The limit of the quantity of lime in solution in a normal puer, in the form of calcium salts, appears to be about 1 grm. per litre. If more lime be presented to the liquor, the goods stop coming down. If now fresh dung be added, they will continue to come down, but the quantity of lime in solution does not increase; the excess of lime is precipitated, partly as phosphate and partly as oxalate, in the way stated above. The type of reaction by which the lime is dissolved is similar to that occurring between ammonium chloride and lime, and is expressed by the formula-- 2NH_{4}Cl + Ca(OH)_{2} = CaCl_{2} + 2NH_{3} + 2H_{2}O and in the case of the calcium carbonate in the skins-- 2NH_{4}Cl + CaCO_{3} = CaCl_{2} + (NH_{4})_{2}CO_{3}, so that every molecule of lime neutralized, liberates two molecules of ammonia. Jean states that the production of ammonia progresses regularly during the puering, and that when the free ammonia reaches 0·2 grm. per litre the bate is unfit for further use. But if the excess of ammonia be neutralized by the addition of phosphoric acid, which also precipitates the lime brought in by the skins, the bate may be kept in constant use for some time. Jean found 0·06 grm. NH_{3} per litre in the puer as made up for use, and, after one lot of skins had been put through, 0·086 grm. per litre; after two lots of skins, 0·135 grm. per litre. In ordinary puer wheels, as used at Trent Bridge, we find-- (_a_) (_b_) Before goods 0·0816 0·0850 grm. NH_{3} per litre After " 0·0833 0·0799 so that there is little or no difference before and after puering in this case. Part of the ammonia formed escapes into the air; a further portion unites with the acids which are being formed by the action of bacteria in the bate (see Chapter IV.), and it is also probable that part combines in some way with the calcium salts in the bate. The actual reactions taking place are of course much more complicated than this simple case, because, instead of ammonia and ammonium chloride, we have a number of complex salts formed by combination of organic acids with substituted ammonia derivatives, such as methylamine, ethylamine, etc. The chief of these are ethylamine and methylamine butyrates, and lactates, and probably propionates, although I have not been able to separate the latter.[33] [33] Fitz (Berichte, 1876–1884, see Herfeldt, J.S.C.I. May 31, 1895), has shown that lactates in dung are decomposed by bacteria, the chief product being propionic acid. The reaction with the butyrate is represented by the equation-- 2C_{2}H_{5}NH_{2}C_{3}H_{7}COOH + CaO = 2C_{2}H_{5}NH_{2} + C_{3}H_{7}COO \ Ca + H_{2}O / C_{3}H_{7}COO by which it will be seen that the amine is set free just in the same way as ammonia in the equation on p. 40.