8 BULLETIN 785, U. S. DEPARTMENT OF AGRICULTURE. 
more applications had been made before testing began. When 
samples could not be taken immediately before and after a spray 
application, estimates have been made from the previous or sub- 
sequent direction of the graph to determine the probable values, 
and such approximations are indicated by X rather than by O in 
plotting. 
The owner of field A, besides guessing at the quantities of ingre- 
dients, followed a faulty method of mixing the spray. In spite of 
frequent applications, the results were very unsatisfactory. If it is 
assumed that adequate protection from some specific celery disease 
is secured by spray residues testing between 35 as a minimum and 
45 as a maximum for a transition zone, it will be seen that this field 
was really protected for only 3 out of 38 days by 6 applications. 
Field B, with two applications, dropped below this theoretical 
transition zone 10 days after the second application and stood with 
insufficient protection during the remaining 15 days of the test 
period. | 
Field C, with four applications, was insufficiently protected during 
a total of five days, but this could have been prevented by slight 
shortening of the intervals between applications. 
Field D was late celery sprayed first on March 1. On the basis 
of the limits assumed for the critical zone, the second application 
should have followed the first in 10 days, and the interval between 
the third and fourth might have been considerably prolonged pro- 
vided large growth development had not occurred. 
DIRECTIONS FOR USE. 
As a guide in the practical use of this method, a condensed state- 
ment of the successive steps may be outlined, as follows: 
Dissolve 3.928 grams of pure crystals of copper sulphate in distilled water and 
make up to 1,000 c. c. 
Prepare in well-stoppered bottles stock dilutions of the above solution containing 
one-half c. c., lc. c., 14. ¢c. c., 2c. c., and so on up to 10 c. c., in 100 c. c. of distilled 
water. These dilutions would then contain from one-half to 10 milligrams of copper 
per 100 c. c. 
Prepare a weak solution of potassium ferrocyanid, about 2 grams in 100 c. c. of 
distilled water. 
All of the above may be prepared in advance, by a pharmacist if desirable, and 
kept for a season’s use. 
Collect at least three representative samples of sprayed leaves, each of 200 grams 
fresh weight. 
Wash each sample separately in 1,000 c. c. of pure water to which has been added 
2 c. c. of chemically pure nitric acid, being careful to use glass or chinaware vessels 
and stirring occasionally for at least three minutes. 
Pour into test tubes or vials of uniform size about 5 c. c. portions of the graded stock 
solutions of copper sulphate, and add to each a few drops of the potassium-ferrocyanid 
solution, sufficient to develop the maximum color reaction. 
