44	QUANTITATIVE AGRICULTURAL
Calibration.—The standard working temperature
At this temperature 1 liter of distilled water, free from
gases and weighed in air with brass weights, has an
weight of 997.18 gm. The simplest and most accurate ]
calibrating is based upon this relation. Flasks or othc
tus, rated to contain any stated volume, are marked £tt
reached by the meniscus of water, taken at the rate c
gm for each cubic centimeter at 20°.
If the temperature of the balance room is not 20° a
weight of water must be taken.    Bearing in mind that
ratus actually has different volumetric capacities for
temperatures it will be seen that the calculated weight
to be used for calibrating at temperatures other than
include corrections for (a) expansion or contraction of
change in density of water and (c) change in density of
air.   All of these corrections are used in compiling the
table.
table II.—temperature corrections

Temperature, deg.

Weight of water, in grui
taken for calibruti

15            ;          997.93
16 | 997.80
17 | 997.66
18 | 997.51
19 | 997.36
20 | 997.18
21 | 996 . 99
22 | 996.81
23 | 996.61
24 | 996.39
25 | 996.16
In calibrating burettes or pipettes the water is delivei
these into a weighing bottle, which is then stoppered
weighed. The marking on a burette is too complicated
it practicable to remark the instrument. Therefore th<