It is usually advisable to run away the first liquor into which butts are brought from the lime-yard, as it is very completely spent, and highly charged with lime salts and impurities. Whether other exhausted liquors are to be retained or rejected is largely a question of climate, and mode of working. In hot weather, such liquors, charged with organised ferments (moulds, _bacilli_, and _bacteria_), are apt to cause ropiness, and other fermentive diseases of the liquors. This danger may be lessened by boiling all spent liquors, so as to kill the ferments, before running on the taps, or prevented by the free use of antiseptics, such as carbolic acid. Small doses of carbolic acid, however, are useless; at least 1/10 per cent. must be employed; and it must be borne in mind that antiseptics prevent souring as well as other fermentations, and hence, where they are employed, other means must be adopted to maintain the necessary acidity. Such liquors are very liable to darken if boiled. The suspender liquors should be acid enough freely to redden litmus-paper. The present author has published a simple volumetric method for the determination of the free acid; 10 _cc._ of the carefully filtered liquor is placed in a beaker, and clear lime-water is run in from a burette till permanent cloudiness is produced. The quantity of lime-water employed is that which the acid is capable of neutralising, without producing discoloration of the leather, and care must be taken that the lime introduced with the butts does not exceed this proportion. The explanation of the reaction is that dark-coloured tannates of lime are formed, which are dissolved by the free acid so long as it remains in excess. It must be remembered that this process estimates all acids capable of retaining tannates of lime in solution, including some so feeble as to have practically no plumping effect. A liquor may have acidity equal to several _cc._ of lime-water, and yet react absolutely alkaline to methyl-orange (see p. 9), a colour which is distinctly reddened by small excess of acids, even so weak as gallic, which is barely acid to the taste. Hence, the acidity of a liquor available for plumping may be taken as represented by the lime-water required to change the red of methyl-orange to yellow, and if the liquor does not redden methyl-orange it is incapable of plumping. If 5 or 10 drops of orange solution be added to the pale filtered liquor from suspenders, there is no difficulty in approximately hitting the point of change, but great accuracy is not to be expected. If the liquor will not filter clear, kaolin (see p. 119) may be used to clear it. It is well to test the lime-water occasionally on 10 _cc._ of decinormal sulphuric or oxalic acid (p. 96), to make certain of its constancy. Lime-water should be kept in a bottle with excess of lime, shaken occasionally, and a small quantity filtered off as required. Liquors are frequently miscalled "sour" which are not acid, but putrid.