preface to fifteenth edition

2.70 SECTION 2
Example 3.
25
1.2500 :
4
To find the specific gravity at 20/4C of a liquid which has a specific gravity of
Since the density of water at 4C, is equalto 1, specific gravity 25/4 d25
1.2500; and,
specific gravity 20/4 d
20.
Substitution in Equation 3, with d
t
1.2500; and, with F t
at 25C, from the table, equal to
0.99705, gives
Sp. gr. 25/25 S 1.2500 0.99705
Substitution of this value for S in Equation 2, with F t
at 20C, from the table, equal to 1.0036,
gives
20
Sp. gr. 20/4 d (1.2500 0.99705) 1.0036 1.2582
15
Example 4. To find the density at 25C of a liquid which has a specific gravity of 1.250015:
Since the density of water at 15C 0.99910,
Substitution in Equation 3, with
15
d sp. gr. 15/15 0.99910 1.2500 0.99910
F t
at 15C, from the table, equal to 1.0102, gives
Sp. gr. 25/25 S (1.2500 0.99910) 1.0102
Substitution of this value for S in Equation 2, with F t
at 25, from the table, equal to 0.99705,
gives
26 t
d d (1.2500 0.99910 1.0102) 0.99705 1.2326
2.1.3 Barometry and Barometric Corrections
In principle, the mercurial barometer balances a column of pure mercury against the weight of the
atmosphere. The height of the column above the level of the mercury in the reservoir can be measured
and serves as a direct index of atmospheric pressure. The space above the mercury in a barometer
tube should be a Torricellian vacuum, perfect except for the practically negligible vapor pressure of
mercury. The perfection of the vacuum is indicated by the sharpness of the click noted when the
barometer tube is inclined. A barometer should be in a vertical position, suspended rather than
fastened to a wall, and in a good light but not exposed to direct sunlight or too near a source of
heat. The standard conditions for barometric measurements are 0C and gravity as at 45 latitude
and sea level. There are numerous sources of error, but corrections for most of these are readily
applied. Some of the corrections are very small, and their application may be questionable in view
of the probably larger errors. The degree of consistency to be expected in careful measurements is
about 0.13 mm with a 6.4-mm tube, increasing to 0.04 mm with a tube 12.7 mm in diameter.
In reading a barometer of the Fortin type (the usuallaboratory instrument for precision measurements),
the procedure should be as follows: (1) Observe and record the temperature as indicated
by the thermometer attached to the barometer. The temperature correction is very important and may
be affected by heat from the observer’s body. (2) Set the mercury in the reservoir at zero level, so
that the point of the pin above the mercury just touches the surface, making a barely noticeable
dimple therein. Tap the tube at the top and verify the zero setting. (3) Bring the vernier down until
the view at the light background is cut off at the highest point of the meniscus. Record the reading.
The corrections to be made on the reading are as follows: (1) Temperature, to correct for the
difference in thermal expansion of the mercury and the brass (or glass) to which the scale is attached.