quently in addition given a light dipping in varnish as a
protection against the decaying influences of the atmosphere.

The only wise preservatives are those long known and employed by our
ancestors; salt, vinegar, and spices are all food preservatives, but
they are at the same time substances which in small amounts are not
injurious to the body. Smoked herring and salted mackerel are
chemically preserved foods, but they are none the less safe and
digestible.

235. The Preservation of Wood and Metal. The decaying of wood and
the rusting of metal are due to the action of air and moisture. When
wood and metal are surrounded with a covering which neither air nor
moisture can penetrate, decay and rust are prevented. Paint affords
such a protective covering. The main constituent of paint is a
compound of white lead or other metallic substance; this is mixed with
linseed oil or its equivalent in order that it may be spread over wood
and metal in a thin, even coating. After the mixture has been applied,
it hardens and forms a tough skin fairly impervious to weathering. For
the sake of ornamentation, various colored pigments are added to the
paint and give variety of effect.

Railroad ties and street paving blocks are ordinarily protected by oil
rather than paint. Wood is soaked in creosote oil until it becomes
thoroughly saturated with the oily substance. The pores of the wood
are thus closed to the entrance of air and moisture, and decay is
avoided. Wood treated in this way is very durable. Creosote is
poisonous to insects and many small animals, and thus acts as a
preservation not only against the elements but against animal life as
well.

CHAPTER XXV

DRUGS AND PATENT MEDICINES

236. Stimulants and Narcotics. Man has learned not only the action
of substances upon each other, such as bleaching solution upon
coloring matter, washing soda upon grease, acids upon bases, but also
the effect which certain chemicals have upon the human body.

Drugs and their varying effects upon the human system have been known
to mankind from remote ages; in the early days, familiar leaves,
roots, and twigs were steeped in water to form medicines which served
for the treatment of all ailments. In more recent times, however,
these simple herb teas have been supplanted by complex drugs, and now
medicines are compounded not only from innumerable plant products, but
from animal and mineral matter as well. Quinine, rhubarb, and arnic