8 BLACK. 



that Black's discovery of fixed air affords no second 

 exception to this rule ; for it is said that Van Helmont, 

 who nourished at the end of the sixteenth and begin- 

 ning of the seventeeth century, had observed its evo- 

 lution during fermentation, 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 aeri- 

 form 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 com- 

 monly 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 is now fit that we see in what manner the subject 

 was treated by scientific men at the period immediately 

 preceding Black's discoveries. The article 'Air' in 

 the French 4 Encyclopedic' was published in 1751, and 



* 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 sub- 

 ject. No one questions Newton's title to the discovery of the different 

 refrangibility of light, and the true theory of the rainbow ; yet, at the 

 beginning of the 17th century, Antonio de Dominis, Archbishop of 

 Spalatro, had really made an ingenious and well-grounded experiment on 

 the similarity of the rainbow colours with those formed by the sun's rays 

 refracted twice and reflected once in a globe filled with water. The doc- 

 trine of universal gravitation was known to both Kepler and Galileo ; and 

 Boulland (Astronomia Philolaica, lib. i., 1645), distinctly stated his belief 

 or conjecture that it acted inversely as the squares of the distances. The 

 famous proposition of equal areas in equal times was known to Kepler. 

 The nearest approach to the Fluxional Calculus had been made by Har- 

 riott and Roberval and Fermat ; and to take but one other example, the 

 electrical explosion of the Leyden jar, discovered in 1747, obtained the 

 name of the coup-foudroyant, and was by Abbe Nollet conjectured to be 

 identical with lightning, Franklin's celebrated experiment being only made 

 in 1752. 



