196 CHEMISTRY 



Without going into much detail concerning the nature of valence, 

 or, what is the same thing, concerning the nature of the forces in- 

 herent in our atoms, we assume briefly that every atom of an ele- 

 ment possesses one, two, three, four, or more such units of force, and 

 we call the element univalent, bivalent, trivalent, quadrivalent, etc., 

 according to the number of such units it possesses. It is by virtue 

 of the existence of these units of force that the compounds made up 

 of the same or of various elementary atoms exist. We assume that 

 in such a molecular compound the atoms are bound one to another 

 in a definite way by means of their affinity units. 



Sirice the development of these ideas concerning the valence of the 

 elements there has been a great deal of work carried on with the ob- 

 ject of determining whether the valence of an element is constant or 

 whether it may vary; the majority of chemists are now convinced 

 that it may vary. The valence of nitrogen may be three or five. The 

 valences of hydrogen, oxygen, and carbon, on the other hand, have, 

 until recently, been assumed always to remain constant, i. e,, one, two, 

 and four, respectively. 



Since the complexity, the very great variety and number of exist- 

 ing compounds containing carbon are unquestionably to be attrib- 

 uted to the peculiar nature of the forces inherent in the carbon atom, 

 let us consider a little more in detail what hypotheses we make in our 

 present system of carbon chemistry concerning this element. We 

 assume, first, that the valence of the carbon atom is always four; 

 second, that the four valences or affinity units of the carbon atom 

 are equivalent; third, that they are distributed in space in three 

 dimensions and act in tetrahedral directions; fourth, that the carbon 

 atoms can unite with one another by means of one, two, or three 

 affinity units to form what we usually call chains. 



These chains may be open, or closed rings or cycles. The number of 

 carbon atoms thus bound to one another may be exceedingly large. 

 The closed chains usually contain three, four, five, six, or seven carbon 

 atoms in the ring. We may have in these chains, whether open or 

 closed, some of the carbon atoms replaced by oxygen, nitrogen, 

 sulphur, or other elements. If now we unite the extra valences of each 

 carbon or other atom - - i. e., those affinity units which are not neces- 

 sary for binding the atoms together in chains - - with other atoms 

 or radicals, it is at once evident that we can represent theoretically, 

 by so-called graphical formulse, molecules of great complexity. It is 

 also at once obvious that with a small number of atoms it must be 

 possible to construct a relatively large number of aggregates which 

 differ from one another simply in the way the atoms are bound to- 

 gether. In 1884, for instance, fifty-five totally different substances of 

 the empyrical-formula C 9 H 10 3 were actually known. We call them 

 isomers. One of the chief problems of organic chemistry since 1858 



