REGULATING THE HEAT 149 



a candle had been burned, also contained fixed air. But 

 neither Dr Joseph Black nor any of his fellow-workers 

 saw that these discoveries threw light on the use of 

 the lungs. 



In 1 77 1, when Dr Joseph Black had become Professor 

 of Chemistry in the University of Edinburgh, Mr Joseph 

 Priestley, a native of Leeds, where he was born in 1733, 

 a clergyman of the Unitarian Church by profession and 

 a natural philosopher by instinct, commenced a series of 

 experiments which led him up almost to the discovery of 

 the real use of the lungs. He filled in his idle moments 

 by inquiring into the nature of the air which men breathed 

 in living rooms. Fie found that air which had been 

 breathed for some time by a mouse under a bell-jar, so 

 that it might no longer support life, could be restored by 

 growing plants within it. He discovered, too, in the 

 course of other inquiries that the gas given off" when 

 red oxide of mercury was heated caused a lighted candle, 

 when placed in it, to burn much more brightly ; a mouse 

 could breathe this new gas and live vigorously. He 

 made a bad guess as to its nature and called it " dephlo- 

 gisticated air " — the element to which Lavoisier, the 

 great French chemist, afterwards gave the name oxygen. 

 Almost at the same time (1774) oxygen was discovered in 

 Sweden by Scheele. Very soon afterwards the Hon. 

 Henry Cavendish, a rich landed gentleman who devoted 

 his life to experimental researches, found out that, when 

 Priestley's dephlogisticated air (oxygen) was burned with 

 certain proportions of " inflammable air " (hydrogen), 

 both gases disappeared and water was formed in their 

 place. He was the first to realise that water was formed 

 during combustion and was made up by the union of 

 definite proportions of two elements — hydrogen and 

 oxygen. 



Thus in England, by 1783, all the facts had been 

 discovered relating to the similarity of the gases formed 

 during the burning of a candle and the breathing of an 

 animal ; but none of the British investigators had suc- 

 ceeded in putting the facts together, so as to show that 



