Henry Cavendish

The English physicist and chemist Henry Cavendish (1731-1810) determined the value of the universal constant of gravitation, made noteworthy electrical studies, and is credited with the discovery of hydrogen and the composition of water.

Henry Cavendish was born on Oct. 10, 1731, the elder son of Lord Charles Cavendish and Lady Anne Grey. He entered Peterhouse, Cambridge, in 1749 and left after 2 years without taking a degree. He never married and was so reserved that there is little record of his having any social life except occasional meetings with scientific friends. His death (Feb. 24, 1810) he faced with the same equanimity with which he faced the unavoidable breaking of apparatus in the course of increasing knowledge. He was buried in All Saints Church, Derby.

Cavendish's work and reputation have to be considered in two parts: the one relating to his published work, the other to the large amount he did not publish. During his lifetime he made notable discoveries in chemistry mainly between 1766 and 1788 and in electricity between 1771 and 1788. In 1798 he published a single notable paper on the density of the earth, but interest in this subject was evidently of long standing.

Contributions to Chemistry

At the time Cavendish began his chemical work, chemists were just beginning to recognize that the "airs" which were evolved in many chemical reactions were distinct entities and not just modifications of ordinary air. Cavendish reported his own work in Three Papers Containing Experiments on Factitious Air in 1766. These papers added greatly to knowledge of the formation of "inflammable air" (hydrogen) by the action of dilute acids on metals. Cavendish also distinguished the formation of oxides of nitrogen from nitric acid. Their true chemical character was not yet known, but Cavendish's description of his observations had almost the same logical pattern as if he were thinking in modern terms, the principal difference being that he used the terminology of the phlogiston theory (that is, a burning substance liberates into its surroundings a principle of inflammability).

Cavendish's other great merit is his experimental care and precision. He measured the density of hydrogen, and although his figure is half what it should be, it is astonishing that he even found the right order of magnitude, considering how difficult it was to manage so intractable a substance. Not that his apparatus was crude; where the techniques of his day allowed, his apparatus (like the splendid balance surviving at the Royal Institution) was capable of refined results.

Cavendish investigated the products of fermentation, showing that the gas from the fermentation of sugar is indistinguishable from the "fixed air" characterized as a constituent of chalk and magnesia by Black (both are, in modern language, carbon dioxide).

Another example of Cavendish's technical expertise was Experiments on Rathbone-Place Water (1767), in which he set the highest possible standard of thoroughness and accuracy. It is a classic of analytical chemistry. In it Cavendish also examined the phenomenon of the retention of "calcareous earth" (chalk, calcium carbonate) in solution, and in doing so he discovered the reversible reaction between calcium carbonate and carbon dioxide to form calcium bicarbonate, the cause of temporary hardness of water. He also found out how to soften such water by adding lime (calcium hydroxide).

In his study of the methods of gas analysis Cavendish made one remarkable observation. He was sparking air with excess oxygen (to form oxides of nitrogen) over alkali until no more absorption took place and noted that a tiny amount of gas could not be further reduced, "so that if there is any part of the phlogisticated air of our atmosphere which differs from the rest, and cannot be reduced to nitrous acid, we may safely conclude, that it is not more than 1/120 part of the whole." As is now known, he had observed the noble gases of the atmosphere.

One of Cavendish's researches on the currently engrossing problem of combustion made an outstanding contribution to fundamental theory. Without seeking particularly to do so, in 1784 Cavendish determined the composition of water, showing that it was a compound of oxygen and hydrogen ("dephlogisticated air" and "inflammable air"). Joseph Priestley had reported an experiment of Warltire in which the explosion of the two gases had left a dew on the sides of a previously dry vessel. Cavendish studied this, prepared water in measurable quantity, and got an approximately correct figure for its volume composition.

Electrical Researches

Cavendish published only a fraction of the experimental evidence he had available to support his theories, but his contemporaries were convinced of the correctness of his conclusions. He was not the first to profound an inverse-square law of electrostatic attraction, but Cavendish's exposition, based in part on mathematical reasoning, was the most effective. He founded the study of the properties of dielectrics and also distinguished clearly between quantity of electricity and what is now called potential.

Cavendish had the ability to make an apparently limited study yield far-reaching results. An example is his study of the origin of the ability of some fish to give an electric shock. He made up imitation fish of leather and wood, soaked in salt water, with pewter attachments representing the organs of the fish which produced the effect. By using Leyden jars to charge the imitation organs, he was able to show that the results were entirely consistent with the fish's being able to produce electricity. This investigation was among the earliest in which the conductivity of aqueous solutions was studied.

Cavendish began to study heat with his father, then returned to the subject in 1773-1776 with a study of the Royal Society's meteorological instruments, in the course of which he worked out the most important corrections to be employed in accurate thermometry. In 1783 he published a study of the means of determining the freezing point of mercury. In it he added a good deal to the general theory of fusion and freezing and the latent heat changes accompanying them.

Cavendish's most elaborate (and celebrated) investigation was that on the density of the earth. He took part in a program to measure the length of a seconds pendulum in the vicinity of a large mountain (Schiehallion). Variations from the period on the plain would show the attraction exerted by the mountain, from which the density of its substance could be calculated. Cavendish also approached the subject in a more fundamental way by determining the force of attraction of a very large, heavy lead ball for a very small, light ball. The ratio between this force and the weight of the light ball would furnish the mass of the earth. His results were unquestioned and unsurpassed for nearly a century.

Unpublished Works

Had Cavendish published all his work, his great influence would undoubtedly have been greater, but in fact he left in manuscript form a vast amount which often anticipated that of his successors. It came to light only bit by bit until the thorough study undertaken by Maxwell (published in 1878) and by Thorpe (published in 1921). In these notes is to be found such material as the detail of his experiments to examine the law of electrostatic force, the conductivity of metals, and many chemical questions such as a theory of chemical equivalents. He had a theory of partial pressures before Dalton.

However, the history of science is full of instances of unpublished works which might have influenced others but in fact did not. Whatever he did not reveal, Cavendish gave his colleagues enough to help them on the road to modern conceptions. Nothing he did has been rejected, and for this reason he is still, in a unique way, part of modern life.

Further Reading on Henry Cavendish

The Scientific Papers of the Hon. Henry Cavendish: Edited from the Published Papers and the Cavendish Manuscripts was published in two volumes: vol. 1, The Electrical Researches, edited by J. Clerk Maxwell (1879), and vol. 2, The Chemical and Dynamical Researches, edited by Sir Edward Thorpe and others (1921). A straightforward account of Cavendish's life and work is A. J. Berry, Henry Cavendish (1960), which includes a useful select bibliography. George Wilson, The Life of the Honble. Henry Cavendish (1851), is available in many libraries. James R. Partington, A History of Chemistry, vol. 3 (1962), contains a very full account of Cavendish's chemical work and some discussion of his electrical work.

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