THE UNIVERSE. DISCOVERIES IN THE CELESTIAL SPACES. 345 



animals, of which the production of animal heat, and the 

 conversion of black into red blood are the result, the 

 processes of combustion and the calcination of metals, are 

 all dependent on these nitro-aerial particles of the atmosphere-, 

 in the antiphlogistic chemistry, they play nearly the part of 

 oxygen. The cautiously doubting Robert Boyle recognised 

 that the presence of a certain constituent of atmospheric air 

 is necessary to the process of combustion ; but he remained 

 uncertain as to its nitrous nature. 



Oxygen was to Hooke and Mayow an ideal object or a 

 fiction of the imagination. The acute chemist and vegetable 

 physiologist Hales, in 1727, first saw oxygen escape as gas 

 in large quantities from the lead which he calcined under 

 an intense heat. He saw the gas escape, but without ex- 

 amining its nature or remarking the vividness of the flame 

 occasioned by it. Hales did not divine the importance of 

 the substance which he had produced. The vivid evolution 

 of light in bodies burning in oxygen gas, and its properties, 

 were discovered, as many believe quite independently, ( 531 ) 

 by Priestley in 1772-1774, by Scheelein 1774-1775, and 

 by Lavoisier and Trudaine in 1775. 



The commencements of pneumatic chemistry have been 

 touched upon in these pages in their historic connection, 

 because, like the feeble beginnings of electric science,, they 

 prepared the way for the enlarged views, which the succeed- 

 ing century has been able to form of the constitution of the 

 atmosphere and of its meteorological variations. The idea 

 of specifically-distinct gases was never perfectly clear to 

 those who in the seventeenth century produced those gases. 

 Men began again to attribute the difference between 

 atmospheric air and the irrespirable, light-extinguishing, or 



