Discovery of the Composition of Water

Date 1781-1784

After discovering “inflammable air,” or hydrogen, Cavendish investigated its properties and found that pure water formed when hydrogen burned in “dephlogisticated air,” or oxygen.

Locale Clapham Common, London, England

Key Figures

• Henry Cavendish (1731-1810), English natural philosopher who is best known for his research on gases and the nature of water
• Joseph Priestley (1733-1804), English scientist and Unitarian minister who discovered oxygen and several other gases
• James Watt (1736-1819), English inventor of an improved steam engine who gave an interpretation of water’s nature
• Antoine-Laurent Lavoisier (1743-1794), French chemist who interpreted water as a compound of hydrogen and oxygen
• Sir Charles Blagden (1748-1820), Cavendish’s assistant during the years that he studied hydrogen and the nature of water

Summary of Event

Although Henry Cavendish lived and worked largely as a recluse, his greatest discoveries cannot be understood apart from the community of scientists to which he belonged. This certainly was the case with his discovery of “inflammable air” and his related research on the composition of water.

Scientists such as Robert Boyle had noticed that a flammable gas was generated when acids were added to metals, but Cavendish was sufficiently intrigued by this gas to study it comprehensively. He prepared it with various metals (iron, zinc, and tin) and acids (what we now call hydrochloric and sulfuric acids). Using two different methods, he determined the gas’s specific gravity, finding it was nearly nine thousand times lighter than water and about one-fourteenth the weight of common air. When he introduced a flame into a mixture of this gas and ordinary air, the gas burned bright blue, and so he called it “inflammable air from the metals,” which was later shortened to “inflammable air” (its modern name is hydrogen). Because he believed in the phlogiston theory, which posited that every combustible material contained a substance called phlogiston, and because inflammable air burned with no residue, Cavendish believed that this new gas (“inflammable air”) was phlogiston.

In 1766, Cavendish published his findings in a tripartite paper in which each part dealt with a specific gas prepared by a certain process: one, inflammable air from metals and acids; two, fixed air (carbon dioxide) from alkalis and acids; and three, “mixed airs” from organic materials by fermentation or putrefaction. Cavendish’s report on inflammable air stimulated Joseph Priestley, who, in 1781, put an electric spark through a mixture of inflammable air and common air and noticed that the inside of the dry glass container became coated with moisture. Neither Priestley nor a colleague who helped him understood what they had done, but Cavendish did understand after he repeated their experiment in a systematic and quantitative way. During the summer of 1781 he found that all the inflammable air and about one-fifth of the ordinary air had ceased being gases in forming what he discovered was pure water.

Cavendish and Priestley routinely interacted, so Cavendish was aware of a new gas, “dephlogisticated air” (oxygen), discovered by Priestley in 1774. Cavendish was therefore curious about what would happen if he sparked various mixtures of inflammable air and dephlogisticated air. After several trials he established that a two-to-one ratio of inflammable to dephlogisticated air led to the complete conversion of these gases to water. Although he was the first scientist to establish this experimental fact, his interpretation of the results was confusing. The obvious explanation was to see water as the union of these two gases, but Cavendish was a phlogistonist and still tied, in a way, to the old idea of water as an element (that is, a basic, indivisible substance rather than a combination of other substances). For him, inflammable air was either phlogiston or “phlogisticated water” (water united to phlogiston). Dephlogisticated air, on the other hand, was water deprived of its phlogiston. Therefore, Cavendish saw water as preexisting in the combining gases, and the spark-induced reaction simply revealed what had previously been hidden.

Even though Cavendish did not publish his experimental results and interpretation until 1784, scientists in England and France learned about them. For example, in 1783, Sir Charles Blagden, Cavendish’s assistant, made a trip to Paris and met Antoine-Laurent Lavoisier, whom he informed of how Cavendish, with his assistance, had made pure water from two new gases. Antoine-Laurent Lavoisier quickly realized the implications of their results for his new theory of chemistry. In November, 1783, Lavoisier reported to the French Academy of Sciences on experiments that he and Pierre Simon de Laplace had performed, which demonstrated that water was not an element but instead was a compound of hydrogen and oxygen. Lavoisier failed to mention the stimulus he had received from the research of Cavendish, who did not publish his results until 1784. In this later publication, Cavendish was able to complete his earlier studies by showing that the gas that was left behind when dephlogisticated air was removed from common air was a colorless gas in which mice died and a candle would not burn (this gas was what Lavoisier called azote and others called nitrogen).

