composing a molecule of chlorine separate. Each atom of chlorine seizes on an atom of hydrogen, and forms a molecule of an entirely new gas, namely, hydrochloric-acid gas. Since each atom of chlorine takes one atom of hydrogen, it is plain that the number of molecules of each gas must be the same. In other words, 1 cubic foot of chlorine requires 1 cubic foot of hydrogen to combine with it; these gases cannot be made to combine in any other proportion. For example, if 3 cubic feet of chlorine were placed in contact with 2 cubic feet of hydrogen, 4 cubic feet of hydrochloric-acid gas would be formed, and the extra cubic foot of chlorine would remain chlorine. The combination of two or more symbols representing the elements in a compound is called a formula. The formula for hydrochloric acid is HCl.
If hydrogen and oxygen are brought in contact and heated, they will combine and form steam (or water). But it will be found that each atom of oxygen seizes two atoms of hydrogen to form a molecule of water, and therefore the volume of hydrogen must be double the volume of the oxygen with which it combines. This is shown by the formula for water, which is H2O; that is, two parts of hydrogen to one part of oxygen. Similarly, the formula for ammonia is NH3; that is, three parts of hydrogen to one part of nitrogen. Again, hydrogen and carbon form a compound in which each atom of carbon seizes four atoms of hydrogen and forms a molecule of marsh gas, CH4.
6. The formula of any compound indicates how the atoms of the elements combine to form the compound. Thus, the formula for water, H20, shows that two atoms of hydrogen and one atom of oxygen unite to form a molecule of water. The formula H2SO4, (sulphuric acid) shows that a molecule of the sulphuric acid contains two atoms of hydrogen, one of sulphur, and four of oxygen.
7. Combination by Weight.—One cubic foot of hydrogen combines with just 1 cubic foot of chlorine. But, on weighing each gas, it is found that the cubic foot of chlorine weighs 35.5 times as much as the cubic foot of hydrogen.
