not spent in compressing the gas within the
tube, for since it is collected over water it contains a certain amount
of water vapor. This water vapor exerts a pressure (as indicated by the
arrow within the tube) in opposition to the upward pressure. It is
plain, therefore, that the effective pressure upon the gas is equal to
the atmospheric pressure less the pressure exerted by the aqueous vapor.
The pressure exerted by the aqueous vapor increases with the
temperature. The figures representing the extent of this pressure (often
called the _tension of aqueous vapor_) are given in the Appendix. They
express the pressure or tension in millimeters of mercury, just as the
atmospheric pressure is expressed in millimeters of mercury.
Representing the pressure of the aqueous vapor by a, formula (5)
becomes

(6) V_{s} = v(p - a)/(760(1 + 0.00366t)).

The following problem will serve to illustrate the method of applying
the correction for the pressure of the aqueous vapor.

The volume of a gas measured over water in a laboratory where the
temperature is 20 deg. and the barometric reading is 740 mm. is 500 cc. What
volume would this occupy under standard conditions?

The pressure exerted by the aqueous vapor at 20 deg. (see table in Appendix)
is equal to the pressure exerted by a column of mercury 17.4 mm. in
height. Substituting the values of v, t, p, and a in formula
(6), we have

(6) V_{s} = 500(740 - 17.4)/(760(1 + 0.00366 x 20)) = 442.9 cc.

~Adjustment of tubes before reading gas volumes.~ In measuring the volumes
of gases collected in graduated tubes or other receivers, over a liquid
as illustrated in Fig. 8, the reading should be taken after raising or
lowering the tube containing the gas until the level of the liquid
inside and outside the tube is the same; for it is only under these
conditions that the upward pressure within the tube is the same as the
atmospheric pressure.

EXERCISES

1. What is the meaning of the following words? phlogiston, ozone,
phosphorus. (Consult dictionary.)

2. Can combustion take place without the emission of light?

3. Is the evolution of light always produced by combustion?

4. (a) What weight of oxygen can be obtained from 100 g. of water?
(b) What volume would this occupy under standard conditions?

5. (a) What weight of oxygen can be obtained from 500g. of mercuric
oxide? (b) What volume would this occupy under standard conditions?

6. What weight of each of the f