Water which is in any way held mechanically may be dislodged by mechanical means, by friction, pressure, centrifugal force, &c. Thus water is squeezed from wet cloth by pressure or centrifugal machines. But objects which in practice are called dry (because they do not feel wet) often still contain moisture, as may be proved by heating the object in a glass tube closed at one end. By placing a piece of paper, dry earth, or any similar object (especially porous substances) in such a glass tube, and heating that part of the tube where the object is situated, it will be remarked that water condenses on the cooler portions of the tube.
The presence of such absorbed, or ‘hygroscopic,’ water is generally best detected in non-volatile substances by drying them at 100°, or under the receiver of an air-pump and over substances which attract water chemically.
By weighing a substance before and after drying, it is easy to determine the amount of hygroscopic water from the loss in weight.13
Only in this case the amount of water must be judged with care, because the loss in weight may sometimes proceed from the decomposition of the substance itself, with disengagement of gases or vapour. In making exact weighings the hygroscopic capacity of substances—that is, their capacity to absorb moisture—must be continually kept in view, as otherwise the weight will be untrue from the presence of moisture.
The quantity of moisture absorbed depends on the degree of moisture of the atmosphere (that is, on the tension of the aqueous vapour in it) in which a substance is situated.
In an entirely dry atmosphere, or in a vacuum, the hygroscopic water is expelled, being converted into vapour; therefore, substances containing hygroscopic water may be completely dried by placing them in a dry atmosphere or in a vacuum.
The process is aided by heat, as it increases the tension of the aqueous vapour.
Phosphoric anhydride (a white powder), liquid sulphuric acid, solid and porous calcium chloride, or the white powder of ignited copper sulphate, are most generally employed in drying gases.
They absorb the moisture contained in air and all gases to a considerable, but not unlimited, extent.
- Phosphoric anhydride and calcium chloride deliquesce, become damp,
- sulphuric acid changes from an oily thick liquid into a more mobile liquid, and
- ignited copper sulphate becomes blue;
after which changes these substances partly lose their capacity of holding water, and can, if it be in excess, even give up their water to the atmosphere.
We may remark that the order in which these substances are placed above corresponds with the order in which they stand in respect to their capacity for absorbing moisture.
Air dried by calcium chloride still contains a certain amount of moisture, which it can give up to sulphuric acid. The most complete desiccation takes place with phosphoric anhydride.
Water is also removed from many substances by placing them in a dish over a vessel containing a substance absorbing water under a glass bell jar.14 The bell jar, like the receiver of an air pump, should be hermetically