In the case of skin gelatine, also, sulphurous acid may fulfil a double function, viz. that of deliming agent as well as of bleaching agent. In such instance it is necessary to use excess of bleaching acid, some acting as deliming material and the remainder as bleaching agent. As it is desirable to get rid of the lime and soda salts, several changes of liquor are given to the goods, possibly with intermediate washing. Here again approximation to a counter-current system is of advantage, as the employment of used bleach liquors for deliming purposes effects considerable economy of sulphurous acid. Indeed, there need be no waste acid at all. Whether the material be for bone or skin gelatine, however, it will be seen that the extraction is conducted in an acid condition and the resulting sol is also acid. Most usually the decolorization and filtration processes are also conducted with such an acid sol. From what has been said (Section IV.) of the value of dibasic inorganic acids as clarifying agents, it will be understood that the presence of sulphurous acid at this stage is of great advantage in the production of a clear and bright gelatine. Indeed, it is well known in trade circles that sulphurous acid gelatines are usually of exceptional clarity and brightness. The disadvantage of sulphurous acid processes is also found in the same fact that both sol, gel and cake are in an acid condition. To complete the bleach it is sometimes necessary to add sulphurous acid to the sol after extraction, or even after evaporation, but this is to be avoided if possible. Usually the ideal attempted is that the bleaching action should be as much as possible before extraction; the excess of sulphurous acid is then washed off just before the extraction, as far as practicable, and the rest is boiled off during extraction. The ideal is practically never attained, for the acid is strongly adsorbed, and the result is that the finished article is always an acid gelatine, and sometimes indeed very decidedly such. The acid condition is objectionable in the case of some forms of filter press on account of the solvent action on the metals, and is objectionable in evaporation for similar reasons. Acid gelatines are also objectionable for many purposes for which gelatine is usually sold, and this limits the commercial possibilities of the product thus obtained.