~Variations in the volume of a gas due to the pressure of aqueous vapor.~ In many cases gases are collected over water, as explained under the preparation of oxygen. In such cases there is present in the gas a certain amount of water vapor. This vapor exerts a definite pressure, which acts in opposition to the atmospheric pressure and which therefore must be subtracted from the latter in determining the effective pressure upon the gas. Thus, suppose we wish to determine the pressure to which the gas in tube A (Fig. 8) is subjected. The tube is raised or lowered until the level of the water inside and outside the tube is the same. The atmosphere presses down upon the surface of the water (as indicated by the arrows), thus forcing the water upward within the tube with a pressure equal to the atmospheric pressure. The full force of this upward pressure, however, is 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?