People/Henry Cavendish
Henry Cavendish
Henry Cavendish was an English philosopher and scientist who was an important experimental and theoretical chemist and physicist. He is noted for his discovery of hydrogen, which he termed "inflammable air". He described the density of inflammable air, which formed water on combustion.
His interest and expertise in the use of scientific instruments led him to head a committee to review the Royal Society's meteorological instruments and to help assess the instruments of the Royal Greenwich Observatory. His first paper, "Factitious Airs", appeared in 1766. Other committees on which he served included the committee of papers, which chose the papers for publication in the Philosophical Transactions of the Royal Society, and the committees for the transit of Venus (1769), for the gravitational attraction of mountains (1774), and for the scientific instructions for Constantine Phipps's expedition (1773) in search of the North Pole and the Northwest Passage.
Henry was an odd man. He never addressed strangers directly and was petrified of women. He had a staircase built into the back of his house to avoid any encounter with the ladies he employed. When it came time for his final oral exams to complete his natural philosophy degree at Cambridge University—that's what they called a science degree before the advent of modern science and specialized degrees—he dropped out of school all together rather than talk in public.
Life's work
Cavendish published no books and few papers, but he achieved much. Several areas of research, including mechanics, optics, and magnetism, feature extensively in his manuscripts, but they scarcely feature in his published work. Cavendish is considered to be one of the so-called pneumatic chemists of the eighteenth and nineteenth centuries
Chemistry research
Cavendish found that a definite, peculiar, and highly inflammable gas, which he referred to as "Inflammable Air", was produced by the action of certain acids on certain metals. This gas was hydrogen, which Cavendish correctly guessed was proportioned two to one in water.
In 1785, Cavendish investigated the composition of common (i.e. atmospheric) air, obtaining impressively accurate results. He conducted experiments in which hydrogen and ordinary air were combined in known ratios and then exploded with a spark of electricity. Furthermore, he also described an experiment in which he was able to remove, in modern terminology, both the oxygen and nitrogen gases from a sample of atmospheric air until only a small bubble of unreacted gas was left in the original sample. Using his observations, Cavendish observed that, when he had determined the amounts of phlogisticated air (nitrogen) and dephlogisticated air (oxygen), there remained a volume of gas amounting to 1/120 of the original volume of nitrogen. By careful measurements he was led to conclude that "common air consists of one part of dephlogisticated air [oxygen], mixed with four of phlogisticated [nitrogen]".
In 1783, he published a paper on the temperature at which mercury freezes and in that paper made use of the idea of latent heat, although he did not use the term because he believed that it implied acceptance of a material theory of heat. He made his objections explicit in his 1784 paper on air. He went on to develop a general theory of heat, and the manuscript of that theory has been persuasively dated to the late 1780s. His theory was at once mathematical and mechanical: it contained the principle of the conservation of heat (later understood as an instance of conservation of energy) and even included the concept (although not the label) of the mechanical equivalent of heat.
Density of the Earth (Cavendish experiment)
The most famous of Henry's experiments, published in 1798, was to determine the density of the Earth and became known as the Cavendish experiment. The apparatus Cavendish used for weighing the Earth was a modification of the torsion balance built by geologist John Michell, who died before he could begin the experiment. The apparatus was sent in crates to Cavendish, who completed the experiment in 1797–1798 and published the results.
The experimental apparatus consisted of a torsion balance with a pair of 2-inch 1.61-pound lead spheres suspended from the arm of a torsion balance and two much larger stationary lead balls (350 pounds). Cavendish intended to measure the force of gravitational attraction between the two. He noticed that Michell's apparatus would be sensitive to temperature differences and induced air currents, so he made modifications by isolating the apparatus in a separate room with external controls and telescopes for making observations.
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. John Henry Poynting later noted that the data should have led to a value of 5.448, and indeed that is the average value of the twenty-nine determinations Cavendish included in his paper. The error in the published number was due to a simple arithmetical mistake on his part. What was extraordinary about Cavendish's experiment was its elimination of every source of error and every factor that could disturb the experiment, and its precision in measuring an astonishingly small attraction, a mere 1/50,000,000 of the weight of the lead balls. The result that Cavendish obtained for the density of the Earth is within 1 per cent of the currently accepted figure.
Cavendish's work led others to the values used for the gravitational constant (G) and Earth's mass. Based on his results, one can calculate a value for G of 6.754 × 10−11N-m2/kg2, which compares favourably with the modern value of 6.67428 × 10−11N-m2/kg2
Electrical research
Cavendish's comprehensive theory of electricity was mathematical in form and was based on precise quantitative experiments. Working with his colleague, Timothy Lane, he created an artificial torpedo fish that could dispense electric shocks to show that the source of shock from these fish was electricity. He published an early version of his theory of electricity in 1771, based on an expansive electrical fluid that exerted pressure. He demonstrated that if the intensity of electric force were inversely proportional to distance, then the electric fluid more than that needed for electrical neutrality would lie on the outer surface of an electrified sphere; then he confirmed this experimentally. Cavendish continued to work on electricity after this initial paper, but he published no more on the subject.
Death
Cavendish died at Clapham on 24 February 1810 (as one of the wealthiest men in Britain) and was buried, along with many of his ancestors, in the church that is now Derby Cathedral. The road he used to live on in Derby has been named after him, as has a road near his house in Clapham, of which the north part is part of the South Circular Road. The University of Cambridge's Cavendish Laboratory was endowed by one of Cavendish's later relatives, William Cavendish, 7th Duke of Devonshire (Chancellor of the University from 1861 to 1891).