vi CARBONIC ACID AND THE CARBONATES , 101 



it showed clearly that the diamond, like charcoal, must be 

 regarded as a variety of carbon. 



The mineral GRAPHITE, also known as PLUMBAGO or 

 BLACK LEAD, was shown by Scheele, in 1779, to give rise to 

 fixed air when fused with nitre. Other workers showed that 

 (if allowance were made for the ash which it always 

 contains) it gave the same proportion of gas as charcoal and 

 diamond, and must therefore be regarded as a third variety 

 of carbon, 



Such varieties of an element as charcoal, diamond, and 

 graphite were described by Berzelius, in 1840 (Jahresbericht^ 

 20, Chem. p. 13), as ALLOTROPES. 



Bi CARBONIC ACID AND THE CARBONATES. 



Bergman (1774) regards fixed air as an acid. It is 



uncertain to what extent Black recognised that fixed air had 

 the properties of a weak acid. He was aware that it was 

 attracted by the caustic alkalis and by lime, and that it 

 blunted and diminished their caustic and alkaline properties. 

 But the small quantity of liquid which could be condensed 

 when distilling chalk or magnesia differed so completely 

 from the mineral acids produced by distilling green vitriol 

 or nitre, that it is not surprising that Black hesitated to 

 describe fixed air as an acid. 



The clear recognition of its acid properties was due to the 

 Swedish chemist Torbern Bergman (1735-1784), who in 

 1774 published a full description of its properties in a paper 

 "On the Aerial Acid " (Bergman's Essay 's, translated by 

 E. Cullen, 1784, pp. 1-90). 



In support of this view he mentions : 



(1) Its solubility in water, as measured by Cavendish 

 (Chapter V). 



(2) The acid taste which it imparts to natural and 

 artificial aerated waters, as noticed by Brownrigg and by 



