Page:Astronomy for Everybody.djvu/100

form light. As we make it hotter it still emits more and more waves of long wave lengths, and also waves of shorter and shorter wave lengths. Thus as we heat up a piece of iron, it appears first as red hot, and afterward as white hot.

The possibility of reaching conclusions about the constitution of a hot body from the light which it emits arises from the fact that different bodies emit light of different wave lengths. If the body is solid, it emits light of all wave lengths, and we cannot tell much about it. But if it is a mass of transparent gas, it only emits light of certain wave lengths, depending on the nature of the gas.

The easiest way of making a gas emit its peculiar light is by passing an electric spark or current through it. Then, if we analyse the light produced by the spark with a prism, we find that the spectrum is composed of one or more bright lines, varying in position according to the nature of the gas. Thus we have a spectrum of hydrogen, another of oxygen, and others of almost all the bodies which we know. Solid bodies, including all the metals, can be made to give their spectrum by being heated so intensely by the electric spark that a small quantity of the body is changed into a gas. Thus we may even form a spectrum of iron, which the practised observer can immediately detect as iron by the position and arrangement of the lines of the spectrum.

The fundamental principle of spectrum analysis is that if the light of an incandescent body passes through