INTRODUCTORY REMARKS. 237 



An organic compound, according to modern views, is sim- 

 ply a compound of carbon generally containing hydrogen, 

 frequently also oxygen and nitrogen, and sometimes other 

 elements. 



Organic chemistry may consequently be defined as the chemistry of 

 carbon compounds. The old familiar terms organic compounds 

 and organic chemistry are, however, still in general use. 



In a strictly systematically arranged text-book of chemistry 

 organic compounds should be considered in connection with the 

 element carbon itself, but as these carbon compounds are so 

 numerous, their composition often so complicated, and the 

 decompositions which they suffer under the influence of heat 

 or other agents so varied, it has been found best for pur- 

 poses of instruction to defer the consideration of these com- 

 pounds until the other elements and their combinations had 

 been studied. 



Elements entering into organic compounds. Organic compounds 

 contain generally but a small number of elements. These are, 

 besides carbon, chiefly hydrogen, oxygen, and nitrogen, some- 

 times sulphur and phosphorus. Other elements, however, enter 

 occasionally into organic compounds, and by artificial means all 

 metallic and non-metallic elements may be made to enter into 

 organic combinations. 



Here the question presents itself: "Why is it that the four ele- 

 ments carbon, hydrogen, oxygen, and nitrogen are capable of 

 producing such an immense number (in fact, millions) of different 

 combinations? To this question but one answer can be given, 

 which is that these four elements differ more widely from each 

 other, in their chemical and physical properties, than perhaps 

 any other four elements. 



Carbon is a black, solid substsnce, which has never yet been 

 fused or volatilized, while hydrogen, oxygen, and nitrogen are 

 colorless gases which can only be converted into liquids with 

 the greatest difficulty. Moreover, hydrogen is highly combus- 

 tible, oxygen is a supporter of combustion, whilst nitrogen is 

 perfectly indifferent. Finally, hydrogen is univalent, oxygen 

 bivalent, nitrogen trivalent, and carbon quadrivalent. These 

 elements are, therefore, capable of forming a greater number, 



