CH. XXVII. CHEMICAL AFFINITY. 229 



Bergmann made a great advance in chemistry by working 

 out the ' chemical affinity ' of many substances, and showing 

 how to make use of it to test or try mineral waters. 



Nearly a hundred years before Bergmann began to study 

 chemistry, Newton, when writing on attraction, had pointed 

 out that when substances are mixed together some kinds 

 attract each other very strongly and join together, making 

 one compound substance. For instance, he said, if you 

 put copper into nitric acid the copper will dissolve and dis- 

 appear; but if you plunge a piece of iron into the liquid the 

 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 made out 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; 



