28 ESSENTIALS OF BIOLOGY 



(the fourth group being on the soHd angle behind) 

 3 



or thus 



and the two substances thus formed would not be, chemically 

 and physically, the same. 



These are instances of isomerism. Two substances might have 

 exactly the same numbers and kinds of atoms, but they will differ 

 if the distribution in space of the atoms is different. And so 

 many rearrangements of the atoms and groups of a protein 

 or carbohydrate are possible that billions of different substances 

 formed from the same numbers and kinds of atoms are possible. 



Other organic chemical substances . The above are the " founda- 

 tion " categories of the chemical substances found in the bodies 

 of plants and animals. There are many others. Proteins, carbo- 

 hydrates, etc., may " conjugate," or join up in many ways. In- 

 organic chemical substances, sulphur, phosphoric acid, etc., may 

 become incorporated in the carbon chains, rings, etc., and so on. 



The organic " keystones " of chemical structure are carbon 

 chains or rings, or nitrogen nuclei (see Section 73^). These 

 two elements, C and N, have such extraordinary '* pivotal " roles, 

 and the existence of isomeric arrangements is so fruitful of results, 

 that the number of possible organic compounds, that is, those 

 pivoted on binding C and N atoms, must be incredibly great. But 

 it is finite. 



And so very great is the power, or facility, of the carbon atom 

 for building up complex chemical structures that huge molecules 

 are formed. Some proteins may consist of tens or hundreds of 

 thousands of atoms and have weights, relatively to the hydrogen- 

 molecule, of hundreds of thousands. There is nothing like this 

 in inorganic chemical substances. 



8c. The Chemistry of Artifacts. Chemically artifacts 

 " may be anything." They are things made consciously or 

 unconsciously by man and other organisms and their chemical 

 nature expresses purpose or tendency of some kind : 

 The metallic alloys of stainless steel — because that does not rust ; 



