HELIOTROPISM

turning of a plant towards light; stems curve towards sun (positice h.), roots and stems of certain tinct turn away from sun (negative HELIUM. A gaseous element with an atomic weight of 3.99, being the lightest of the known elements with the exception of Hydrogen, which has the atomic weight 1.00. It was discovered in the sun, by means of the spectroscope, long before its existence on the earth was demonstrated. The discovery was made by Jannsen and Lockyer in 1868 and it was later found, by Ramsay, in the gases evolved from the mineral cleveite. It also occurs in the air to the extent of about one part, by volume, to 250,000 parts of air. It is believed to be the final product in the disintegration of radium, the so-called alpha rays having been identified by Ramsay as particles of helium. Because of its lightne and its non-inflammapbility and general chemical inertne , the suggestion was made by English chemists during the European war, that it should be used in place of hydrogen for filling airships and balloons. Up to that time, however, it had been prepared only on a laboratory scale, and there was no known source of supply from which it could be obtained in large quantities. The co-operation of the U. S. Bureau of Mines was sought, and under their direction the Jeffries-Norton and Linde proce es for the extraction of helium from natural gas were developed. The principal constituents of natural gas are nitrogen, methane and helium, and the separation of the gases is brought about by freezing to extremely low temperatures. At 318° F. below zero, nitrogen and mehtane become liquified, while helium remains in a gaseous condition. The separation then becomes an easy matter, the the helium being drawn off from the liquid gases. The proce was developed too late to be of value during the war, although when the Armistice was signed quantities of the gas were awaiting shipment to France. Since the war, however, considerable experimental work has been carried out with the gas, and the results are considered satisfactory. Although helium is denser than hydrogen, its greater safety more than counterbalances the lo in buoyancy.