use, as was dramatically demonstrated when the zeppelin Hindenburg exploded and was then destroyed by fire at Lakehurst, New Jersey, in 1937. The inertness of helium firmly established it as the lifting gas of choice for most applications.
The properties of helium that make it desirable for application as a lifting gas (i.e., its chemical inertness and low mass) also underlie its use for many other commercial applications. There are, however, two other properties of helium that are important in some special industrial processes. First, the thermal conductivity of gaseous helium is five to six times greater than that of other gases (the exception is hydrogen, which is comparable in thermal conductivity). Second, helium atoms share with hydrogen the ability to diffuse with relative ease through many solid materials, especially at elevated temperatures. An example in which these properties play a vital role is the manufacture of optical fibers, where the high thermal conductivity is important during the heat-treatment phase of fabrication, and the rapid diffusion of helium through the glass ensures that there are no trapped bubbles that would destroy the desired properties of the fibers.
Helium's low liquefaction temperatures make it desirable for purging, pressurization, and cryogenic applications. The fact that it remains a gas at liquid hydrogen temperatures makes it especially useful in the purging and pressurization of liquid hydrogen rocket propulsion systems. Another important use for liquid helium is in the cooling of superconducting magnets, an application of increasing importance with the advent of such technologies as magnetic resonance imaging (MRI) and superconducting cavities for high-energy accelerators (see Chapter 3).
Liquid helium is also a subject of great scientific interest in itself. Liquid 4He undergoes a phase transition to a superfluid state when the temperature is lowered below 2.2 K. The superfluid properties of liquid 4He are generally believed to be a manifestation of the phenomenon known as the Bose-Einstein condensation, where the whole liquid exhibits macroscopic quantum properties such as quantization of the superfluid flow field. A practical aspect of superfluid helium is its extremely high thermal conductivity, which is orders of magnitude greater than that of other excellent thermal conductors (e.g., copper). Another important application for liquid helium is the 3He-4He dilution refrigerator, which employs both isotopes and allows routine access to temperatures within a few tens of millidegrees above absolute zero. Studies of materials at liquid helium temperatures or using liquid-helium-cooled apparatus play a central role in modern materials research. These cryogenic applications depend crucially on the unique properties of helium and can be expected to ensure a demand for helium for the foreseeable future.
Helium was essentially unknown before the twentieth century. It was first detected in the spectra of solar prominences observed during the solar eclipse of August 18, 1868. The art of spectroscopic observations and interest in them had advanced sufficiently since the preceding total solar eclipse that at least six separate observers were able to identify a new spectroscopic line in the atmosphere of the Sun. The line was initially thought to be that of sodium, but it was concluded later that year that it was actually evidence of a new element. This element was named helium, after the Greek word for Sun.
Nearly 30 years passed before the element helium was detected from terrestrial sources. It was not until 1895 that Sir William Ramsay and Lord Rayleigh reported the spectroscopic observation of helium in gas evolved from uranium and thorium ores. Following this report,