CONSTITUTION OF COMPLEX MOLECULES. 25 



of this kind with tri-elementary mineral substances in which the 

 number of atoms seldom exceeds 10 or 12, you perceive that the 

 compounds presented for our consideration appear at the first 

 glance to be possessed of a highly complicated nature. 



(23.) But in the majority of instances a minute chemical ex- 

 amination of these apparently complex organic bodies has led to 

 the conclusion that they are formed, if I may so say, by the 

 agglomeration of certain less complex molecules. Taking salicin 

 and populin as illustrations, we find that salicin readily breaks 

 up into the less complex molecules known as saligenin and glu- 

 cose or grape sugar, while populin breaks up into a molecule 

 of saligenin, a molecule of grape-sugar, and a molecule of ben- 

 zoic acid. I have here a specimen of saligenin or salicylic 

 alcohol, a beautiful crystalline body, which even when in very 

 weak solution is capable of being recognised by its action on 

 perchloride of iron. Thus, on adding tincture of iron to a dilute 

 solution of saligenin, we get a deep purple colour developed by 

 the mutual reaction of the two bodies, as you observe. Salicin, 

 then, by an absorption of water, breaks up into the less complex 

 bodies saligenin and glucose, as shown in this equation, 



Salicin Water Saligenin Glucose 



C I3 H I8 7 + H,0 = C 7 H 8 Z + C 6 H I2 6 ; 



while, under similar circumstances, populin, with its 50 consti- 

 tuent atoms, breaks up into saligenin, glucose, and benzoic acid, 

 thus : 



Populin Water Saligenin Glucose Benzoic acid 



+ aH 3 = C 7 H 8 O a + C 6 H IS! 6 + C 7 H 6 O a 



(24.) Confining our attention to salicin, the point I wish to 

 impress upon you in relation to this body is, that it does not 

 really contain either saligenin or glucose in the state of a com- 

 plete molecule. Adding together the number of atoms of carbon, 

 hydrogen, and oxygen in saligenin, and the number of atoms of 

 the same elements in glucose, we find they are in excess of the 



