The Cambridge History of English and American Literature in 18 Volumes (1907–21).
Volume XIV. The Victorian Age, Part Two.

VIII. The Literature of Science

§ 18. Priestley and Cavendish

Among those who investigated the phenomena of combustion in the eighteenth, and early nineteenth, century, Priestley and Cavendish are pre-eminent. Black was the first chemist to make an accurate, quantitative examination of a particular, limited, chemical change, and, by so doing, to give clearness to the expression “a homogeneous substance.” The atomic theory was Dalton’s gift to science. From the many chemists who amplified the work of Dalton, and used the conceptions of atom and molecule to connect and explain new classes of chemical facts, Williamson and Frankland may be selected as the representatives. As workers in the borderland between chemistry and physics, Graham and Faraday are specially to be remembered. The investigations of Davy touched and illuminated every side of chemical progress.

Besides these men, who greatly enriched and advanced the science of chemistry in the period under review, there were many workers whose contributions cannot be considered here. References are given in the bibliography to the writings of some of them.

Joseph Priestley was a man of many gifts and a very versatile mind. When a youth at an academy, he tells us that he “saw reason to embrace what is generally called the heterodox side of almost every question.” When about twenty-eight years of age, he taught, in a school at Warrington, languages (he had a great natural gift of tongues), oratory and criticism, elocution, logic, natural phenomena, civil law and anatomy.

In the seventies of the eighteenth century, Priestley turned his attention to different kinds of airs. He obtained and partially examined many gases, but rarely troubled about separating them completely from impurities. In August, 1774, Priestley obtained a large lens with which he concentrated the sun’s rays on whatever substance happened to come to his hand, with the object of finding what air could be extracted from it. When he thus heated mercurius calcinatus per se (now called oxide of mercury), he obtained an air in which a candle burned with a “remarkably vigorous flame.” This result, he says, “surprised me more than I can well express.” The new air was subjected to many tests; it always behaved in a very unexpected manner. He placed a mouse in his new air; the mouse remained lively, and the air did not become “noxious.” The results of other experiments caused Priestley to lie awake through the night “in utter astonishment.” At last, he concluded that the new air was “between four and five times as good as common air.” He regarded the new air as a very superior kind of common air.

Priestley thought alchemically, not as a chemist. To the alchemist, the properties of things were external wrappings which might be removed from one thing and put round another, without affecting the essential substance of either thing, which substance it was the business of properties to hide from the uninitiated. Priestley thought of different airs as identical, or nearly identical, in substance, and only apparently different because of superficial differences in the mantles, the properties, by which the essential substance was concealed. When he obtained the air from burnt mercury, he thought he had removed from common air something which made it “noxious, vitiated, depraved, corrupt.” He had not learnt, what Black’s experiments, made twenty years before 1774, might have taught him, that each particular, material thing is known only by its properties. Priestley’s forced explanation of the facts which he himself discovered helped to convince investigators that the notion of identity of substance hidden under differences of properties is a great hindrance to the acquirement of accurate knowledge of natural events.

Priestley could not get over his astonishment at the behaviour of the new air. In science, one does well to be astonished; but, to astonishment one must add investigation, to investigation, reasoning, and, to reasoning, more investigation. Stopping at astonishment, Priestley made his facts square with the theory that dominated him, the theory of phlogiston. The phlogisteans taught that something, which they had named phlogiston, the principle of fire, rushes out of a burning substance as it burns. Phlogiston was never captured. Priestley held that the elusive phlogiston is a great corrupter of your airs or gases. He supposed that he had deprived common air of this depraving principle; he named his new gas dephlogisticated air. He invented many very ingenious hypotheses to account for facts observed by himself. Had he made a few accurate quantitative experiments, he might have broken the toils of his favourite theory.

The French chemist Lavoisier saw the importance of Priestley’s discovery of dephlogisticated air, and, by a series of rigidly quantitative experiments with tin and mercury, proved that, when a substance burns in air, it combines with a constituent of the air, which air-constituent is the gas prepared by Priestley. Lavoisier called his gas oxygen, because many of its compounds are acids.

Priestley’s insatiable curiosity, his mental alertness, his impatience of details, were required for the advancement of chemistry, no less than the passionless determination and the scrupulous accuracy of Cavendish.

Henry Cavendish, of Peterhouse, was bred in the theory of phlogiston, as Priestley was, and remained faithful to that theory, as Priestley did. He thought of many airs, or gases, as more or less phlogisticated forms of a few particular substances. Cavendish described the explosion of a mixture of common air and inflammable air (obtained by the action of acids on zinc) as one of the ways of phlogisticating air. This process is accompanied by a decrease in the volume of the interacting gases. Cavendish tried to discover the cause of this decrease. He exploded accurately measured volumes of dephlogisticated air (oxygen) and inflammable air (hydrogen), and found that water was the sole product of the change when dephlogisticated air was mixed with twice its volume of inflammable air. The explanation which Cavendish gave of this fundamentally important fact was confused and vague, because he insisted on making the facts uphold the phlogistic theory. Without knowing exactly what he had done, Cavendish had determined the quantitative volumetric composition of water. When the phlogistic theory had been swept away, the very great importance of the accurate work of Cavendish became manifest.