[63] A particle of gamboge in a colloidal solution has a mass 10^9 times that of a molecule of hydrogen. *Hydrogen ion Concentration of Puer Liquors.*--It has already been shown, in Chapter II., that fresh puer liquors have a certain acidity (7 c.c. N/10 per 100 c.c.) at the commencement of the operation, but that at the end they are alkaline (3 c.c. N/10 per 100 c.c.). If an artificially acid liquor be made by diluting hydrochloric acid until it shows the same number of c.c. by titration, it will be found far too “strong,” and will swell the skins. This brings us to the consideration of what is meant by the strength of acids. Procter and Jones[64] have drawn attention to the point in their paper on “Acids in Tan Liquors.” As is well known, the ionic theory affirms that degree of acidity depends on the concentration of hydrogen ions, a strongly acid solution being one in which the hydrion concentration is great, an alkaline solution one in which it is extraordinarily minute, and if we adopt pure water as our standard of neutrality, a neutral solution is one in which the hydrion concentration is approximately 10^{-7} normal. A normal solution of hydrogen ions would contain 1 gram of hydrogen ions per litre; in the case of hydrochloric acid this would equal 1·35 N/1 HCl. [64] Acids in Tan Liquors, Journ. Soc. of Chem. Ind., 1910, p. 1354. Sand and Law,[65] and Wood, Sand and Law,[66] have described the mode of estimating the hydrogen ion concentration in tan liquors directly by means of the electrometric method, and this method is especially applicable to the estimation of the hydrogen ion concentration in puer liquors. It can also be used to titrate the liquors, and we have already given some of the results in Chapter II. [65] Journ. Soc. Chem. Ind., 1911, p. 3. [66] Ibid., 1911, p. 872. The method is based on the theory of Nernst, that the difference of potential between a metal plate and the solution of one of its salts into which the metal is dipping, depends on the osmotic pressure of the free ions of that metal in the solution (in other words, on the concentration of the solution). In the case of hydrogen ions, we use a plate of platinum coated with platinum black and saturated with hydrogen, and the difference of potential depends, therefore, on the concentration of the hydrogen ions in solution. [Illustration: Fig. 9.--Diagram of Connexions of Electrometric Apparatus.] The accompanying figure (Fig. 9) shows the hydrogen electrode I, and the auxiliary electrode II, drawn to scale,[67] on the right, whereas the electrical apparatus is explained diagrammatically on the left. [67] An improved form of hydrogen electrode devised by Dr. H. Sand may now be obtained from Universitäts-mecanikes Fritz Köhler, Leipzig. It is shown at I in Fig. 10.