STEPHEN HALES 443
something volatile to which he gave the name of "gas," a word he coined on purpose to designate this "spirit" of wood and other kin- dred spirits. Gas sylvestre, because it came from or was produced by the burning of wood, was the first name under which carbon dioxide became a chemical concept in the minds of men of science.
The next contributions to accurate notions about breathing and hence about the necessity of ventilation, were made by Thomas Willis (1621 to 1675), who distinctly laid it down that three things cooperated in the act of respiration. These were: (1) a free and continuous access of air; (2) a constant supply of combustible material, and (3) the necessity for the continuous removal of the products of the combustion, for Willis clearly identified the burning of a flame in air and respiration in a living animal body ; it is certain that he believed that they werei chemically the same thing. It was in 1660 that the Hon. Bobert Boyle, who did so much for the early mathematico-physical study of the atmos- phere and of gases, performed the fundamental experiment as regards ventilation, namely, to exhaust the air aroimd a living animal. He showed that long before the vacuum was perfect, a sparrow and a mouse had both died, and the flame of a candle had gone out. Boyle also understood that there was something besides watery vapor that rendered expired air unfit for further breathing by animals. 6. A. Borelli about twenty years later was the first to estimate what we now know as the "tidal air," that is the quantity of air taken in and sent out at each breath, a most important datuml as regards the supply of fresh air per person.
The Comishman, Bichard Lower, clearly perceived before 1669 that the blood in the lungs was arterialized by absorbing something from the inspired air, what we now know to be oxygen. Lower was also quite certain that the expired air was noxious and ought to be removed; were there no need for this change, he writes, " we should breathe as well in the most filthy prisons as amongst the most delightful pastures." Lower held it as an axiom that where a fire bums readily, there an animal can breathe easily. The full significance of Lower's conclusions was not grasped by his contemporaries ; even so great a physiologist as Haller failed to see all that they involved.
The next step was taken also by an Englishman, an Oxford man of science, John Mayow. Working between 1668 and 1674, Mayow vir- tually discovered oxygen in a physiological sense. He named it " nitro- aerial particles," for he identified the substance which, absorbed from the air in breathing, produces animal heat, with the substance niter that appeared to be the cause of the combustion of gunpowder. Mayow died in 1679 ; and in England nothing was done as regards respiration or ventilation until Hales arose to rediscover much that Lower and Mayow had known well. In some respects Hales was less of a chemist than Mayow, but he caused hygiene to advance to a vastly greater ex*
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