xi RESPIRATORY EXCHANGES 373 



three, a few years after the publication of his early experiments. 

 The great importance of his unfinished work was overlooked, and 

 it hardly had any influence upon the progress of physiology. 



II. Before Mayow's notions of the composition of the air could 

 be regarded as a definite achievement, it was necessary to perfect 

 the methods of investigating the chemical study of gases, more 

 especially the art of manipulating them like solid or fluid bodies. 

 Many workers contributed to the building up of this technique, 

 in particular Hales (1678-1761), who was the promoter of the so- 

 called pneumatic chemistry, by inventing the method of collecting 

 gases in an inverted test-tube, suspended in a vessel of water or 

 mercury into which the gas was passed by means of bent tubes. 

 By this method, known as the eudiometric, Joseph Black, Professor 

 at Glasgow in 1757, again isolated and studied the properties of 

 that gas which van Helmont termed gas silvestre, to which Black 

 gave the name of " fixed air." He showed experimentally that it 

 was a product of the respiration of man and other animals. On 

 blowing through lime water or a solution of caustic alkali, he saw 

 that the lime was precipitated and the alkali was rendered mild. 

 Bergmann of Stockholm subsequently (1772) continued Black's 

 investigations on " fixed air," which he termed " ae'real acid," 

 finding it, though in small quantities, in the atmosphere. Black 

 and Bergmann, in these weighty experiments, were the immediate 

 precursors of Priestley and Lavoisier, who are usually accredited 

 with the prestige of being the founders of modern chemistry. 



Priestley (1733-1804) eagerly pursued the researches of his 

 predecessors ; in 1772 he experimented with the object of seeing 

 whether it were possible to restore the vital properties of air that 

 had deteriorated in consequence of animal respiration and com- 

 bustion, and after many fruitless experiments he discovered that 

 plants thrive in this air and renew it for purposes of animal life 

 and combustion. In 1775 he discovered that red precipitate of 

 mercury on calcination developed a gas which was exceedingly 

 favourable for combustion and animal respiration ; this (in accord- 

 ance with Stahl's doctrine which then predominated) he termed 

 dephlogisticated air, i.e. air free from the imaginary principle 

 known as phlogiston. This was the same gas that Mayow called 

 nitro- ae'real or igneo - ae'real, and which Priestley succeeded in 

 obtaining pure, and isolated from the other atmospheric gases. 

 By various other processes he also isolated the irrespirable gas 

 obtained from air, after burning coal or sulphur, and dissolving 

 the products of combustion in water. He studied the properties 

 of this and called it phlogistic air, i.e. air charged with phlogiston. 

 Lastly, in continuation of the experiments above described, of 

 Fracassati and Lower (the results of which had been confirmed 

 by Cigna and Hewson in 1773), Priestley showed that the de- 

 phlogisticated air which he had discovered was essential for the 



