From a drawing by Alexander in the Print Room of the British Museum.
Henry Cavendish was born at Nice in 1731, and died in London in 1810. He was a natural philosopher in the widest sense of that term, and occupied himself in turn with nearly every branch of physical science. He was a capable astronomer and an excellent mathematician, and he was one of the earliest to work on the subject of specific heat, and to improve the thermometer and the methods of making thermometric observations. He also determined the mean density of the earth. He made accurate observations on the properties of carbonic acid and hydrogen, greatly improved the methods of eudiometry, and first established the practical uniformity of the composition of atmospheric air. His greatest discovery, however, was his determination of the composition of water. He was the first to prove that water is not a simple or elementary substance, as supposed by the ancients, but is a compound of hydrogen and oxygen. In certain of his trials he found that the water formed by the union of oxygen and hydrogen was acid to the taste; and the search for the cause of this acidity led him to the discovery of the composition of nitric acid. He was the first to make a fairly accurate analysis of a natural water, and to explain what is known as the hardness of water.
Phlogistonism may be said to have dominated chemistry during three-fourths of the eighteenth century. Although radically false as a conception and of little use in the true interpretation of chemical phenomena, it cannot be said to have actually retarded the pursuit of chemistry. Men went on working and accumulating chemical facts uninspired and, for the most part, uninfluenced by it. Even Priestley, perhaps one of the most conservative of the followers of Stahl, regarded his dogma with a complacent tolerance; and as its inconsistencies became apparent he was more than once on the point of renouncing it. Of one thing he was quite convinced, and that was that Stahl had greatly erred in his conception of the real nature of phlogiston. Perhaps the most signal disservice which phlogiston did to chemistry was to delay the general recognition of Boyle’s views of the nature of the elements. The alchemists, it will be remembered, regarded the metals as essentially compound. Boyle was disposed to believe that they were simple. Becher and Stahl and their followers, until the last quarter of the eighteenth century, also regarded them as compounds, phlogiston being one of their constituents. On the other hand, what we now know to be compounds—such as the calces, the acids, and water itself—were held by the phlogistians to be simple substances.
The discovery, in 1774, of oxygen—the dephlogisticated air of Priestley—and the recognition of the part it plays in the phenomena which phlogiston was invoked to explain, mark the termination of one era in chemical history and the beginning of another. Before entering upon an account of the new era it is desirable to take stock of the actual condition of chemical knowledge at the end of the phlogistic period, and to show what advances had been made in pure and applied chemistry during that time.
During the eighteenth century greater insight was gained into the operations of the form of energy with which chemistry is mainly concerned, and views concerning chemical affinity and its causes began to assume more definite shape, chiefly owing to the labours of Boerhaave, Bergman, Geoffroy, and Rouelle. It was clearly recognised that the large group of substances comprised under the term “salts” were compound, and made up of two contrasted and, in a sense, antagonistic constituents, classed generically as acids and bases.
On the practical side chemistry made considerable progress. Analysis—a term originally applied by Boyle—greatly advanced. It was, of course, mainly qualitative; but, thanks to the labours of Boyle, Hoffmann, Marggraf, Scheele, Bergman, Gahn, and Cronstedt, certain reactions and reagents came to be systematically applied to the recognition of chemical substances, and the precision with which these reagents were used led to the detection of hitherto unknown elements. The beginnings of a quantitative analysis were made even before the time of Boyle, but its principles were greatly developed by him, and were further extended by Homberg, Marggraf, and Bergman. Marggraf accurately determined the amount of silver chloride formed by adding common salt to a solution of a known weight of silver, and Bergman first pointed out that estimations of substances might be conveniently made by weighing them in the form of suitably prepared compounds, which, it was implicitly assumed, were of uniform and constant composition. The foundations of an accurate system of gaseous analysis were made by Cavendish; and various forms of physical apparatus were applied to the service of chemistry.
To the elements which were known prior to Boyle’s time, although not recognised as such, there were added phosphorus (Brand, 1669), nitrogen (Rutherford), chlorine (Scheele, 1774), manganese (Gahn, 1774), cobalt (Brandt, 1742), nickel (Cronstedt, 1750), and platinum (Watson, 1750). Baryta was discovered by Scheele, and strontia by Crawford. Phosphoric acid was discovered by Boyle, and its true nature determined by Marggraf; Cavendish first made known the composition of nitric acid. As already stated, Scheele first isolated molybdic and tungstic acids and determined the existence of a number of the organic acids (p. 75). Other discoveries—such as the true nature of limestone and magnesia alba and their relations respectively to lime and magnesia by Black, the many gaseous substances by Priestley, and the compound nature of water by Cavendish—have already been referred to.
Technical chemistry also greatly developed during the eighteenth century, thanks to the efforts of Gahn, Marggraf, Duhamel, Reaumur, Macquer, Kunkel, and Hellot; and many important industrial processes—such as the manufacture of sulphuric acid by Ward of Richmond, and subsequently by Roebuck at Birmingham, and the Leblanc process of conversion of common salt into alkali—had their origin during this period.