222 
water would not be wholly free from oxidizable matter ; 
hence in a little while there will be a depreciation in the 
tint of the permanganate solution. If the tint be strong the 
error arising from this cause might be neglected, but when the 
tint is feeble the error is appreciable. If the depreciation in 
tint in the two solutions compared were proportional to the 
depth of tint in each, this source of error might be ne- 
glected ; but inasmuch as the same quantity of water has 
been used in making each solution, an equal amount of 
depreciation is introduced into each. Some of the foregoing 
experiments will be affected by this source of error. Small 
sources of error which may be neglected when we deal with 
ponderable quantities become of sufficient magnitude to 
vitiate results when we deal with traces. 
For experiments with a yellow solution I took a solution 
of bichromate of potash. Standard solution 10 in 500 cc. of 
water, depth 8*3. 
A 
B 
C 
15 
5-4 
15-4 
20 4-4 18-8 
30 3*5 23-7 
40 
2-5 
34-0 
50 
1*7 
48'8 
60 
1-5 
55-3 
Still stronger solutions of the bichromate were compared 
with this standard, but the discrepancies were considerable, 
Some of the results also given' in the last table are not very 
good; this more especially applies to the third and fourth 
results. On another occasion I repeated some of these ex- 
periments with the following results — standard solution 
same as last : 
ABC 
30 
3 ..... 
27*6 
40 
3-2 
37-7 
50 
1-6 
51 *8 
60 
1-4 
59*3 
