HISTORICAL INTRODUCTION 475 



whose admirable work, published in the years 1668 and 1674, 

 has been neglected, especially in his own country, and has only 

 recently received the recognition which it deserves. He it was 

 who showed that air was a mixture, and that one of its constituents, 

 which he called the nitro-aerial gas, was necessary for the existence 

 of all forms of life ; this gas, which is now known as oxygen, was 

 necessary for the support of a flame ; it combined with substances 

 such as sulphur to form acids, and with metals during calcination. 

 Respiration and combustion were analogous, and the function 

 of respiration was the absorption of the nitro-aerial gas and the 

 removal of the vapours arising from the blood. The respiration 

 of the foetus was recognised ; the placenta served as a lung from 

 which the blood in the umbilical vessels took up the nitro-aerial 

 gas and conveyed it to the foetus. The embryo chick absorbed 

 the nitro-aerial gas through .the porous shell of the egg. Evi- 

 dence for the absorption of the gas by the blood was found 

 in the fact that blood exposed to a vacuum gave of! minute 

 bubbles of gas, which Mayow thought were composed of the nitro- 

 aerial gas. 



Mayow's views were too advanced for the knowledge of his 

 time, they were not accepted, and after the early death of their 

 author at the age of thirty-five were soon forgotten. 



The next advance was the recognition of the waste products 

 which are given off from the lungs during respiration. Stephen 

 Hales, about the year 1726, showed that " noxious vapours " were 

 produced by repeatedly breathing air in a bladder, and that these 

 vapours were removed by potash, and thus the air could be breathed 

 again. He even suggested the use of a bladder of air and such 

 an absorbent as potash for use in the foul air of coal-mines, and 

 thus anticipated the modern employment of oxygen and soda- 

 lime, whereby the members of a rescue-party are enabled to 

 penetrate a mine after an explosion. 1 His knowledge of respira- 

 tion was, however, very defective, for, although he had demon- 

 strated that animals in a closed vessel absorbed air and a similar 

 change was effected by a burning candle, he rejected the views 

 of Mayow, and maintained that the function of respiration was 

 to cool the blood and remove aqueous and noxious vapours. 



1 It is only necessary to recall the fact that the poisonous nature of the air 

 after an explosion is due to lack of oxygen and to the presence of carbon mon- 

 oxide, not to excess of carbon dioxide. 



