cal Measurements may be consulted.

=To Calibrate Apparatus.=--The glass tubes of which graduated apparatus
is made are, as already stated, very rarely truly cylindrical
throughout their entire lengths. It follows that the capacities of equal
lengths of a tube will usually be unequal, and therefore it is necessary
to ascertain by experiment the true values of equal linear divisions of
a tube at various parts of it.

A burette may be calibrated by filling it with distilled water, drawing
off portions, say of 5 c.c. in succession, into a weighing bottle of
known weight, and weighing them.

Great care must be taken in reading the level of the liquid at each
observation. The best plan is to hold a piece of white paper behind the
burette, and to read from the lower edge of the black line that will be
seen. Each operation should be repeated two or three times, and the mean
of the results, which should differ but slightly, may be taken as the
value of the portion of the tube under examination.

If the weights of water delivered from equal divisions of the tube are
found to be equal, the burette is an accurate one, but if, as is more
likely, different values are obtained, a table of results should be
drawn up in the laboratory book showing the volume of liquid delivered
from each portion of the tube examined. And subsequently when the
burette is used, the volumes read from the scale on the burette must be
corrected. Suppose, for example, that a burette delivered the following
weights of water from each division of 5 c.c. respectively:--

C.C. Grams.

0 to 5 gave 4.90
5 " 10 " 4.91
10 " 15 " 4.92
15 " 20 " 4.93
20 " 25 " 4.94
25 " 30 " 4.95
30 " 35 " 4.96
35 " 40 " 4.97
40 " 45 " 4.98
45 " 50 " 4.99

and that in two experiments 20 c.c. and 45 c.c. respectively of a liquid
re-agent were employed. The true volumes calculated from the table would
be as 19.66 to 44.46.

If the temperature remained constant throughout the above series of
experiments, and if the temperature selected were 4 deg. C., the weights of
water found, taken in grams, give the volumes in cubic centimetres, for
one gram of water at 4 deg. C. has a volume of one cubic centimetre. If the
temperature at which the experiments were made was other than 4 deg. C., and
if great accuracy be desired, a table of densities must be consulted,
with the help of which the v