Because the discovery of the compound nature of water was so significant, and because so many people contributed in one way or another to that discovery, a “water controversy” soon developed. It was basically a priority dispute. Both Priestley, who could have made a claim but never did, and Cavendish, whose introverted personality ill suited him to controversy, stayed on the sidelines. The contending parties in the first phase of the water controversy were James Watt and Lavoisier. Watt became involved because Priestley told him about his dew-forming experiments and Watt then circulated his interpretation of Priestley’s results to Royal Society members. When Watt learned of Cavendish’s and Lavoisier’s reports on water’s nature, he accused Cavendish of plagiarizing his ideas and Lavoisier of plagiarizing Cavendish’s experiments. For his part, Cavendish was willing to give credit to Lavoisier for interpreting the composition of water in terms of the oxygen theory. Although most historians of science appreciate Lavoisier’s contributions, they criticize him for neglecting to credit Cavendish. These scholars also find Watt’s claims confused and his interpretation derivative. Indeed, they bestow on Cavendish, the least contentious of the claimants, the lion’s share of the honor for finding water’s true nature.

Significance

Some scholars consider Henry Cavendish to be Britain’s preeminent eighteenth century scientist, who lived and worked between the time of Sir Isaac Newton in the seventeenth century and James Clerk Maxwell in the nineteenth century. Cavendish’s studies of what he called “factitious airs” (those contained in solids) were models of a rigorously quantitative approach to chemistry. Future chemists would use his methods for generating, collecting, transferring, and measuring gases and for determining their unique characteristics. He used these methods to help discover the composition of water but also to help clarify the nature of compounds such as nitric acid. His quantitative studies of the specific combining volumes of the gases necessary to form water constituted an important step toward the law enunciated by Joseph-Louis Gay-Lussac in 1809, which states that the ratios of the volumes of reacting gases are always small, whole numbers. Cavendish’s experimental contributions were much more important than his theoretical contributions, and his adherence to the phlogiston theory hampered his understanding of his experimental results almost to the end of his life, when he finally began to see some value in the new chemistry of Lavoisier.

The water controversy was significant because of what it revealed about the changing nature of science. Before the eighteenth century scientists tended to work alone, and their discoveries were often seen as a consequence of their individual genius. In the eighteenth century scientific discoveries increasingly involved many talented individuals working in concert with others or working with the knowledge that many others were working on the same or similar projects. Inevitably, more than one scientist would make the same conclusion or discovery simultaneously.

Some scholars attribute the water controversy to the casual way in which scientific data were then gathered, dated, and reported. Other scholars point to nationalism as a factor in the water controversy, especially as it continued in the nineteenth century after the deaths of the original contenders. French and British scholars, using newly available primary sources, argued about the credit that should be given to Watt and Cavendish. One significant by-product of the study of Cavendish’s papers was the role that some of his data played in the discovery of a new element in 1894. When Cavendish had removed oxygen and nitrogen from ordinary air, he found a small bubble of gas still remaining. In the late nineteenth century this bubble of gas was shown to be argon, a new noble gas, a belated testimony to the meticulousness of Cavendish’s experimental prowess.

Bibliography

Jaffe, Bernard. Crucibles: The Story of Chemistry. New York: Dover, 1998. This popular history of chemistry told through the lives and achievements of the great chemists has a chapter on Cavendish. Includes a section of sources and an index.

Jungnickel, Christa, and Russell McCormmach. Cavendish: The Experimental Life. Lewisburg, Pa.: Bucknell, 1999. This biography is an extensive revision of the authors’ earlier biography published by the American Philosophical Association. With updated primary and secondary sources, this edition offers an extremely rich view of the context and contributions of Cavendish to physics and chemistry. Illustrated, with an extensive bibliography and a detailed index.

Miller, David Philip. Discovering Water: James Watt, Henry Cavendish, and the Nineteenth Century “Water Controversy.” Burlington, Vt.: Ashgate, 2004. Describes how Cavendish’s (and Watt’s and Lavoisier’s) discovery that water was a compound of “airs,” and not a combination of elements, became an issue of controversy among nineteenth century scientists.

Partington, J. R. A History of Chemistry. Vol. 3. London: Macmillan, 1962. The eighth chapter of this comprehensive history of chemistry is on Cavendish’s life and contributions to chemistry. The chapter’s footnotes include many references to both primary and secondary sources. Illustrated, with indexes of names and subjects.

Strathern, Paul. Mendeleyev’s Dream: The Quest for the Elements. New York: Berkeley Books, 2000. This popular history of chemistry treats Cavendish’s achievements in chapter 5, “Trial and Error.” Illustrated, with a further-reading section and an index.