144 
Journal of Agricultural Research 
Vol. II, No. 2 
In direct contrast with the irregularity of the add numbers of the 
various fractions there is a remarkable regularity of the ester numbers 
from year to year. There is also a most notable regularity of increase 
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Fig. ii. —Ester-number curves of hop oils from the crops of 1907. 
6 7 
in ester numbers as the particular fractions are approached in which the 
boiling point most nearly corresponds to those of the esters present. A 
correspondingly regular decrease is noted in each succeeding fraction 
from this point to the highest boiling fraction. This is clearly shown by 
