relative to Black, Watt, and Cavendish. I 23 



is condensed over water, but not over mercury. You indeed 

 tell us that Black "found this gas incondensable" but he has 

 nowhere told us as much himself; and you might with more 

 safety have presumed the contrary ; the true statement being, 

 that he and his predecessors had found it condensable, and 

 that Cavendish found the conditions under which it is not 

 condensed. 



In the same spirit of liberality you take "the capital disco- 

 very" that the air of the atmosphere is not the only air per- 

 manently elastic, from its ancient owners, to appropriate it 

 to Black, and expend much learned pains in setting forth 

 the originality and importance of the "doctrine" which you 

 ascribe to him. " The great step," you say, " was now made, 

 that the air of the atmosphere is not the only permanently 

 elastic body, but that others exist, having perfectly different 

 qualities from atmospheric air, and capable of losing their elas- 

 ticity by entering into chemical union with solid and with 

 liquid substances, from which, being afterwards separated, 

 they regain the elastic or aeriform state." "In order to esti- 

 mate the importance of this discovery, and at the same time to 

 show how entirely it attends the whole face of chemical science, 

 and how completely the doctrine was original, we must now 

 examine the state of science which philosophers had previously 

 attained to. It has often been remarked, that no great disco- 

 very was ever made at once, except perhaps that of logarithms: 

 all have been preceded by steps which conducted the disco- 

 verer's predecessors nearly, though not quite, to the same 

 point. Some may perhaps think that Black's discovery of 

 fixed air affords no second exception to this rule; for it is 

 said that Van Helmont, who flourished at the end of the six- 

 teenth and beginning of the seventeenth century, had observed 

 its evolution during fermentation, and gave it the name of gas 

 sijlvcstre, spirit from wood, remarking that it caused the phse- 

 nomena 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 rea- 

 son for affirming that they considered it as differing from at- 

 mospheric air, except by having absorbed or become mixed 

 with various exhalations or impurities. Accordingly a cen- 

 tury later than Van Helmont, Hales, who made more experi- 

 ments upon air than any of the old chemists, adopts the com- 

 monly received opinion, that all elastic fluids were only differ- 

 ent combinations of the atmospheric air with various exhala- 

 tions or impurities : and this was the universal opinion upon 



the subject, both of philosophers and the vulgar." "It is 



now fit that we see in what manner the subject was treated by 



K2 - 



