This section is from the book "Health", by W. H. Coefield.
Under the head of Climate let us first consider moisture in the atmosphere. The air dissolves water just in the same way as water dissolves sugar or salt. The air dissolves very different quantities of water, according to its temperature. Cold air dissolves very little water, warm air dissolves a great deal Air dissolves water out of all proportion to the rate of the increase of its temperature, so that warm air dissolves a great deal more water than we should expect from its temperature, if we begin by considering how much cold air would dissolve; and so it follows, if you take warm air with water dissolved in it, and cool it, at a certain temperature it will no longer hold all the water-in solution, so that some will be deposited ; and that temperature is called the dew point You can prove that by taking a tumbler, and putting a lump of ice into it; after a time water will be deposited on the outside, and that comes from the moisture that was dissolved in the air; the air in the tumbler has cooled the air around to such a degree that it can no longer hold as much water dissolved in it as it held before; and so some of that water is deposited in the form of drops, or, as we call it, in the form of dew. Now, there are several ways of finding out how much moisture there is in the air; one way is to weigh the air ; moist air, of course, weighs heavier than the same bulk of dry air: but that is a very difficult process. Then there are instruments for directly showing the dew point; these are called hygrometers, by means of which the result is obtained very quickly by calculations from tables, telling how much moisture was in the air at the time of the experiment.
The most widely known hygrometer is that which goes by the name of De Saussure's. I name this particular one because it is one you will recognise at once. Its principle is founded on the fact that a piece of catgut is longer when it is wet than when dry; and so, if you take a piece of catgut, you can make it move a needle as it shortens or lengthens. You will recognise this instrument at once, when I tell you that you have all seen it in the form of a little toy that is sold for telling the weather. You see it very often on mantelpieces- a little house with two doors; a lady comes out in fine weather, and a gentleman in wet. This is produced by the shortening or lengthening of the piece of catgut.
The way, however, in which the amount of moisture is ordinarily determined, is by means of the wet and dry bulb thermometers. This instrument consists of two precisely similar mercurial thermometers. One has some cotton wound round the bulb, the other end of which dips into a vessel containing water; the water rises up the threads of cotton by capillary attraction, and keeps the bulb continually wet. When the air around these two thermometers is saturated with moisture, then the mercury stands at the same height in them both. You will see why, because one is continually moist, and the other is as moist as it can be, because it is in air containing as much moisture as the air will dissolve. But suppose the air, at the time of the experiment, does not contain as much moisture as it can hold, then the moisture evaporates from the wet cotton into the air around. Whenever evaporation takes place heat is required to convert the moisture of this cotton into vapour, which is dissolved by the air around. That heat must come from somewhere, and it is taken from the mercury in the thermometer, which contracts, and therefore falls in the tube. Whenever air does not contain as much water as it can hold in solution, the mercury stands at a lower point in the wet than it does in the dry bulb thermometer.
Now, from the difference in the height we are able to calculate the amount of moisture in the air.
In inhabited places the rule is that the difference between the height of the mercury of these two thermometers should not be less than 4° nor more than 5° Fahr. If less than 4° the air is too moist; if more than 5° it is too dry.
You will see, if you notice the reports in the papers, that the amount of moisture in the air is stated as so much per cent of saturation.
Whenever the air contains as much moisture dissolved in it as it can hold, no matter what the temperature, it is said to be saturated with moisture, and its state is represented by the number 100 ; and if ever you see in weather reports, under the head of moisture of the air, the number 100, you will know that the air contains as much as it can hold. The amount of moisture that the air contains when it is not saturated at any given time is expressed in numbers proportional to the ratio between the amount that it does contain and the amount it would contain if it were saturated. Suppose, for instance, it contains half as much water as it can hold in solution, then, no matter what the temperature may be, its state of saturation is represented by 50. So that you see if 80 is represented as the state of the saturation of the air at a certain time on a certain day, you know then that the amount of moisture in the air at that time was iWrths, or iths of what it could contain at that temperature.
At 64° Fahr. air will hold in solution 6 J grains of moisture per cubic foot, and if it has that quantity it is saturated.
Now, if on any given day it is found that the temperature is 64°, and there are 6£ grains of moisture per cubic foot, the state of saturation is represented by the number 100.
The freezing point of water is 32° Fahr., and air at that temperature will hold a little over 2 grains of water in solution. If on any day the thermometer is at the freezing point, and the air contains a little over 2 grains of water in each cubic foot, then the saturation of that air is also represented by 100, so that the number 100 means very different things with different temperatures.
As to the influence of moisture dissolved in air- animals in a dry atmosphere lose weight much faster than they do in a moist one. This is because they cannot get rid of moisture by their respiratory organs, or by their skin, fast enough in a moist atmosphere.