62 OUTLINES OF PHYSICAL CHEMISTRY
is only moderately narrow and carries a mark. By means of a platinum wire attached to the bends, the apparatus can be suspended on a balance. The substance whose specific gravity is to be determined is filled into the apparatus by aspiration. The instrument and its contents are then brought to the desired temperature by immersion for a few minutes in a water bath at that temperature. If, after assuming this temperature, the meniscus of the Hquid in the branch b is not exactly at the mark, it can very easily be adjusted. To remove an excess of the Hquid it is only necessary to bring a piece of filter paper in contact with the capillary end c ; or, if it be necessary to fill a little more liquid into the apparatus, then a glass rod with a drop of the liquid hanging at the end is brought in contact with the end of c. The weight p of the empt instrument, p f of the instrument filled with water l at t°, and the weight p" full of substance at t° are then deter- mined. The weight of the substance is then m = p" - p, and the weight of an equal volume of water is w =■ p' — p. For ordinary determinations the specific gravity or density,
d, at t° is equal to — .
w
But in scientific work it is usual to take as standard of comparison not water at t° but water at its maximum density (at 4°C). Accuracy to the fourth decimal place can be obtained by reducing the weighings to vacuum. This double correction can be made by means of the formula :
4° w
where q denotes the density of water at t° and \ the density of the air during the time of the experiment.
q can be obtained from chemical tables, and for X it is generally sufficient to take the average value 0'0012.
The pycnometric method is very exact, but requires con- siderable time. We can obtain results much more quickly,
\ It is preferable to take boiled water as free as possible from air.
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