LECTURE II.] HISTORY OF CHEMISTRY. 19 



that period. Priestley failed to recognise this, because his 

 general chemical knowledge was small ; 7 because he placed no 

 value upon the results obtained by others ; and because he uni- 

 formly defended, with the most dogged persistence, the ideas 

 which he once adopted. 



What, on the other hand, were Scheele's theoretical views ; 

 how did he regard oxygen ? Scheele the ideal of a pure ex- 

 perimental chemist, the discoverer of numberless substances, a 

 man who carried out the most difficult investigations with the 

 most slender resources, who possessed in the highest degree the 

 faculty of observation, so that an error can scarcely be found in 

 any of his very numerous researches ; Scheele, who does not, 

 as happens to-day even with the best and most capable observers, 

 overlook the half of the points, but grasps the phenomena in 

 their entirety and examines them one by one, and for whom 

 every new experiment forms a mine of great discoveries what 

 intellectual progress did he introduce into our science ? 



I must, unfortunately, reply that this was very small. His 

 general views are so confused that I only enter with reluctance 

 upon the task of giving an outline of them. 



Scheele laid down his views chiefly in a small work on " Air 

 and Fire." The principal difficulty in giving an account of his 

 opinions is due to the fact that phlogiston, the basis of them, is 

 an unknown substance, to which he can assign every possible 

 property ; so that sometimes he endeavours to identify it with 

 an element which is known to us, while at others he seeks to 

 place it side by side with the medium which the physicists call 

 ether. In consequence, it often seems as if Scheele adopted 

 the hypothesis of Cavendish and Kirwan, and by phlogiston 

 understood hydrogen, 8 and yet, on the other hand, this is not in 

 accord with many of his other views. With him phlogiston is, 

 generally speaking, a subtle substance weighing but littlej and 

 concerning which he assumes that it is capable of penetrating 

 the walls of his vessels. He regards oxygen as a compound of 

 water with a hypothetical saline substance, 9 in which compound 



7 Kopp, Geschichte. I, 239. 8 Ibid. I, 262. 9 Ibid. I, 261, 



