutral | 40 c.c. acid Fresh | 11 c.c. alkali | Neutral ----------+----------------+-------------- These figures were obtained by the ordinary colorimetric method described on p. 32. Adding together the alkalinity of the bate run away, and the acidity of the new bate which is neutralized, the total for 1 and 2 = 38 c.c. N/1 acid, and for 3 and 4, 51 c.c. N/1 acid. Multiplying by 810 (volume of bate less volume of dry skin), we find that the skins have neutralized 30·78 and 41·31 litres N/1 acid respectively. Since one litre of N/1 acid will neutralize 28 grm. of lime, it follows that the lime removed by the bate was in one case 862 grm., in the other case 1157 grm. The 500 kilos, of wet washed grains introduced into the paddle contain approximately 0·5 per cent. of lime CaO, equivalent to 2500 grm. altogether. As will be seen from the figures given in table, p. 35, the free acids present in the fresh bate are only capable of neutralizing 10 × 810 c.c. of normal alkali, and this is equivalent to 225 grm. of CaO, or 9 per cent. of the total lime.[30] [30] Eitner, Der Gerber, 566, p. 77, states that little or no neutralization of lime by acids or acid salts takes place in the bate.