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 444 THE SCIENTIFIC MONTHLY

tent because he applied what little theoretical knowledge he had to the solving of problems of very definite practical utility. He knew some- thing of the work of his predecessors Borelli^ Lower and Mayow. He knew that of his contemporaiy Boerhaave; but, much less technically learned than all of these, he became the pioneer sanitarian of the first half of the eighteenth centuiy. He was not a physiological chemist like Lower and Mayow, but he was the discoverer of a method of sustain- ing respiration in the absolutely irrespirable atmospheres of coal-mines or burning houses. He suggested that the apparatus might be service- able for divers. He was the father of all such as descend into "fire- damp *' and " choke-damp '* and " black damp " provided with an inde- pendent supply of air in an apparatus capable of absorbing the exhaled carbon dioxide. He was the Jubal of all such as handle rescue-appa- ratus. He was a pioneer in a great deal else that does not concern us now, for he was the first person in this or any other country to obtain by experiment on the living animal a demonstration of the magnitude of the pressure of the blood in arteries ; and he is the father of vegetable physiology in England, and he is much else. But we must not imagine that, although Hales devised an artificial respiration apparatus, he was acquainted with all the properties of carbon dioxide. For just as Van Helmont in the seventeenth century worked with the carbon dioxide of combustion without being aware of all its properties, so Hales in the eighteentli worked with ihe carbon dioxide of respiration without real- izing all that was involved in his researches. This sort of thing had happened again and again in science. Bespiratory carbon dioxide was discovered by Professor Joseph Black at the University of Glasgow in 1754. In point of time nitrogen was the next constituent of the atmos- phere to be identified: this was also by a Scotsman, Professor Daniel Butherford (1740-1819) of the Chair of Botany at Edinburgh. The year of this was 1772, the man the maternal uncle of Sir Walter Scott. Within two years more, oxygen was separated by Joseph Priestley from mercuric oxide under the name of " dephlogisticated air.'' By 1775 Priestley had found that this gas supported both combustion and respi- ration. Had it not been for the phlogiston theory, to which he clung with fatal tenacity, Priestley would have been the undisputed discoverer of the gaseous basis of life ; as a matter of fact, Lavoisier, as we are all aware, was the man who knew what he had got when by the end of 1774 he had isolated oxygen and so named it.

But we must not forget that Priestley also worked much on the properties of carbon dioxide, it was the gas he first studied near a brewery at Warrington. As early as 1772 he read a paper to the Eoyal Society showing experimentally that, while animal life could not be supported by this gas, plants, on the other hand, restored the wholesomeness to air rendered putrid by animal breathing. He also demonstrated that by both combustion and respiration the air loses one

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