4 ELEMENTARY AGRICULTURAL CHEMISTRY 



When two elements combine together to form a chemical 

 compound they do so by the union of a certain number of atoms 

 of one with a certain number of atoms of the other element, 

 and the proportion between these numbers is usually a very 

 simple one, and is always the same for a given compound. To 

 take an example, water is composed of hydrogen and oxygen, 

 and the numbers of atoms are in the proportion of two and 

 one respectively. This can be briefly expressed by the formula 

 HjjO, which conveys the information that a molecule of water 

 consists of two atoms (having a relative weight of 2) of hydro- 

 gen and one atom (with a relative weight of 16) of oxygen. 



It is important to realise the essential difi'erences between 

 a mechanical mixture and a chemical compound. 



From the theoretical aspect this may be stated thus : In 

 a mixture the molecules of the constituents remain intact, 

 and by a sufficiently delicate means might conceivably be 

 detected lying side by side, and each constituent retains its own 

 characteristic properties, so that the mixture has properties 

 intermediate between those of its components. In a compound, 

 on the other hand, all the molecules are alike, and no investi- 

 gation, however searching, would be able to detect the original 

 components in the compound without destroying its charac- 

 teristic properties. From the practical aspect it is to be noted 

 that the mixing of the two subjects generally produces no 

 evolution or absorption of heat, and yields a product inter- 

 mediate in properties between its components, and capable, 

 by merely mechanical means, of being separated into its 

 constituents ; whilst with the formation of a compound there 

 is us.ually much heat evolved, and the product has properties 

 totally unlike those of its constituents. No mechanical means, 

 however delicate, can separate, or even detect the existence of, 

 ihe constituents. 



The following experiment will perhaps help to render the 

 distinction clear. If about 10 grammes of fine copper filings 

 are mixed in a mortar with half their weight of sulphur, 

 a dirty orange powder results. Under the microscope red 



