ACIDS. 11 
no breathing animal can live for a moment in any air 
or gas which has not in it a mixture of oxygen ; every 
kind of combustible burns with great splendour in it, 
and without it ceases to burn. It unites with a great 
number of substances, and changes both their appear- 
ance and properties in a very remarkable manner. Of 
the metals it entirely destroys the metallic lustre, and 
gives them an earthy form and texture. Substances in- 
this state have the name of oxides. 
Lead, for instance, combined with oxygen becomes 
the well-known red and heavy substance used by 
painters, under the name of minium or red lead (239), 
This, if deprived of its oxygen, loses its red colour, 
and returns to its former metallic state. Some of the 
metals are oxidized by merely being exposed to 
moisture. Thus the rust which is so readily contracted 
by iron is an oxide of that metal, produced by its at- 
tracting oxygen from the air or from water. 
ACIDS. 
22, It is one of the most remarkable properties of 
oxygen to impart to most of those bodies called acids 
their peculiar character of acidity. Oxygen does not 
itself possess the properties of an acid, nor is it an 
essential ingredient in all acids, though it is the acidify- 
ing principle in the greater number of them. 
23. Acid is a word originally synonymous with 
sour. It has, however, been gradually extended in 
its signification, and now comprehends all substances 
possessed of the properties of exciting upon the tongue 
the sensation called sour ; of changing the blue colours 
of vegetables to red; of uniting with water in almost 
any proportion ; of combining with alkalies (4-2), me- 
