A pressure of one atmosphere, or 14.7 pounds per square inch, is exerted by a column of water 33.947 feet high at 16.5° C. A column of water at 16.5° C, one foot high, presses on the base with a force of 0.433 pounds, or 6.928 ounces per square inch.

Explanation of Chemical Terms.

Atom

This is the smallest particle of matter that can enter into combination; anything extremely small.

Molecule

A molecule is the smallest mass into which any substance can be subdivided without changing its chemical nature; one of the invisible particles supposed to constitute matter of any kind.

Density

Density is denned as the amount of matter in a unit volume; the proportion or mass or quantity of matter to bulk or volume; thus, a body having twice the quantity of matter of another of the same bulk is said to have twice its density.

Gravity

Gravity is that force which tends to make bodies move downward toward the centre of the earth, and which prevents their being moved upward.

Specific Gravity

This is the ratio of the weight of a body to the weight of an equal volume of some other body taken as the standard or unit. This standard is distilled water for solids and liquids, and air for gases. Thus 19, the specific gravity of gold, expresses the fact that, bulk for bulk, gold is 19 times heavier than water.