6 BULLETIN 771, U. S. DEPARTMENT OF AGRICULTURE 
The materials used in these tests were placed in open tubes and 
heated for 18 hours at the temperature given. The results show that 
an exposure to a temperature of 120° C. (248° F.) for 18 hours does 
not noticeably injure pyrethrum powder, but that a temperature of 
130° C. (266° F.) or 140° C. (284° F.), for the same length of time, 
either destroys or drives off the active principle. 
SUMMARY 
1. Whole and ground flower heads kept in sealed fruit j jars for 150 
ees were not injured. 
. Ground flower heads kept in tightly closed glass containers for 
oy years lost practically all of their effectiveness. 
3. Whole flower heads kept in tightly closed glass containers for 
54 years were practically unhurt. 
4. Whole flower heads exposed in an open dish in a room for 150 
weeks were not injured. . 
5. Ground flower heads were not injured by an exposure for 34 
weeks in an open dish in a room. Their value was materially re- 
duced by an exposure of 136 weeks and they were practically worth- 
less at the end of 150 weeks. 
6. Whole and ground flower heads were uninjured by an exposure 
to the weather of 12 weeks, but an exposure of 21 weeks greatly re- 
duced their efficiency. 
7. Powdered flower heads heated at 120° C. for 18 hours were 
practically uninjured, but a temperature of 130° or 140° C. for the 
same length of time destroyed practically all of their effectiveness. 
8. Ground flower heads were slightly injured by soaking for 24 
hours in cold water, and materially injured by soaking for the same 
length of time in hot water. 
