Correction for Moisture.—The "vapour tension" of water has been accurately determined for various temperatures, and it may be looked upon as counteracting the barometric pressure. For example, at 15° C. the vapour tension equals 12.7 millimetres of mercury; if the barometer stood at 750 mm., the correction for moisture would be made by subtracting 12.7 from 750, and taking 737.3 mm. to be the true barometric pressure.
The vapour tensions for temperatures from 0° C. to 20° C. are as follows:—
| Temp. | Tension. | Temp. | Tension. | Temp. | Tension. |
| 0° | 4.6 mm. | 7° | 7.5 mm. | 14° | 11.9 mm. |
| 1° | 4.9 mm. | 8° | 8.0 mm. | 15° | 12.7 mm. |
| 2° | 5.3 mm. | 9° | 8.6 mm. | 16° | 13.5 mm. |
| 3° | 5.7 mm. | 10° | 9.2 mm. | 17° | 14.4 mm. |
| 4° | 6.1 mm. | 11° | 9.8 mm. | 18° | 15.3 mm. |
| 5° | 6.5 mm. | 12° | 10.5 mm. | 19° | 16.3 mm. |
| 6° | 7.0 mm. | 13° | 11.2 mm. | 20° | 17.4 mm. |
The correction for pressure is:—Multiply the volume by the actual pressure and divide by 760.
The correction for temperature:—Multiply the volume by 273 and divide by the temperature (in degrees Centigrade) added to 273.
For all three corrections the following rules hold good. To reduce to 0° C. and 760 mm. dry.
Volume × 0.3592 × (Pressure-tension)
Corrected volume = ———————————————————
Temperature + 273
To find the volume, which a given volume under standard conditions would assume, if those conditions are altered.
Volume × 2.784 × (Temperature + 273)
Resulting volume = ——————————————————
Pressure - tension
As an example, we will suppose that it is desired to enclose in the apparatus referred to on p. 45, a volume of air, which, when dry (at 0° C. and 760 mm.), shall measure 100 c.c., whilst the actual temperature is 15° C., and the pressure 750 mm.