Chemistry plays a part in every phase of life; in the arts, the industries, the household, and in the body itself, where digestion, excretion, etc., result from the action of the bodily fluids upon food.  The chemical substances of most interest to us are those which affect us personally rather than industrially; for example, soap, which cleanses our bodies, our clothing, our household possessions; washing soda, which lightens laundry work; lye, which clears out the drain pipe clogged with grease; benzine, which removes stains from clothing; turpentine, which rids us of paint spots left by careless workmen; and hydrogen peroxide, which disinfects wounds and sores.

In order to understand the action of several of these substances we must study the properties of two groups of chemicals—­known respectively as acids and bases; the first of these may be represented by vinegar, sulphuric acid, and oxalic acid; and the second, by ammonia, lye, and limewater.

202.  Acids.  All of us know that vinegar and lemon juice have a sour taste, and it is easy to show that most acids are characterized by a sour taste.  If a clean glass rod is dipped into very dilute acid, such as acetic, sulphuric, or nitric acid, and then lightly touched to the tongue, it will taste sour.  But the best test of an acid is by sight rather than by taste, because it has been found that an acid is able to discolor a plant substance called litmus.  If paper is soaked in a litmus solution until it acquires the characteristic blue hue of the plant substance, and is then dried thoroughly, it can be used to detect acids, because if it comes in contact with even the minutest trace of acid, it loses its blue color and assumes a red tint.  Hence, in order to detect the presence of acid in a substance, one has merely to put some of the substance on blue litmus paper, and note whether or not the latter changes color.  This test shows that many of our common foods contain some acid; for example, fruit, buttermilk, sour bread, and vinegar.

The damage which can be done by strong acids is well known; if a jar of sulphuric acid is overturned, and some of it falls on the skin, it eats its way into the flesh and leaves an ugly sore; if it falls on carpet or coat, it eats its way into the material and leaves an unsightly hole.  The evil results of an accident with acid can be lessened if we know just what to do and do it quickly, but for this we must have a knowledge of bases, the second group of chemicals.

203.  Bases.  Substances belonging to this group usually have a bitter taste and a slimy, soapy feeling.  For our present purposes, the most important characteristic of a base is that it will neutralize an acid and in some measure hinder the damage effected by the former.  If, as soon as an acid has been spilled on cloth, a base, such as ammonia, is applied to the affected region, but little harm will be done.  In your laboratory experiments you may be unfortunate