52 THE BEGINNINGS OF LIFE. 



Seeing that in bodies of this class a compound radicle, 

 such as cyanogen (CN) or ammonium (NH 4 ), may 

 replace one of the simple metallic elements, and that 

 two such salts may combine together to constitute a 

 double salt ; or that the metallic element may be re- 

 placed by a more complex radicle, such as urea 

 (C 2 N 2 H*O 2 ) or kreatinine (C 8 H 7 N 3 O 2 ), or even by 

 one of the still more complex bodies known as alka- 

 loids \ we may be somewhat amazed at the marvellous 

 atomic complexity which is to be attained even by 

 crystallizable bodies known as salts. 



In each case we have to do with atomic and mole- 

 cular properties, and looked at in this light, the differ- 

 ences between what are called simple and complex 

 substances gradually vanish. All alike have a mole- 

 cular constitution, though the molecules may be simple 

 or compound, and made up of like or unlike units. 



What has been said concerning crystallizable bodies 

 obtains also with regard to the compounds known as 

 colloids. We will not recapitulate what has been al- 

 ready said 2 concerning these remarkable compounds; 

 we will merely state that they are supposed to be 

 generally characterized by the large size and complexity 



constituent atoms of the simple molecules are the last to part com- 

 pany. 



1 The composition of narcotine, for instance, is said to be Qg H 2 s 

 NO U , and that of morphine, C 34 H 19 NO 6 4 2 HO. Both bodies have 

 distinct crystalline forms. 



2 See vol. i. pp. 88-91. 



