A point of probably much greater importance than the quantity of lime used is the length of time during which a lime is worked without change of liquor. An old lime becomes charged with ammonia and other products of the action of lime upon the skin, such as tyrosin, leucin (amidocaproic acid), and some caproic acid, the disagreeable goaty odour of which is very obvious on acidifying an old lime-liquor with sulphuric acid, by which considerable quantities of partially altered gelatin are at the same time precipitated (compare p. 64). Lime has considerable antiseptic power, and a new lime is practically sterile, but very old limes, especially in hot weather, often contain large numbers of active bacteria, which may be seen in the microscope under a good ¹⁄₆-inch objective. Their presence is always an indication that putrefaction is going forward, and if their number be very excessive, the leather out of such limes will generally prove loose, hollow and dull-grained, and in extreme cases hides may be totally destroyed. Spherical concretions of calcium carbonate may also be seen under the microscope, resembling on a smaller scale those found in Permian limestone, and caused perhaps in both cases by crystallisation from a liquid containing much organic matter. It is hardly probable that in many tanneries the ammonia would pay for recovery from the lime-liquors, though it could be easily done by steaming the old limes in suitable vessels, and condensing the ammoniacal vapours in dilute sulphuric acid. Its quantity rarely exceeds 0·1 per cent. of NH₃. For methods of estimation of ammonia, see L.I.L.B., p. 30.