XXX CONSTITUTION OF CHEMICAL COMPOUNDS, 

 what different character. These have been designated as 



PJT3 p 2TT5 p2TT5 



acetones or ketones, as 0:C;^g 3 O.-C.^jp O.-C^gs etc. 



Aldehydes are, hence, compounds which consist of an 

 alcohol-residue and a hydrogen-atom united by means of 

 the group CO: and acetones consist of two alcohol-residues 



united by the group CO. It will thus be seen that the 

 jj 



body 0:C. jj may be considered as the simplest representa 

 tive of both classes of compounds. 



In addition to the various classes of compounds, there 

 are others, but they are all variations on these principal 

 classes, and demand here no special explanation. We 

 have, for instance, compounds which partake of the pro 

 perties of both acids and bases, acids and alcohols, alde 

 hydes and acids, aldehydes and alcohols, etc. etc., but 

 with the aid of the few principles laid down these will be 

 readily understood. We have, further, instances, especially 

 among carbon-compounds, in which atoms of the same 

 kind are united with each other by means of more than 

 one affinitjr, and also those in which each carbon-atom of 

 the compound is united with two other carbon-atoms, on 

 the one hand with one, on the other with two affinities. 

 etc. etc.; the constitution of such compounds can, how 

 ever, be easily comprehended by the application of the 

 fundamental principles. 



The main question which now presents itself is: What 

 grounds have we for the acceptation of these fundamental 

 principles ? It can only be answered, they have been 

 proposed and accepted as affording the simplest explana 

 tion of innumerable investigations concerning the proper 

 ties of chemical compounds. At present no facts are 

 known that conflict with their acceptation. They are by 

 no means established beyond a doubt, but, as they simplify 

 known facts, and have been exceeding^ fruitful in widen 

 ing the field of observation, they are worthy of our most 

 careful study. 



It would lead too far in this place to recall the indi 

 vidual investigations and the methods of reasoning which 

 have led to the acceptation of our present ideas concern 

 ing the constitution of chemical compounds. In order to 

 draw our conclusions we must know the methods of forma 

 tion, the decompositions, and all the varied changes which 

 individual compounds or groups of compounds undergo. 



