MODIFIED BY TEMPERATURE. 249 



starch, gelatine, albumen, and some salts, are 

 more soluble in water than in alcohol ; why sul- 

 phur, phosphorus, resins, &c. are more soluble 

 in alcohol than in water ; and why some bodies 

 are soluble in oils that are insoluble in other 

 liquids ; why one acid dissolves metals which 

 another will not ; and why some metals are more 



a modification of that universal principle of action, which I have 

 endeavoured to shew, is capable of producing all the contractions 

 and expansions of matter. 



As yet, philosophers are but partially acquainted with the 

 circumstances which determine the numerous diversities in the 

 forms of the same substance : why, for example, there should be 

 several hundred varieties in the crystalline arrangement of car- 

 bonate of Jime. But it has been ascertained by the recent 

 researches of Mitscherlich and Haidinger, that various pro- 

 portions of water unite with many substances at different tempe- 

 ratures, during the process of crystallization, producing a corres- 

 ponding diversity of forms ; that seleniate of zinc unites with 

 three different portions of water, and assumes three different forms, 

 according as its temperature is cold, lukewarm, or hot; and so 

 of other solutions. They also found that moderate degrees of 

 temperature, such as that of the solar rays, produced a decided 

 change in the molecular arrangement of solid crystals. 



Not less remarkable and unexpected was the discovery of 

 Mitscherlich, that many bodies differing in chemical composition, 

 assume the same crystalline forms, which are determined by the 

 relative number and position of their atoms ; in other words, that 

 the same number of atoms, united in the same way, produce the 

 same crystalline form; that the protoxides of iron, copper, zinc, 

 nickel, and manganese, crystallize in the same form ; which is 

 also true of the neutral phosphate of soda, arseniate of soda, and 

 many other compounds that vary in chemical composition. 

 Such bodies have been therefore termed isomorphous, meaning 

 identity of form. 



