332 BLACK. 



altered the whole face of chemical science, and how 

 completely the doctrine was original, we must now 

 examine the state of knowledge which philosophers 

 had previously attained upon the subject. 



It has often been remarked that no great discovery was 

 ever made at once, except perhaps that of logarithms ; 

 all have been preceded by steps which conducted the dis- 

 coverer's predecessors nearly, though not quite, to the 

 same point. Some may possibly think that Black's dis- 

 covery of fixed air affords no second exception to this 

 rule ; for it is said that Van Helmont, who flourished 

 at the end of the sixteenth and beginning of the seven- 

 teenth century, had observed its evolution during fer- 

 mentation, and given it the name of gas silvestre, spirit 

 from wood, remarking that it caused the phenomena 

 of the Grotto del Cane, near Naples. But though he 

 as well as others had observed an aeriform substance 

 to be evolved in fermentation and in effervescence, 

 there is no reason for affirming that they considered it 

 as differing from atmospheric air, except by having 

 absorbed, or become mixed with, certain impurities. 

 Accordingly, a century later than Van Helmont, Hales, 

 who made more experiments on air than any other of 

 the old chemists, adopts the commonly received opinion 

 that all elastic fluids were only different combinations 

 of the atmospheric air with various exhalations or 

 impurities ;* and this was the universal belief upon 

 the subject, both of philosophers and of the vulgar. 



* It may safely be affirmed that Van Helmont's observation, 

 which lay for a century and a half barren, threw no light of any 

 value upon the subject. No one questions Newton's title to the 

 discovery of the different refrangibility of light, and the true theory 



