22 ORGANIC EVOLUTION CONSIDERED 



vessel. An atom of sulphur and an atom of iron 

 throughout the mass — atom for atom— crash together 

 and hold each other in their grasp. 



The result of the chemical action is a new substance 

 that looks like neither sulphur nor iron. Before the 

 burning match was placed in the mixture the sulphur 

 and iron could be readily separated by mechanical 

 means — the iron could be removed with a magnet, or 

 the sulphur could be dissolved out with bisulphide of 

 carbon. The compound formed looks like neither 

 sulphur nor iron, and its mechanical and chemical 

 properties are different from those of the two 

 elements that compose it. 



The whole of the two elements used is present in 

 the compound, known by the name, sulphide of iron, 

 and by various chemical means they may be separated 

 from each other and obtained again in the simple 

 form. 



The smallest amount of this compound that can 

 exist is represented by one atom of sulphur com- 

 bined with one atom of iron, which is expressed by the 

 formula Fe S. This smallest conceivable amount of a 

 compound is called by the chemist a molecule. A 

 single molecule may contain from two up to hundreds 

 of atoms. The molecule is the unit of the compound 

 on which its qualities as a distinct substance depend. 



We are unable to tell in advance from the proper- 

 ties of the elements what will be the properties of the 

 compounds produced by their union. How totally we 

 would be unable to predict that by putting a spark 

 to a mixture of two volumes of hydrogen and one 

 volume of oxygen they would unite with a tremendous 

 explosion to form a volume of water more than 

 eighteen hundred times less than that of the two 

 gases; or that by decomposing common salt we would 

 find it composed of a soft metal and a very poisonous 



