546 SCIENCE PROGRESS 



minerals which were never quite pure and could not be obtained 

 artificially, while Mitscherlich, starting from the same pure 

 material, was able to obtain either form of the sodium di- 

 hydrogen phosphate. 



Two years later, in 1 823, he found in sulphur another instance 

 of dimorphism and showed that the element separates from a 

 solution in carbon bisulphide in orthorhombic-bi-pyramids, 

 from the state of fusion in monoclinic prisms. 



The next advance was the discovery that the temperature 

 at which the crystals are produced frequently determines the 

 appearance of one or other modification of a polymorphic sub- 

 stance. This was first recognised by Mitscherlich in the case of 

 mercuric iodide. He showed that the alteration of the red into 

 the yellow form on heating was accompanied by a change in 

 crystalline form from tetragonal to orthorhombic. 



It is worth recording that Brodie in 1854 first observed in 

 the case of sulphur that polymorphic forms may have different 

 melting-points. 



During the next few decades many new instances of poly- 

 morphism were recorded among inorganic substances, but it 

 was a long time before similar examples were discovered among 

 organic compounds. 



This extension was made by Jungfleisch, who obtained two 

 modifications of 1 :2 :4-chlorodinitrobenzene, one melting at 

 50 , the second of a different crystalline habit melting at 43 °. 

 The crystals of the latter could be kept unaltered in a closed 

 vessel, but on being touched with a crystal of the form melting 

 at 50 ° they changed into this, becoming opaque like wax. 



Benzophenone was soon afterwards added to the list by 

 Zincke. Chancel, who first prepared this compound in 1849 

 by heating calcium benzoate, obtained it in orthorhombic 

 crystals melting at 46 . Zincke by oxidising diphenylmethane 

 obtained it in monoclinic crystals melting at 26 . The latter 

 form was unstable with respect to the former at all tempera- 

 tures and changed into the higher-melting variety when warmed 

 or touched with a crystal of it. 



Other examples were soon discovered, and from a study of 

 all then known Lehmann in 1877 was able to group them in 

 two classes. He recognised more clearly than had hitherto 

 been done that whilst in some pairs of polymorphs one form was 

 unstable in contact with the second at all temperatures, in 



