GELATINE AND GLYCERINE 223 



likely to attain supersaturation in the " cake " and con- 

 sequently a dull product. Further, lyotrope influences can 

 never strengthen a gel very much, but may and often do 

 weaken it very considerably. Hence the aim of most 

 manufacturers in the preliminary treatment is so to delime 

 that a nearly neutral and salt-free product is obtained. An 

 exception is the case of skin gelatine in which excess of 

 sulphurous acid is used. This process has for its object not 

 only deliming and purifying, but also a bleaching action. 



In the case of bone gelatine, the raw material is such 

 that there are much longer and more elaborate preparatory 

 processes. This arises from the fact that about half the 

 bones of animals consists of mineral matter, chiefly calcium 

 phosphate. Bones, of course, vary in composition to some 

 extent, and those from younger animals contain distinctly 

 less of the mineral constituents. Approximately speaking, 

 bones have the following average composition : 



Gelatinous matter . . . . . . 21 J per cent. 



Fat i2i 



Calcium phosphate . . . . . . 48 ,, ,, 



Calcium carbonate .. .. .. 3 ,, 



Alkali salts, silica, etc. . . . . 2\ ,, 



Water . . . . . . . . . . 12$ 



ioo 



It will be seen, therefore, that the manufacture of bone 

 gelatine and of a comparatively large proportion of phos- 

 phate involves the recovery and purification of much fatty 

 matter. The manufacturing processes are naturally subject 

 to considerable variation. One respect in which they differ 

 is the stage in which grease is removed. Sometimes this is 

 simply done as the need and occasion arise, and it is skimmed 

 out in the acid or water extractions, but it is now more usual 

 to have a special " degreasing " process. There are, more- 

 over, two quite distinct types of manufacture. In one of 

 these (the boiling process) the routine bears some resemblance 

 to that for skin gelatine. In this process the bones are 



