MICROSCOPICAL EXAMINATION OF FLOUR. 9 
The variation in the counts on these slides naturally raises the ques- 
tion of the limits of accuracy in weighing out the test portion of flour. 
Since the amount of flour used on a slide is 5 milligrams, it is desirable 
to determine how great is the error due to weighing the test portion of 
flour. The balance employed in this investigation was a fine assay 
balance. In weighing the sample the vibration method was used, 
and the quantity of flour was so adjusted as to produce a deviation 
of approximately not more than one-fourth of a space on each side of 
the zero point of the scale. This is equivalent. to not more than 1/40 
milligram, or one-half of 1 per cent, on the basis of the portion of flour 
used (5 milligrams) . Hence any error in weighing can not be accepted 
as an explanation of the difference in slide counts. 
VARIATION IN HOMOGENEITY OF BULK SAMPLE. 
The question has been raised as to whether or not a portion of the 
slide variation might not be accredited to lack of uniformity of the 
bulk sample, due to the fact that any grade of flour is usually the 
component result of several constituent streams which vary more or 
less- among themselves. The fact that in general practice the flour 
stocks are subjected to a certain degree of purification, however, 
leaves this factor little chance to figure to any great extent. This 
point was tested by passing a certain sample of flour which had an 
average count of 32 bran particles and 64 hairs through a 30-mesh 
sieve and making up and counting 12 slides. The bulk sample was 
then passed through the sieve once more (making two times for the 
sample), and another series of slides made and counted. Finally, 
the sample was put through the sieve twice more (making four times 
for the sample) , and a third series of 12 slides made and counted. The 
results of these tests are given in Table 6, the counts in which are the 
average of the results obtained by two persons. 
Table 6. — Effect of variation in homogeneity of sample on count. 
Sample passed through 30-mesh sieve- 
Once. 
Twice. 
Four times. 
Bran 
particles. 
Hairs. 
Bran 
particles. 
Hairs. 
Bran 
particles. 
Hairs. 
31 
48 
38 
33 
41 
36 
35 
35 
37 
35 
30 
34 
73 
73 
58 
53 
64 
64 
74 
75 
57 
76 
66 
60 
26 
37 
25 
22 
27 
32 
32 
27 
34 
26 
39 
30 
67 
70 
58 
54 
61 
57 
83 
64 
61 
72 
67 
67 
21 
22 
30 
32 
23 
29 
36 
34 
39 
38 
33 
37 
76 
53 
56 
81 
79 
66 
60 
• 56 
70 
52 
65 
48 
135 
218 
166 
2 23 
129 
217 
165 
2 29 
131 
2 18 
163 
2 2,1 
i Average. 
112350°— 22— Bull. 839- 
» Variation 
