Structure of Simpler Organic Compounds. 47 



in its properties from, a substance, silver cyanate, which 

 had been described by Wohler a year before. Somewhat 

 later (in 1828), Wohler himself succeeded in converting 

 ammonium cyanate into urea, a substance with identical 

 chemical composition, although widely differing in its pro- 

 perties. At the date at which these discoveries were 

 made, it was not possible to determine the molecular 

 complexity ; but later researches have shown that sub- 

 stances in which this constant is the same, may differ 

 widely from one another in chemical and physical pro- 

 perties. Thus, for example, three different substances are 

 known to possess the simple formula C 5 H 12 , all of which 

 have not only the same chemical composition, but also the 

 same vapour density, and consequently the same molec- 

 ular weight. Such substances are said to be isomeric 

 with one another, and the individual members of the 

 group are designated isomerides. It is now known that 

 the differences between isomerides is due to differences in 

 the relative arrangement of the atoms to one another, 

 and the determination of this arrangement constitutes the 

 chief problem of organic chemistry. It is one also of 

 fundamental importance to the physiologist, for isomerides 

 exhibit, as a rule, totally different reactions in the bodies 

 of living organisms. 



In the present chapter the relative arrangements of the 

 atoms to one another in some of the simpler organic com- 

 pounds will be discussed. Such an arrangement constitutes 

 ^vhat is known as the " structure " of a compound, and is 

 represented by a graphical method. It is not possible in 

 this place to trace out the history of the evolution of this 

 modern method of representing the structure (or " con- 

 stitution ") of organic substances ; it will suffice to state 

 that, although now firmly established, it was only reached 

 after several decades of controversy, which was often 



