CHEMISTRY 211 



mark them off sharply from other substances. 

 Thus carbon bears relatively little resem- 

 blance to its neighbors silicon or titanium, 

 nitrogen to phosphorus or vanadium, oxy- 

 gen to sulphur or chromium; while hydro- 

 gen, of course, has a place quite apart in the 

 classification, and as an element appears to 

 be correspondingly unique. 



It is, therefore, in the highest degree prob- 

 able that compounds made from elements of 

 such positive chemical characteristics and 

 very unusual properties will be unlike com- 

 pounds formed from other elementary sub- 

 stances. In this manner the periodic classi- 

 fication confirms our confidence in the results 

 of many decades of experience, which lead us 

 to believe that other elements are exceedingly 

 unlikely readily to form compounds com- 

 parable in number, variety, and complexity 

 with those of organic chemistry as we know it. 



For more evidence we may turn to certain 

 further data of organic chemistry. I refer 

 principally to the character of the organic 

 radicals composed exclusively of carbon and 

 hydrogen. In making a rational classifica- 

 tion of the carbon compounds it has been 

 found convenient to commence with that 

 series of hydrocarbons, called paraffines, with 

 which the present discussion was begun. 



