Helium, when pure, has a density of only 1.98, that of oxygen being taken as 16. Its atomic weight cannot be directly determined, because helium has not yet been made to com bine with any other substance, although it has been subjected to the same experimental at tempts as were tried in the case of argon (q.v.). The only approach to this was in establishing the fact that zinc vaporized in helium has a vapor density 12 per cent greater than when vaporized in nitrogen under identical condi tions. It has been found, however, that the ratio of the specific heat of the gas at constant pressure to the specific heat at constant volume is about 1.65, and this indicates that helium is a monatomic gas (see GASES, KINETIC THEORY OF), and that its atomic weight is about 2X1.98=3.96; the atomic weight of oxy gen being taken as 16. It therefore has the smallest atomic weight of any of the known elements except hydrogen. The chemical sym bol He has been assigned to helium, although, as has been already noted, no compounds of it have as yet been obtained.
Dewar thought he had liquefied helium at the temperature of melting hydrogen (about 436° F. below zero), but this was not confirmed by sub sequent experiments. Success was finally achieved in 1909 by Onnes, using the Linde process. Liquid helium is colorless and very mobile. It has a density of 0.122, being there fore the lightest liquid known. It boils at 4.5° absolute. By evaporating it rapidly a tem perature of —454° F. has been attained, but the helium did not solidify. Its critical tem perature being nearer to the absolute zero than that of hydrogen, the gas is well adapted for use in thermometers intended for the measure ment of exceedingly low temperatures. It has, in fact been used for this purpose with success, in studying the properties of hydrogen.
A most remarkable and previously unparal leled fact in connection with helium remains to be recorded. It has been known for some time that helium occurs in cleveite, and in other min erals in which the newly discovered element radium is found; but whether this was to be re garded as a mere coincidence, or whether it has some actual physical and chemical significance, has been a subject of considerable discussion. The most striking experiment bearing upon this matter is one that is due to Huggins, who caused the radiation from radium to pass through a spectroscope provided with a quartz prism and to fall upon a sensitive photographic plate. Upon developing the plate after a pro longed exposure, he found that cold radium gives a line spectrum when treated in this man ner; and he made the further discovery that nearly all of the lines in the spectrum so ob tained are apparently coincident with lines in the spectrum of helium. The full significance of
this fact is not yet known; but when taken in connection with the observations of Soddy and Ramsay, which indicate that helium occurs in the gaseous emanation that is given offby radium, it is considered by no means impossible that we have here an instance in which one ele ment is into slowly but continuously trans formed nto another one. If this inference is corroborated by future experiments, it will throw an altogether new light upon the nature of the chemical elements, and upon their rela tion to one another. The case is the more note worthy, since helium has a smaller atomic weight than any other element save hydrogen, and radium has a greater atomic weight than any element save uranium and thorium. Radium, moreover, appears to be metallic in nature, while helium, by Its chemical inertness, resembles nitrogen.
Helium is prepared by separating it from its mixture with nitrogen, argon, krypton, etc. Several methods are in use. If atmospheric nitrogen is passed over heated magnesium (with or without lime) the nitrogen is absorbed and the inert gases collected, to be afterward sepa rated. Helium may also be obtained from liquid air by fractional distillation. The method most commonly used is to heat to redness the powder of helium-bearing minerals in a tube of iron or hard glass, having previously ex hausted the air from the apparatus. The gases are collected over mercury. By this process one ounce of cleveite yields about two gallons of helium. The separation of helium from the mixed gases is accomplished by filtering through quartz. At a temperature of 1,850° to 2,200° F. quartz is permeable to helium, A tube of quartz is so arranged in an atmosphere of the mixed gases that it may be heated to the required degree, and at the same time the in terior of this tube is exhausted by an air pump. Under these conditions the helium passes through the quartz into the tube, from which it may be pumped into a collector. Or the gas mixture is passed into a vacuum tube containing charcoal at the temperature of liquid air. All the gases but neon and helium are con densed, and these are readily separated by the difference of their vapor pressures.