By ignoring these considerations, errors of 3 or 4 per cent. are easily made; but, fortunately, the corrections are simple, and it is easy to construct a piece of apparatus by means of which they may be reduced to a simple calculation by the rule of three. The volume of a gas is, in practice, usually reduced to that which it would be at a temperature of 0° C., when the column of mercury in the barometer is 760 mm. high. But, although convenient, this practice is not always necessary. The only thing required is some way of checking the variations in volume, and of calculating what the corrected volume would be under certain fixed conditions. Suppose that at the time a series of standardisings is being made, 100 c.c. of air were confined in a graduated tube over moist mercury. These 100 c.c. would vary in volume from day to day, but it would always be true of them that they would measure 100 c.c. under the same conditions as those under which the standardisings were made. If, then, in making an actual assay, 35.4 c.c. of gas were obtained, and the air in the tube measured 105 c.c., we should be justified in saying, that if the conditions had been those of the standardising, the 105 c.c. would have measured 100 c.c., and the 35.4 c.c. would have been 33.7; for 105: 100:: 35.4: 33.7. The rule for using such a piece of apparatus for correcting volumes is:--_Multiply the c.c. of gas obtained by 100, and divide by the number of c.c. of air in the apparatus._ If it is desired to calculate the volumes under standard conditions (that is, the gas dry, at 0° C. and 760 mm. barometric pressure) the calculations are easily performed, but the temperature and pressure must be known. _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.