Henry Cavendish (10 October 1731 – 24 February 1810)
Henry Cavendish was a British chemist and physicist who made important experimental discoveries, notably the discovery of hydrogen or what he called “inflammable air” and the density of earth.
Cavendish was born in 1731 in Nice, France, into an aristocratic English family. His mother died when he was only two. He attended Hackney Academy, a private school near London. At age 18 he entered the University of Cambridge, but left three years later without taking a degree (a common enough practice in those days). He then lived with his father in London, where he soon had his own laboratory. His father used to take him to attend the meetings of the Royal Society, of which he soon became a member.
Cavendish made hydrogen by dissolving metals in acids. By dissolving alkalis in acids, he made “fixed air” (carbon dioxide). These gases he collected in bottles inverted over water or mercury. He then measured their solubility in water, and their specific gravity and combustibility. The Royal Society awarded him the Copley Medal for this paper. He next published a paper on the production of water by burning hydrogen in “dephlogisticated air” or what is now known as oxygen.
He developed a general theory of heat, which contained the principle of the conservation of heat (understood later as an instance of conservation of energy) and even contained the concept (though not the label) of the mechanical equivalent of heat.
The apparatus Cavendish used for weighing the Earth was a modification of the torsion balance built by Englishman and geologist John Michell, who died before he could begin the experiment. Using this equipment, Cavendish calculated the attraction between the balls from the period of oscillation of the torsion balance, and then he used this value to calculate the density of the Earth. Cavendish found that the Earth’s average density is 5.48 times greater than that of water. The result that Cavendish obtained for the density of the Earth is within 1 percent of the currently accepted figure. Cavendish’s work led others to accurate values for the gravitational constant (G) and Earth’s mass.
Among Cavendish’s discoveries in electricity were the concept of electric potential (which he called the “degree of electrification”), an early unit of capacitance (that of a sphere one inch in diameter), the formula for the capacitance of a plate capacitor, the concept of the dielectric constant of a material, the relationship between electric potential and current (now called Ohm’s Law), laws for the division of current in parallel circuits (now attributed to Charles Wheatstone), and the inverse square law of variation of electric force with distance, now called Coulomb’s Law.
Cavendish was a shy man who did not socialize much. He always dressed in an old-fashioned suit, was taciturn and solitary and regarded by many as eccentric. He never married. He died in 1810.