Fortunately for manufacturers of gelatine, the colouring matter to be attacked is very susceptible both to reduction and to oxidation, and both types of bleach are widely used in practice. It is somewhat curious that the same colouring matter should be destructible both by reduction and by oxidation, but there is no doubt that each type gives a perfectly satisfactory bleaching action and can result in a practically colourless gelatine. On the other hand, the reduction is the more unstable reaction, for the glue colour slowly develops again in the gelatine on keeping it, even in a dried condition. Gelatine bleached by oxidation, however, retains its colour quite well, and even tends to improve with keeping. It is quite possible that quite different reactions are involved in the two processes, but in the light of the above facts it is somewhat surprising to observe Rideal's statement that reduction followed by oxidation has been successful in practice. Although there is a wide choice of reducing and of oxidizing agents, those which are suitable for application to gelatine cover a very limited field. This limitation arises not so much from the ineffectiveness of the bleach, as from the other effects of these substances upon the purity of the product and upon the elasticity of the gel which it can yield. Especially important is the lyotrope influence of the bleaching agent. Many reactive substances are ruled out simply because they either insolubilize the gelatine or weaken the gel it makes. Others are inadmissible on account of their poisonous nature. It must never be forgotten that whatever is used in bleaching is, like the gelatine itself, much concentrated during evaporation and drying. Its possible percentage in the finished product should be considered, and also the possibility that in these finishing operations what is present may not remain in solution, owing to supersaturation. =Bleaching by Reduction.=--Of all the reducing agents suggested, sulphurous acid has proved to be much the most suitable and successful. It has been used with equal success both for bone and for skin gelatine, but on the whole has proved more suitable for the former. Sulphurous acid can fulfil in this instance a double function, viz. that of acid solvent for the bone phosphate, and that of bleaching agent also. As it penetrates the bone material, dissolving the phosphate, it also exercises its bleaching influence on the gelatinous part of the material. Changes of liquor tend to complete both actions, so that a counter-current system is found most convenient. The "acid process" for the manufacture of bone gelatine has been previously described (Section II.), and the use of sulphurous acid in this connection is typified in the Bergmann process. In this process bleaching is in effect merely a continued treatment.