232 ANIMAL PROTEINS 



also of iron, cast or wrought, the former being cheaper, less 

 liable to corrosion, but more liable to fracture. In the case 

 of iron vessels the heating may also be done by a steam coil 

 beneath a false bottom, but it is sometimes arranged that 

 iron vats are heated by a steam jacket, and even by a hot- 

 water jacket. Heating in either wood or iron vessels has 

 been brought about by direct application of raw steam, but 

 the results are both uncertain and unsatisfactory owing to 

 local overheating. Whatever appliances are used agitation 

 of the material or liquor is advantageous. 



Extraction in closed vats is also used. This is generally 

 associated with extraction at higher temperatures, and more 

 often also with the manufacture of glue than of gelatine. 

 It has been used on the Continent for skin glue, and in this 

 country for bone gelatine and glue by the " boiling " process. 

 In this system of working the vessels are usually made of 

 f -inch steel plates, and will take a charge of 3 to 5 tons of 

 material. It is claimed for the system that there is a lessened 

 steam consumption as well as lesser manipulation, that strong 

 liquors are more easily and quickly obtained, and that 

 the material may be more thoroughly exhausted. Extrac- 

 tion is sometimes made by steam and water playing alter- 

 nately on the material, but many manufacturers prefer the 

 use of direct steam, keeping the pressure at 15 Ibs. for about 

 2 hours. The pressure is then reduced considerably and the 

 process finished off by spraying the material with water. 

 From such a procedure a 20 per cent, glue sol may be obtained. 

 It is common to work such extractors in couples or in 

 batteries of four to six. It will be readily understood that the 

 process is suitable for making bone glue when the phosphate 

 has not been dissolved. The high temperature is in this 

 case almost necessary to ensure thorough extraction. It 

 will be equally clear that the process is not so suitable in the 

 manufacture of a strong gel. 



As alternatives to the systems of fractional extraction, 

 several processes have been devised in which the extraction is 

 continuous. 



Amongst these is the tower system, in which the material 



