SECT. in. THE POLYATOMIC THEORY. 107 



Ammonia consists of three equivalents of hydrogen 

 and one of nitrogen ; now, when the radical phenyle, 

 which consists of twelve equivalents of carbon and five 

 of hydrogen, is put in the ammonia for one equivalent 

 of hydrogen, the result is aniline, whence most of the 

 coal tar colours are obtained. In like manner carb- 

 azotic acid, a beautiful yellow dye from coal tar, is 

 carbolic acid, three of whose equivalents of hydrogen 

 have been replaced by three equivalents of an oxide of 

 nitrogen. 



Compound radicles, consisting of carbon and the three 

 elementary gases, have been discovered which enter into 

 combination in definite proportions as simple atoms, and 

 all compound radicles travel in the galvanic circuit as 

 equivalents to the elementary substances. Hitherto they 

 have been regarded as representatives or equivalents of 

 one atom of hydrogen. Now it is generally admitted 

 that each has the property of replacing two, three, or 

 more atoms of hydrogen by equivalent substitution. 

 This multiple equivalency among compound radicals 

 forms the basis of what is called the polyatomic theory, 

 now so much employed by MM. Hofmann, Berthelot, 

 and other great modern chemists. 



Water is the most common radicle both in the inor- 

 ganic and organic world. Though a compound of oxygen 

 and hydrogen, it enters, according to the law of definite 

 proportion, into the composition of various amorphous 

 bodies in a dry state, that is in the form and proportion 

 of its gases. It is an essential element in the greater 

 number of crystals, and abounds in organic matter. In 

 certain cases the same substance crystallizes at different 

 temperatures, unites with different quantities of water 

 under the form of oxygen and hydrogen, and assumes 

 corresponding forms. For example, the seleniate of zinc 

 unites with three different portions of water and takes 

 three different forms, according as its temperature is 



