82 ESSAYS BIOGRAPHICAL AND CHEMICAL 



to saturate the former, and five the latter ; and Black was 

 very near the truth ; indeed his error was only about four 

 per cent. He proved, by addition of sulphuric acid, that 

 the caustic alkali contained no lime, and therefore that 

 its causticity was not due to an admixture of that 

 substance. 



To prove that lime-stone, or magnesia, ' loses its air ' 

 when dissolved in an acid, but regains it on addition of a 

 mild alkali, the acid in which the lime was dissolved 

 passing to the alkali, Black added caustic ley to a solution 

 of Epsom salt, the result being a precipitate of magnesia ; 

 this dissolved in vitriol without effervescence, showing 

 that no fixed air had taken part in the change. He also, 

 on adding caustic alkali to a solution of chalk in spirit of 

 salt (or hydrochloric acid), produced lime, which on being 

 dissolved in water produced lime-water, indistinguishable 

 from that produced from quick-lime and water. He goes 

 on to say that ' had we a method of separating the fixed 

 alkali from an acid, without at the same time saturating 

 it with " air " we should then obtain it in a caustic form.' 

 It can be done, it is true, by heating nitre with charcoal, 

 but the alkali is then found saturated with air; and 

 again, by heating the alkali-salts of vegetable acids, the 

 same occurs. Black conjectures that the fixed air must 

 be derived either from the nitre or the charcoal in the 

 first case (indeed it is derived from both, the nitre supply- 

 ing the oxygen to the carbon); and in the second, he 

 remarks that the vegetable acid is not really separated, 

 but rather destroyed by the fire. How nearly he came 

 to the discovery that fixed air is produced from carbon ! 



Such was Black's research on fixed air. And now 

 having shown that a gas can be retained by a solid, and 

 can be made to escape by treatment with acid or by heat, 

 he attacked somewhat later the problem of the cause of 

 this fixation. He discovered it to be due to what he 



