4:10 RESPIRATION. 



time to time have been developed, bear so close a relation to 

 discoveries of the properties of certain gases, particularly 

 carbonic acid and oxygen, that it seems desirable to give at 

 least a rapid sketch of these discoveries, and follow the ad- 

 vances in our knowledge of the processes of respiration, with 

 which they are necessarily connected. 1 



In the latter part of the fifteenth century, Leonardo da 

 Vinci, the great painter, mathematician, and naturalist, made 

 a discovery which conclusively proved the fallacy of the idea 

 that the air simply cooled the blood in respiration. He dis- 

 covered that fire consumed the air, and that animals could 

 not live in a medium which was incapable of supporting 

 combustion. This is the first statement in the history of 

 science which points to the fact that the function of the air 

 in respiration depends on its composition, and not on its 

 physical properties. 



About the middle of the seventeenth century, Yan Hel- 

 mont discovered some of the properties of what is now known 

 as carbonic acid gas. He showed that a gas, the result of 

 fermentation, or of the combustion of carbon, and formed by 

 the action of vinegar on certain carbonates, was incapable of 

 supporting combustion or maintaining animal life. He rec- 

 ognized this as the gas which is found in the lower part of 

 the celebrated Grotto del Cane, near Naples, into which a man 

 may enter with impunity, but which will asphyxiate a small 

 animal, as it is brought under the influence of the lower strata. 



A few years later (1670), Boyle, the founder of the Royal 

 Society of London, by some experiments published in the 

 Philosophical Transactions, attempted to show that air was 

 necessary to the life of all animals, even those which live 

 under water. In a remarkable paper entitled Suspicions 

 about some Hidden Qualities of the Air, he pointed to the 



1 The reader is referred to the elaborate work of MILNE-EDWARDS (Lemons sur 

 la Physiologic, tome i., p. 375 et seq.) for a complete and highly interesting history 

 of the physiology of respiration, from which we have taken most of the historical 

 facts to which reference will be made. 



