228 EIGHTEENTH CENTURY. PT. in, 



copper will re-appear and fall to the bottom of the glass, 

 because the iron attracts the nitric acid more strongly than 

 the copper does, and so it takes it up out of the liquid, 

 setting the copper free. 



Chemists had till now neglected this observation of 

 Newton's, but Bergmann followed it out, and by a number 

 of experiments he drew up a table of those substances 

 which seemed to have the greatest affinity for each other, 

 and which would unite whenever the conditions would allow 

 them. This he called a table of elective affinities' 



It is easy to see how this could . be used for testing or 

 trying what substances lie hidden in mineral waters. Iron, 

 for instance, in the case given by Newton, would show when 

 copper was dissolved in a liquid containing nitric acid. 

 Boyle, too, had shown that a blue liquid extracted from the 

 lichen called litmus turns to a bright red directly it touches 

 an acid ; so that blue litmus is a sure test of an acid. Again, 

 common salt put into a clear liquid containing silver, turns 

 it cloudy ; while tincture of gall-nuts makes a purple cloud 

 in a solution containing iron. Bergmann worked out a 

 number of these tests, and by means of them analysed or 

 separated out the substances contained in mineral waters ; 

 he even dissolved solid minerals in acids and tested them in 

 the same way. 



One of the first uses that he made of his tests was to try 

 Black's * fixed air.' When he heard of this gas he suspected 

 that it must be an acid, because it joined itself to lime, 

 which is an alkali, that is, a substance in all respects unlike 

 an acid ; and he had found that unlike substances nearly 

 always attract each other most strongly. So he made 

 some ' fixed air ' and tested it with blue litmus, and, as the 

 litmus turned red directly, he knew that he was right in 

 supposing it to be an acid, and. he called it 'aerial acid,' or 



