ings when the pressure is 30 inches of mercury; also the number of gallons of water equivalent to the same weight of water vapor in

36

35

390

FIGURE 3.-Curves of equal relative humidity for varying temperatures and depressions of wet bulb

100,000 cubic feet of air at different temperatures and different degrees of saturation when the barometric pressure is 30 inches of mercury.

Exact results can be obtained from these figures when the barometric pressure at the time the wet and dry bulb readings are made

35

30

FIGURE 4.-Curves of equal relative humidity for varying temperatures and volumes of water carried

equals 30 inches of mercury. For practical purposes in determining the moisture content of the air of a mine, however, these curves may be used even when the true barometric pressure is much more

49530°-26- -2

or less than 30 inches of mercury. That the barometric pressure may often be neglected is shown by the tabulation below of the relative humidity of atmospheres when the wet-bulb reading is 67° F. and the dry-bulb reading is 75° F., at different barometric pressures:

Relative humidity with different barometric pressures Barometric pressure, inches of mercury:

25___

27_

29_

30_.

Per cent

68

67

66

66

Results of gas analysis, as determined in the laboratory and reported, are on the dry or moisture-free basis; all air, however, contains a certain proportion of moisture. This quantity can be determined and included in the analyses, but the percentage of each of the other constituents will, of course, be lowered thereby. If the sample contains 2 per cent of moisture, a not uncommon proportion, and the oxygen as determined by analysis is 20 per cent, the true percentage of oxygen, when allowance is made for the water vapor, becomes 19.60 per cent. The proportions of oxygen and nitrogen are affected most.

Tables have been carefully worked out to show the pressure of aqueous vapor at saturation for different temperatures. Hence, if the degree of saturation of the air with aqueous vapor is obtained by means of the sling psychrometer, the aqueous tension of the water vapor present in the air can be determined and the percentage by volume of the aqueous vapor found from the ratio of the partial pressure of the water vapor to the total barometric pressure. Thus, if the air is 41 per cent saturated at 20.6° C., the pressure of the aqueous vapor is 7.86 mm. of mercury. If the total barometric pressure is 746 mm. of mercury, the percentage of aqueous vapor by volume is

LABORATORY APPARATUS FOR THE DETERMINATION OF MOISTURE IN MINE AIR

Figure 5 shows an apparatus assembled by the authors for the determination of moisture in gas samples. It works on the same principle as Pettersson's apparatus, as described by Hempel,10 except that the gas sample is passed over phosphorus pentoxide instead of being confined in a tube containing the phosphorus pentoxide under pressure until the dehydrating action is complete. Hempel

10 Hempel, Walther, Methods of gas analysis. 3d German ed., trans. by L. M. Dennis, 1910, p. 346.