160 MISC. PUBLICATION 5 4 0, U. S. DEPT. OF AGRICULTURE 



duly considered. There are, however, certain fundamental principles 

 which should be considered if these tests are to yield reliable informa- 

 tion for plant control (fig. 69). 



Methods 



An amount of dehydrated material suitable to the equipment that 

 is available is weighed directly into a glass beaker. Distilled water 

 is added and the beaker is placed immediately on a heater. The 

 content is brought to the boil at a fixed rate of heating and boiled for 

 the scheduled number of minutes. In many instances soaking at room 

 temperature before boiling produces a product of higher quality. 

 The soaking time may vary from 1 hour to overnight. At the end 

 of the boiling period the dish is removed and the contents quickly 

 filtered. As soon as draining is complete the moist sample is weighed. 

 The following procedure is used at the Western Regional Research 

 Laboratory : 



Weigh two 10-gm. samples of dry material on a torsion balance. Place in 

 600 ml. pyrex beakers, add 80 to 150 ml. of distilled water, cover each with a 

 watch glass, place on electric heaters, bring to the boil within 3 minutes, and 

 boil 5 minutes. The precise amount of water will vary with the material, time, 

 and rate of boiling ; excessive amounts of water should not be used. Remove 

 from the heater and dump into a 75 mm. Buchner funnel which is covered with 

 a coarsely porous filter paper. Apply gentle suction and drain with careful 

 stirring for one-half to 1 minute, or until the drip from the funnel has almost 

 disappeared. Do not dry by long suction. Remove from the funnel and weigh. 

 Set the drained sample aside in a covered porcelain evaporating dish for 

 quality tests. Repeat this test, and then rehydrate six other 10-gm. samples, 

 boiling two for 10 minutes, two for 20 minutes, and two for 30 minutes. It 

 will be necessary to use 20 to 30 ml. more of water for the last two tests than 

 for the shorter boiling tests. Only small pieces will rehydrate in 5 minutes. 

 Samples requiring more than 30 minutes of boiling are not likely to pass in- 

 spection for Government purchase. 



A skillful technician will be able to operate four heating units 

 and by staggering the time the filtering can be accomplished with 

 one Buchner funnel. These dry materials are "water hungry" and 

 1 minute of variation in the time it takes to bring the sample to the 

 boiling point, to remove the sample from the heater, or to complete 

 the filtering, will cause significant errors in the final drained weights. 



Calculations that can be made will vary with the data available. 

 The rehydration ratio (table 14) can always be calculated. When 

 the percentage of water in the dry sample is known, the percentage of 

 water in the rehydrated sample can be calculated. When the per- 

 centages of water in the dry sample and in the undried sample are 

 known, the coefficient of restoration of weight can be calculated. 

 Calculations are illustrated in table 14. 



