The teeth should be thoroughly cleansed night and morning, by means of a tooth-brush dipped in tepid water; once a day soap should be used, or a little very finely powdered chalk sprinkled on the brush. The weak alkali of the soap or chalk is useful. A large proportion of a tooth consists of carbonate of calcium, which readily dissolves in weak acids; and decomposing food particles lodged between the teeth develop acids, which eat away the tooth slowly but surely. Hence all food particles should be carefully removed from between the teeth; as this cannot always be effected completely it is important to brush the teeth with alkaline substances which will neutralize and render harmless any acid. Good manners forbid the public use of a tooth-pick, but on the earliest privacy after a meal a wooden or quill tooth-pick should be employed systematically and carefully to dislodge all food remnants which may have remained wedged between the teeth.

Where is the pulp cavity open? What things pass through the opening?

When and how should the teeth be cleansed? What substance forms a large part of the teeth? In what is this substance soluble? Why should food particles be carefully removed from between the teeth? Why are weak alkaline substances useful in cleaning the teeth?

* Enamel will strike fire with flint.

Acid medicines should always be sucked up through a glass tube and swallowed with as little contact as possible with the teeth. After each dose the mouth should be thoroughly rinsed with water.

Once a slight cavity has been formed, the process of decay is apt to go on very fast; first, because the exposed deeper layer of the tooth is more easily dissolved than its natural surface; and, second, because the little pit forms a lodging-place for bits of food, which, in decomposing, form acids and hasten the corrosion. Small eroded cavities are very apt to be overlooked; the teeth should, therefore, be thoroughly examined two or three times a year by a dentist.