The Cambridge History of English and American Literature in 18 Volumes (1907–21).
Volume XIV. The Victorian Age, Part Two.
§ 19. Black
Joseph Black graduated as doctor of medicine in the university of Edinburgh in 1755, presenting a thesis entitled Magnesia alba, Quicklime, and other alcaline substances. That thesis is probably the earliest example of a genuinely scientific chemical investigation. Black proved that mild magnesia (now called magnesium carbonate) loses weight when it is calcined; he determined the loss of weight; he proved that the solid substance which remains after calcination has properties of its own which distinguish it from mild magnesia; he showed that, during calcination, an air, or gas, is given off, different from any air, or gas, then known; he examined, accurately, the properties of this gas, which he called fixed air; and he reproduced the original quantity of mild magnesia by dissolving the calcined magnesia in acid, and adding fixed alkali (now called potassium carbonate), a substance which he proved to give off fixed air when it is calcined. By his experiments, Black proved mild magnesia to be composed of fixed air united with calcined magnesia, and showed that each of these three substances is a particular and definite kind of matter, distinguished from all other kinds of matter by constant qualities. He also proved that the change which happens when chalk is burned is exactly similar to the calcining of mild magnesia; fixed air is driven out of the chalk, and burnt lime—a perfectly definite homogeneous substance—remains.
The work of Black prepared the way for the penetrative, experimental analysis of the phenomena of combustion; it taught chemists to use accurately observed properties of bodies as the only means of distinguishing one body from another; it showed that, if chemical investigation is to produce results of permanent value, it must be quantitative; incidentally, by isolating and examining fixed air, it began a new branch of chemistry, the study of the changes of composition and properties which happen when homogeneous gases interact.
Black and Cavendish were painstaking, methodical, unemotional, eminently clear-headed. Priestley was flighty, flitting from one thing to another in his laboratory, always curious, never working out his discoveries, unable to think chemically outside of the theory which dominated him. Black, Cavendish and Priestley greatly advanced the science of chemistry.
So long as chemists formed vague generalisations founded on introspective speculations, they made little progress. It was by concentrating their attention on a few limited occurrences, and accurately examining these by quantitative experiments, that chemists gradually gained clear conceptions which could be directly used in the investigation of more complicated chemical changes. “True genius,” Coleridge said, “begins by generalising and condensing; it ends in realising and expanding.” The vague generalising of the alchemists was followed by the condensing work of Black and Cavendish, and by the suggestive discoveries of Priestley. The time was approaching for realising and expanding.