BLACK. 333 



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 'Encyclopedic' was published in 1751, and 

 written by D'Alembert himself. It is, as might be 

 expected, able, clear, elaborate. He assumes the sub- 

 stance of the atmosphere to be alone entitled to the 

 name of air, and to be the foundation of all other per- 

 manently elastic bodies : " L'air elementaire, ou 1'air 

 proprement dit," he says. He describes it as " homo- 

 gene," and terms it " 1'ingredient fondamental de tout 

 1'air de 1'atmosphere, et qui lui donne son nom." 

 Other substances or exhalations mix with it, he says, 

 but these he terms " passageres," passing vapours, and 

 not permanent : the air alone (that is, the atmo- 

 spheric air) he calls "permanent," or permanently 

 elastic (vol. i. p. 225). So little attention had the ob- 

 servation of Van Helmont respecting the Grotto del 



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 doctrine 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 

 Harriott and Roberval and Fermat ; and to take but one other ex- 

 ample, 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. 



