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Water for Drinking.   225

the same principle, but less efficiently than the casks. Sand, charcoal, sponge, and wool, are the substances most commonly used in properly constructed filters : peat charcoal is excellent. Charcoal acts not only as a mechanical filter for solid impurities, but it has the further advantage of absorbing putrid gases. (See below, " Putrid Water.") Snow is also used as a filter in the Arctic regions. Dr. Rae used to lay it on the water, until it was considerably higher than its level, and then to suck the water through the snow.

Alum.-Turbid water is also, in some way as yet insufficiently explained, made clear by the Indian plan of putting a piece of alum into it. The alum appears to unite with the mud, and to form a clayey deposit. Independently of this action, it has an astringent effect upon organic matters : it hardens them, and they subside to the bottom of the vessel, instead of being diffused in a glairy, viscous state, throughout the water. No taste of alum remains in the water, unless it has been used in great excess. Three thimblefuls of alum will clarify a bucketful of turbid water.

Putrid Water should always be purified by boiling it together with charcoal or charred sticks, as low fevers and dysenteries too often are the consequences of drinking it. The mere addition of charcoal largely disinfects it. Bitter herbs, if steeped in putrid water, or even rubbed well about the cup, are said to render it less unwholesome. The Indians plunge hot iron into putrid water.

Thirst, to relieve.-Thirst is a fever of the palate, which may be somewhat relieved by other means than drinking fluids.

By exciting Saliva.-The mouth is kept moist, and thirst is mitigated, by exciting the saliva to flow. This can be done by chewing something, as a leaf; or by keeping in the mouth a bullet, or a smooth, non-absorbent stone, such as a quartz pebble.

By Fat or Butter.-In Australia, Africa, and N. America, it is a frequent custom to carry a small quantity of fat or butter, and to eat a spoonful at a time, when the thirst is severe.