HISTORICAL. 569 



that air that had been repeatedly inspired was no longer capable 

 of maintaining life. Robert Hooke (1635-1703) introduced a 

 method of artificial respiration by means of a bellows, and demon- 

 strated by sending a continuous stream of air through the lungs 

 that the respiratory movements of these organs are in themselves, 

 as a mechanical process, in no wise an essential feature of respiration. 

 John Mayow in 1688-1674 discovered that air is not a simple ele- 

 ment, but contains a definite substance necessary to life and to 

 combustion. He designated this substance as the nitro-aerian 

 vapor or nitrous particles, because he believed that the same 

 substance is present in condensed form, as it were, in common niter, 

 having found that combustion is possible even in a vacuum in the 

 presence of niter. 



In the eighteenth century, as is shown in the work of the great 

 physiologist, Haller, the theories of respiration were in many 

 respects in a most unsatisfactory state. The new facts that had 

 been discovered made the old views untenable, but were not in 

 themselves sufficient to explain clearly what actually takes place. 

 Such periods of uncertainty and dissatisfaction are frequent enough 

 in the history of science. In certain parts of physiology to-day 

 we can recognize a similar state of affairs and from the history 

 of respiration we may assume that periods of this character are 

 necessary transitions to a fuller and more satisfactory knowledge. 

 In 1757 Joseph Black rediscovered carbon dioxid, calling it fixed 

 air, and showed that it is present in expired air. A little later 

 Priestly discovered and isolated oxygen and nitrogen; but, under 

 the influence of an erroneous view of combustion that had been 

 advanced by Stahl, was unable to give his discoveries a clear and 

 satisfactory application. The final step in this progress was made 

 by the wonderful work of Lavoisier between the years 1771 and 

 1780. He made correct analyses of air and of carbon dioxid, he 

 explained combustion as an oxidation with the formation of C0 2 

 and H 2 0, he showed that in respiration the same process occurs 

 and that the blood takes oxygen from the air and gives back to it 

 in expiration the carbon dioxid and water formed by combustion 

 within the body. He gave us the essential facts in the modern 

 theories of respiration and physiological oxidations. 



After Lavoisier the chief positive advances that have been made 

 have been in reference to the condition of the gases in the blood. 

 By means of the gas-pump Magnus (1837) obtained these gases 

 quantitatively and thus procured data which, as Liebig showed, 

 demonstrate that the oxygen is held in the blood, not in simple 

 solution, but in some form of chemical combination, probably 

 with the red corpuscles. Finally it was shown by Stokes and 

 Hoppe-Seyler that the oxygen is held in definite chemical com- 



