Joseph Louis Gay-Lussac (1778-1850) was a French chemist and physicist known for his studies on the physical properties of gases. His first major investigation concerned the thermal expansion of gases. He showed that a common thermal-expansion coefficient applied to all gases. This was significant in the establishment of the Kelvin temperature scale later in the century. 1804, Gay-Lussac and Jean-Baptiste Biot ascended to a height of approximately 13,000 feet to study variations in the Earth's electro-magnetic intensity relative to altitude. In a later solo ascent, he climbed to 23,000 feet (a record held for fifty years) and experienced the effects of oxygen deprivation but still managed to collect air samples, study the variation of pressure and temperature, and repeat his earlier electro-magnetic observations. In 1805, he collaborated with Alexander von Humboldt in determining the proportions of hydrogen and oxygen present in water. In 1810 he published a paper which contains some classic experiments on fermentation. His last great piece of pure research was on prussic acid in 1815 when he described cyanogen as a compound radicle, prussic acid as a compound of that radicle with hydrogen alone, and the prusiates (cyanides) as compounds of the radicle with metals. The proof that prussic acid contains hydrogen but no oxygen was an important support to the hydrogen-acid theory. His work gave support to Dalton's atomic theory, and formed the basis for Avogadro's law. Collaborating with Louis Jacques Thenard, he was the first to isolate the element boron, and studied the newly-isolated elements sodium, potassium, and iodine. He died in 1850 at the age of 71. His is one of the 72 names of scientists inscribed on the Eiffel Tower.