xi RESPIRATORY EXCHANGES 371 



an experimental fact of the highest importance, although Leonardo 

 adduces no evidence. 



The first real discovery in the field of experimental chemistry 

 was made by Jean Baptiste van Helinont (born at Brussels, 1577 ; d. 

 1044), noblest of the experimental alchemists, precursor of Priestley 

 and Lavoisier, the founders of modern chemistry. He found that 

 on burning coal and in the fermentation of wine, a gas which he 

 called gas silvestre escaped, which is incapable of maintaining a 

 flame, and produces asphyxia and death in animals. This gas 

 may develop in the heart of the earth, as in the famous Grotta del 

 Cane near Naples ; it bubbles up in certain mineral waters, as at 

 Spa, and can also be evolved from the calcareous concrements 

 formed in the crab's stomach (the so-called " crab's eyes ") by dis- 

 solving them in vinegar. In short, van Helmont's gas silvestre is 

 nothing else than the carbonic acid of modern chemists. Haller 

 used this discovery of the Belgian alchemist to refute the Aristo- 

 telian theory of respiration which Cesalpinus and Harvey had 

 sought to resuscitate. 



In the year 1670 another distinguished philosopher and 

 investigator, Robert Boyle (1626-1691), the leader of the group of 

 scientific men who formed the nucleus of the Royal Society, proved, 

 with the help of the pneumatic machine introduced by the 

 Magdeburg physicist, von Guericke, that not only do all land 

 animals perish in a vacuum, but all water animals as well, showing 

 that these equally require the air which is dissolved in the water 

 they inhabit. He concluded from many experiments that the air 

 contains a vital substance thus adumbrating the oxygen of 

 modern chemistry, which enters into the phenomena of combustion, 

 respiration, and fermentation. He also confirmed, experimentally, 

 the fact already advanced by Leonardo da Vinci and van Helmont, 

 to the effect that the air becomes unbreathable through respiration 

 not because it gets heated, but because it suffers chemical 

 change. 



Robert Hook, friend and contemporary of Boyle, pointed out 

 the need of incessant renewal of the air in the lungs for the 

 maintenance of life. Vesalius had noticed, a hundred years 

 previously, that in order to prolong the life of a dog after opening 

 of the thorax and consequent retractation of the lungs, it was only 

 necessary to inflate them rhythmically with air ; but he brought 

 forward no conclusions of importance in regard to the physiology 

 of respiration. Hook perfected the method of artificial respiration 

 in the dog with opened thorax by rhythmically blowing air from 

 a bellows into the lungs, or by continuous insufflation after 

 making an opening on the surface of each lung. In both cases he 

 saw that the animal could be kept alive for a prolonged period, 

 and only died when the air stagnated in the lungs from cessation 

 of the rhythmical or continuous ventilation. He concluded that 



