346 COSMOS. 



saltpetre on clay walls in contact with the atmosphere, ap* 

 pear to have contributed jointly to the adoption of this view. 

 The nitrous particles of the air influence, according to Mayow, 

 ^he respiration of animals, the result of which is to generate 

 animal heat, and to deprive the blood of its dark color ; and, 

 while they control all the processes of combustion and the 

 calcination of metals, they play nearly the same part in the 

 antiphlogistic chemistry as oxygen. The cautious and doubt- 

 ing Robert Boyle was well aware that the presence of a 

 certain constituent of atmospheric air was necessary to coni- 

 bustion, but he remained uncertain with regard to its nitrous 

 nature. 



Oxygen was to Hooke and Mayow an ideal object — a delu- 

 sion of the intellect. The acute chemist and vegetable phys- 

 iologist Hales first saw oxygen evolved in the form of a gas 

 when, in 1727, he was engaged at Mennige in calcining a 

 large quantity of lead under a very powerful heat. He ob- 

 served the escape of the gas, but he did not examine its na- 

 ture, or notice the vivid burning of the flame. Hales had no 

 idea of the importance of the substance he had prepared. 

 The vivid evolution of light in bodies burning in oxygen, and 

 its properties, were, as many persons maintain, discovered in- 

 dependently — by Priestley in 1772-1774, by Scheele in 1774- 

 1775, and by Lavoisier and Trudaine in 1775.* 



The dawn of pneumatic chemistry has been touched upon 

 in these pages with respect to its historical relations, because, 

 Hke the feeble beginning of electrical science, it prepared the 

 way for those grand views regarding the constitution of the 

 atmosphere and its meteorological changes which were mani- 

 fested in the following century. The idea of specifically dis 

 tinct gases was never perfectly clear to those who, in the sev- 

 enteenth century, produced these gases. The difference be- 

 tween atmospheric air and the irrespirable light-extinguishing 

 or inflammable gases was now again exclusively ascribed to 

 the admixture of certain vapors. Black and Cavendish first 

 showed, in 1766, that carbonic acid (fixed air) and hydrogen 

 f combustible air) are specifically difTerent aeriform fluids. So 

 long did the ancient belief of the elementary simplicity of the 

 atmosphere check all progress of knowledge. The final knowl- 

 edge of the chemical composition of the atmosphere, acquired 

 by means of the delicate discrimination of its quantitative re- 



* Priestley's last complaint of that which " Lavoisier is considered to 

 have appropriated to himself," is put forth in his little memoir entitled 

 *"The Doctrine of Phlogiston Established;' 1800, p. 43. 



