INTRODUCTORY REMARKS. 441 



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

 gases which can only be converted into liquids with difficulty. More- 

 over, hydrogen is very combustible, oxygen is a supporter of combus- 

 tion, while nitrogen is perfectly indifferent. Finally, hydrogen is 

 univalent, oxygen bivalent, nitrogen trivalent, and carbon quadri- 

 valent. These elements are, therefore, capable of forming a greater 

 number and a greater variety of compounds than would be the case 

 if they were elements of equal valence and of similar properties. 



It will be shown later that carbon atoms have, to a higher degree 

 than the atoms of any other element, the power of combining with one 

 another by means of a portion of the affinities possessed by each atom, 

 thus increasing the possibilities of the formation of complex compounds. 



The number of thoroughly investigated organic compounds is estimated at 

 150,000, and each year is increased by 8000 to 9000. 



General properties of organic compounds. The substances 

 formed by the union of the four elements just mentioned have prop- 

 erties in some respects intermediate to those of their components. 

 Thus, no organic substance is as permanently solid l as carbon, nor 

 as permanently gaseous as hydrogen, oxygen, and nitrogen. 



Some organic substances are solids, others liquids, others gases ; 

 generally they are solids when the carbon atoms predominate ; they 

 are liquids or gases when the gaseous elements, and especially hydro- 

 gen, predominate ; likewise, it may also be said that compounds con- 

 taining a small number of atoms in the molecule are gases or liquids 

 which are easily volatilized ; they are liquids of high boiling- 

 points, or solids, when the number of atoms forming the molecules 

 is large. 



The combustible property of carbon and hydrogen is transferred 

 to all organic substances, every one of which will burn when suffi- 

 ciently heated in atmospheric air. (If carbon dioxide, carbonic acid 

 and its salts be considered organic compounds, we have an exception 

 to the rule, as they are not combustible.) 



The properties possessed by organic compounds are many and 

 widely different. There are organic acids, organic bases, and organic 

 neutral substances; there are some organic compounds which are 

 perfectly colorless, tasteless, and odorless, while others show every 

 possible variety of color, taste, and odor ; many serve as food, while 

 others are most poisonous ; in short, organic substances show a greater 

 variety of properties than the combinations formed by any other 

 four elements. 



1 Non-volatile organic substances are decomposed by heat with generation of volatile 

 products. 



