Methods of Practical Hygiene Volume 1 (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1893 Excerpt: ... highest possible and the absolute moisture m--a is called the deficiency of saturation = d. 1 In mines, caverns, Sec, lower values for oxygen have been found down to 14 per cent. But such cases are too rare to require special notice. If the proportion of the moisture really present, and the highest possible moisture, is expressed in percentages--lnn., a. 100 a: m = r: 100, then r = m the so-called relative moisture 100--r = t is, according to Rubner, the relative dryness. Other magnitudes are not used in practical hygiene. An expression often used, but which leads less readily to a completely clear apprehension instead of absolute and maximum moisture, speaks of absolute and maximum tension or vapour pressure. If we introduce a small quantity of water into the dry vacuum of a barometer the mercury sinks at once very slightly, because a part of the atmospheric pressure supports water instead of mercury. This sinking is only Jj of the column of water which has entered, or for 4 mm. only 0-3 mm. Then there sets in a further sinking, occasioned by the development of watery vapour in a vacuum. The higher the temperature in the vacuum so much the more water evaporates, and the pressure rises the more strongly. To each'temperature there corresponds a certain tension and a certain fall of the mercury. If the water in a space containing air is evaporated the atmospheric pressure is increased by the same tension, but more slowly than in a vacuum. It is a matter of course that also the mercurial column, the length of which serves us in the barometer as a measure of the atmospheric pressure, is supported not only by the pressure of the air, but by this pressure and the tension of the watery vapour in the atmosphere. We should, therefore, strictly speaking, always dedu...