streams, one of which consisted exclusively of oxygen, the other of hydrogen atoms; we shall call the streams into which the original stream is split up the electric spectrum of the atoms, and we can by means of it analyse a stream of atoms, just as a beam of light is analysed by sending it through a spectroscope and observing the different rays into which it is divided.
By means of the electric spectrum we can prove in a very direct and striking way some of the fundamental truths of the Atomic Theory. For example, when we form the electric spectrum of a mixture of gases, such as the air, we get a limited number of sharply-divided streams, which show no tendency to merge into each other. This shows that the gas contains only a few kinds of particles, and that all the particles of one kind have exactly the same mass, for if there had been any variation in the masses the streams would have been fuzzy. This shows that all the atoms of an element are alike; this had sometimes been questioned, and it had been suggested that there might be considerable variations in the masses of the atoms of the same element; ordinary chemical analysis could not settle this question, for it gives nothing more than the average mass of billions of atoms. The electric spectrum can be applied to prove the existence of molecules as well as of atoms, for when we take the electric spectrum of pure hydrogen, for example, we find that we get two streams, and that the mass of the particles in one stream is twice that of those in the other; thus the heavier particles consist of two of the lighter ones, and in hydrogen there must be some systems with two atoms, others with one. In the majority of gases the spectrum consists of two streams; there are however some gases, such as helium and mercury vapour, where there is only one stream