Henry Cavendish
Henry Cavendish was an English 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, in a 1766 paper, On Factitious Airs. Antoine Lavoisier later reproduced Cavendish's experiment and gave the element its name.
A shy man, Cavendish was distinguished for great accuracy and precision in his researches into the composition of atmospheric air, the properties of different gases, the synthesis of water, the law governing electrical attraction and repulsion, a mechanical theory of heat, and calculations of the density of the Earth. His experiment to measure the density of the Earth has come to be known as the Cavendish experiment.
Early life
Cavendish was born on 10 October 1731 in Nice, where his family was living at the time. His mother was Lady Anne de Grey, fourth daughter of Henry Grey, 1st Duke of Kent, and his father was Lord Charles Cavendish, the third son of William Cavendish, 2nd Duke of Devonshire. The family traced its lineage across eight centuries to Norman times, and was closely connected to many aristocratic families of Great Britain. Henry's mother died in 1733, three months after the birth of her second son, Frederick, and shortly before Henry's second birthday, leaving Lord Charles to bring up his two sons. Henry Cavendish was styled as "The Honourable Henry Cavendish".From the age of 11, Henry attended Newcome's School, a private school near London. At the age of 18 he entered the University of Cambridge in St Peter's College, now known as Peterhouse, but left three years later on 23 February 1751 without taking a degree. He then lived with his father in London, where he soon had his own laboratory complete with dog-room.
Lord Charles spent his life firstly in politics and then increasingly in science, especially in the Royal Society of London. In 1758, he took Henry to meetings of the Royal Society and also to dinners of the Royal Society Club. In 1760, Henry Cavendish was elected to both these groups, and he was assiduous in his attendance after that. He took virtually no part in politics, but followed his father into science, through his researches and his participation in scientific organisations. He was active in the Council of the Royal Society of London.
His interest and expertise in the field 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, for the gravitational attraction of mountains, and for the scientific instructions for Constantine Phipps's expedition in search of the North Pole and the Northwest Passage. In 1773, Henry joined his father as an elected trustee of the British Museum, to which he devoted a good deal of time and effort. Soon after the Royal Institution of Great Britain was established, Cavendish became a manager and took an active interest, especially in the laboratory, where he observed and helped in Humphry Davy's chemical experiments.
Chemistry research
About the time of his father's death, Cavendish began to work closely with Charles Blagden, an association that helped Blagden enter fully into London's scientific society. In return, Blagden helped to keep the world at a distance from Cavendish. 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, along with, for example, Joseph Priestley, Joseph Black, and Daniel Rutherford. 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.Although others, such as Robert Boyle, had prepared hydrogen gas earlier, Cavendish is usually given the credit for recognising its elemental nature. In 1777, Cavendish discovered that air exhaled by mammals is converted to "fixed air", not "phlogisticated air" as predicted by Joseph Priestley. Also, by dissolving alkalis in acids, Cavendish produced carbon dioxide, which he collected, along with other gases, in bottles inverted over water or mercury. He then measured their solubility in water and their specific gravity, and noted their combustibility. He concluded in his 1778 paper "General Considerations on Acids" that respirable air constitutes acidity. Cavendish was awarded the Royal Society's Copley Medal for this paper. Gas chemistry was of increasing importance in the latter half of the 18th century, and became crucial for Frenchman Antoine-Laurent Lavoisier's reform of chemistry, generally known as the chemical revolution.
In 1783, Cavendish published a paper on eudiometry. He described a new eudiometer of his invention, with which he achieved the best results to date, using what in other hands had been the inexact method of measuring gases by weighing them. Then, after a repetition of a 1781 experiment performed by Priestley, Cavendish published a paper on the production of pure water by burning hydrogen in "dephlogisticated air".
Cavendish concluded that rather than being synthesised, the burning of hydrogen caused water to be condensed from the air. Some physicists interpreted hydrogen as pure phlogiston. Cavendish reported his findings to Priestley no later than March 1783, but did not publish them until the following year. The Scottish inventor James Watt published a paper on the composition of water in 1783; controversy about who made the discovery first ensued.
In 1785, Cavendish investigated the composition of common 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 and dephlogisticated air, 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 , mixed with four of phlogisticated ".
In the 1890s two British physicists, William Ramsay and Lord Rayleigh, realised that their newly discovered inert gas, argon, was responsible for Cavendish's problematic residue; he had not made an error. What he had done was perform rigorous quantitative experiments, using standardised instruments and methods, aimed at reproducible results; taken the mean of the result of several experiments; and identified and allowed for sources of error. The balance that he used, made by a craftsman named Harrison, was the first of the precision balances of the 18th century, and as accurate as Lavoisier's. Cavendish worked with his instrument makers, generally improving existing instruments rather than inventing wholly new ones.
Cavendish, as indicated above, used the language of the old phlogiston theory in chemistry. In 1787, he became one of the earliest outside France to convert to the new antiphlogistic theory of Lavoisier, though he remained sceptical about the nomenclature of the new theory. He also objected to Lavoisier's identification of heat as having a material or elementary basis. Working within the framework of Newtonian mechanism, Cavendish had tackled the problem of the nature of heat in the 1760s, explaining heat as the result of the motion of matter.
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 and even included the concept of the mechanical equivalent of heat.
Density of the Earth
Following his father's death, Henry bought another house in town and also a house in Clapham Common, at that time to the south-west of London. The London house contained the bulk of his library, while he kept most of his instruments at Clapham Common, where he carried out most of his experiments. The most famous of those 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. 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 accurate values for the gravitational constant 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.
Books often describe Cavendish's work as a measurement of either G or the Earth's mass. Since these are related to the Earth's density by a trivial web of algebraic relations, none of these sources are wrong, but they do not match the exact word choice of Cavendish, and this mistake has been pointed out by several authors. Cavendish's stated goal was to measure the Earth's density.
The first time that the constant got this name was in 1873, almost 100 years after the Cavendish experiment. Cavendish's results also give the Earth's mass.