in steam under
pressure. Unless this is done, the filters may become incubators for
bacteria.

[Illustration: FIG. 62.--PASTEUR WATER FILTERS.]

273. Distillation.--When an unquestionably pure water supply is
desired, distillation should be resorted to. There are many forms of
stills for domestic use which are easily manipulated and produce
distilled water economically.[93] The mineral matter of water is in no
way essential for any functional purpose, and hence its removal through
distillation is not detrimental.

[Illustration: FIG. 63.--WATER STILL.]

274. Chemical Purification.--Purification of water by the use of
chemicals should not be attempted in the household or by inexperienced
persons. When done under supervision of a chemist or bacteriologist, it
may be of great value to a community. Turneaure and Russell,[94] in
discussing the purification of water by addition of chemicals, state:

"There are a considerable number of chemical substances that may be
added to water in order to purify it by carrying down the suspended
matter as well as bacteria, by sedimentation. Such a process of
purification is to be seen in the addition of alum, sulphate of
iron, and calcium hydrate to water. Methods of this character are
directly dependent upon the flocculating action of the chemical
added, and the removal of the bacteria is accomplished by
subsidence."

275. Ice.--The purity of the ice supply is also of much importance.
While freezing reduces the number of organisms and lessens their
vitality, it does not make an impure water absolutely wholesome. The
way, too, in which ice is often handled and stored subjects it to
contamination, and foods which are placed in direct contact with it
mechanically absorb the impurities which it contains. For cooling water,
ice should be placed around rather than in it. Diseases have frequently
been traced to impure ice. The only absolutely pure ice is that made
from distilled water.

276. Mineral Waters.--When water is charged with carbonic acid gas
under pressure, carbonated water results, and when minerals, as salts of
sodium, potassium, or lithium, are added, artificial mineral waters are
produced. Natural mineral waters are placed on the market to some
extent, but most mineral waters are artificial products and they are
sometimes prepared from water of low sanitary character. Mineral waters
should not be used extensively except under medi