ELEMENTS ATOMIC WEIGHTS MOLECULES, ETC. 7 



of temperature and pressure, all gases have the same number 

 of molecules in equal volumes." While it is quite impos- 

 sible to isolate and weigh a single molecule, yet if we select 

 as a standard of molecular weight a molecule of oxygen, 

 consisting of two atoms, and assign to it a weight of 32, 

 we may, by assuming the Avog^dro hypothesis, obtain 

 the weight of the molecule of any gas. This is done in 

 the case of ammonia by weighing equal volumes of ammonia 

 and of oxygen under the same conditions of temperature and 

 of pressure, and solving for the molecular weight of ammonia 

 in the proportion: 



Weight of volume of oxygen : weight of same volume of 



ammonia = 32 : molecular weight of ammonia. 



In this manner we may determine accurately the weight 

 of the molecule of any element or of any compound which 

 may be weighed in a gaseous condition. 



8. Oxidation and Reduction. An understanding of the 

 principles of chemistry involved in the processes of oxidation 

 and of reduction is important. It is impossible to obtain 

 a working knowledge of chemistry without gaining a mastery 

 of these processes. A simple case of oxidation takes place 

 when a bright strip of iron is heated over a flame. The 

 oxygen of the air, more active at the high temperature, 

 combines with the metallic iron, rusting it into a black 

 oxide of i.-on. If a silver spoon is exposed to the fumes 

 of sulphur, the silver combines with the sulphur and black 

 silvsr sulphide is formed on the surface of the metal. This 

 is also a case of oxidation, although here there is no oxygen 

 involved. 



Oxidation may be defined as the combination of oxygen, 

 or of other elements that act chemically in the same way 

 as oxygen, with other material. Sometimes oxidation takes 

 place in two or more stages. Thus metallic mercury will 

 be oxidized by combining with one atom of chlorine, form- 

 ing white mercurous chloride or calomel, which, in turn, 



