The Romance of Modern Chemistry

INVISIBLE SUBSTANCES the proverbial feather, for its weight is only between one five-hundredth and one six-hundredth of an ounce. Hydrogen is, in fact, the lightest substance known. The fact that hydrogen is so much lighter than air is of great importance in the manipulation of balloons. In order that a balloon may itself rise in the air and carry as well a load in its car, it must be filled with something which is considerably lighter than air. For this purpose hydrogen is the ideal substance, but coal gas, which con- tains a good deal of hydrogen, is often employed. Bulk for bulk, coal gas is about half as heavy as air. We have been comparing air with hydrogen, but it is important to bear in mind that whereas hydrogen is an element, air is a mixture chiefly of the two elements, oxygen and nitrogen, in the proportion of one volume of the former to four volumes of the latter. Air is not a chemical compound of oxygen and nitrogen, and from what has been said already about the essential difference between a mechanical mixture and a chemical compound of two elements, it will be understood that the properties of air are a sort of cross between the properties of oxygen and those of nitrogen. Both these gases are without colour or smell, but in their chemical behaviour they are widely different. Oxygen is a very active element, eager to enter into chemical combination with all sorts of bodies, and its power of supporting life is simply one phase of its activity. Nitrogen, on the other hand, is a neutral, sluggish, and inert gas, without any ambitions in the direction of chemical union. This being so, it is not surprising that air acts like diluted oxygen, the nitrogen, as it were, chilling the enthusiasms of the more active gas. Many things burn in air—that is, they combine chemically with the oxygen which it contains—but the combustion is 42