which had been propelled through the thin glass tube into the outer tube. Most of the alpha particles are propelled with such force that they penetrate some distance into the walls of the outer tube and some of these gradually diffuse out into the exhausted space. The presence of helium in the spectrum tube can be detected after a shorter interval if a thin cylinder of lead is placed over the emanation tube, since the particles fired into the lead diffuse out more rapidly than from glass. A still more definite proof of the identity of the alpha particle with the helium atom was obtained by removing the outer glass tube _T_ and placing a cylinder of lead over the emanation tube in the open air. Helium was always detected in the lead after it had remained several hours over the thin tube containing a large quantity of the emanation. In order to test for the presence of helium in the lead, the gases present were released by melting the lead in a closed vessel. There can thus be no doubt that the alpha particle becomes a helium atom when its positive charge is neutralized. Thus the chemist was afforded the experience of the building up of at least one element under his observation, and both the analysis and synthesis of matter have been revealed through the discoveries of radio-activity. Discovery of Helium It is of interest at this point to learn something of the history of helium and its occurrence. In 1868 there was discovered by Janssen and Lockyer a bright yellow line in the spectrum of the sun's chromosphere. Because of its origin the name helium was given to the supposed new element causing it. Later it was found in the spectra of many of the stars, and because of its predominance in some of these they were called helium stars. Its existence on our planet was not detected for nearly thirty years. In 1895, in connection with the discovery of argon in the atmosphere, a search was made to see if the latter element could be obtained from mineral sources. In analyzing certain uranium minerals Hillebrand had found considerable quantities of a gas which he took to be a peculiar form of nitrogen. Ramsay made a further examination of the gas coming from these minerals and the spectroscope revealed the yellow line of helium, thus at last proving the presence of this element on the earth. It is known now to be present in thorium minerals, in the waters of radio-active wells, and in minute amounts in the atmosphere. Its occurrence in every case, in the light of the experiment described above, would seem to be due to the presence of radio-active changes. Characteristics of Helium