1919] 



Hall-Goodspcrd : Ch rysil 



221 



clear that previous to extraction the finest possible subdivision of the 

 plant material was desirable. Preliminary tests indicated that after 

 prolonged grinding in the pebble mill a greater reduction in size of 

 particles was attained tliau by our usual final grinding in the coffee 

 mill. Examination of tlie pebbles at the end of their action showed 

 particles of rubber as well as woody and fibrous tissue adhering to 

 their surfaces. We therefore made a number of analyses to determine, 

 if possible, just what proportion of the rubber was left behind on the 

 pebbles after pebble mill grinding. The following table indicates the 

 relation in each case between the amount of rubber extracted from a 

 sample of the ground material as it came out of the pebble mill and 

 the amount extracted from another portion of the same rough ground 

 material which had been reduced in the coffee mill. Attention should 

 be called to the fact that no rubber was found to adhere to the 

 metal grinding plates of the feed mill or coffee mill and that both 

 were taken apart and l)ruslird out after each grinding, the material 

 remaining in the mills being added to the ground substance which 

 had gone through them. Tn a few cases material was ground in a 

 pebble mill operated in the Food and Drug Laboratory of the Uni- 

 versity of California. This mill was larger than ours and the product 

 was somewhat more finely divided than the product of our pebble 

 mill. 



Table 3. — Influence Upon the Results of Axalysls of Final Grinding in a 



Coffee Mill and in a Pebble Mill 



