withdrawn for testing, shake the flask to ensure mixing, and then take
out a drop with the test-rod; the neglect of these precautions may give
a finishing-point too early. This is generally indicated by a sudden
finish, in which case on shaking the flask and again testing no reaction
is got. Do not remove the drop on the point of the burette with the
test-rod; let it remain where it is or drop it into the solution by
carefully opening the clip.

Generally the methods of working are as follows:--

(1) _When the finishing-point depends on a change of colour in the
solution._--Increase the bulk of the assay up to from 100 to 150 c.c.
with water. Boil or cool, as the case may be. Run in the standard
solution from a burette speedily, until the re-agent appears to have a
slower action, and shake or stir all the time. Then run 1 c.c. or so at
a time, still stirring, and finally add drops until the colour change is
got.

(2) _When an outside-indicator is used._--Pour the standard solution
from a burette into the assay until 5 or 6 c.c. from the
finishing-point; then run in 1 c.c. at a time (stirring and testing on
the plate between each) until the indicator shows the change wanted, and
deduct 0.5 c.c. for excess. When greater accuracy is sought for a
duplicate assay is made. In this case the standard solution is run in
close up to the end, and the operation is finished off with a few drops
at a time.

(3) _Where the finishing-point depends upon the absence of a precipitate
and no outside-indicator is used._--As in the last case, run in the
standard solution up to within a few c.c. of the end, then run in 1 c.c.