238 ANIMAL PROTEINS 



supplemented by the use of the filter-press. This well- 

 known appliance can easily be adapted to the local require- 

 ments of the manufacturer. As speed of working is an 

 essential requirement it is necessary to have a large filtering 

 surface, and this may be done either by increasing the 

 number of plates in the press or by increasing the area of the 

 plates used. The large plates, however, are often cumbrous 

 and inconvenient, and if of metal are very heavy. The plates 

 may be constructed of well-seasoned wood, or in the case of 

 alkaline gelatine and glues, even of iron. The framework 

 is in any case usually iron. Acid gelatines and glues may 

 have wooden plates, but " acid-proof " alloys are sometimes 

 used to make them. Where it is essential to filter quickly 

 two presses may be arranged in parallel, thus doubling the 

 active filtering surface. When it is essential to obtain the 

 highest possible clarity, two presses may be worked in series, 

 which, m effect, means that the sol is filtered twice. In 

 using the filter press for gelatine and glue it is most necessary 

 to observe the most scrupulous cleanliness, and the plates 

 must be frequently washed and sterilized. Rideal recom- 

 mends weak chlorine water or bleaching powder solution 

 for this purpose. 



The process of decolorizalion, by which colouring matters 

 are removed without being chemically altered or destroyed, 

 usually precedes or takes place concurrently with the filtra- 

 tion. The underlying principle of this operation is adsorp- 

 tion. The colouring matters are usually in colloidal solution 

 and most frequently are emulsoids, hence they are substances 

 which are known to be exceedingly susceptible to positive 

 adsorption. It is probable, also, that in a gelatine sol are 

 particles which cause turbidity, though not coloured, and 

 which are capable of being adsorbed. Hence the adsorption 

 of colouring matters not only makes the sol more colourless, 

 but in all probability makes it brighter and clearer. 

 Further, decolorization by adsorption probably also involves 

 the removal of the last traces of emulsified grease. It will 

 be clear, therefore, that in the improvement in bright- 

 ness and colour of a gelatine sol, adsorption fulfils a triple 



