When we wish to express the number of atoms in a compound we write the number underneath when more than one; thus water is H_{2}O. Sulphuric acid H_{2}SO_{4}. As we proceed we will give the various formulæ when considering the chief elements. In chemistry we have acids, alkalis, and salts, with metallic oxides, termed _bases_, or bodies, that when combined with _acids_ form _salts_. Alkalis are bases. ACIDS are compounds which possess an acid taste, impart red colour to vegetable blues, but lose their qualities when combined with bases. Hydrogen is present in all acids. There are insoluble acids. Silicic acid, for instance, is not soluble in water, has no sour taste, and will not redden the test litmus paper. On the other hand, there are substances (not acids) which possess the characteristics of acids, and most acids have only one or two of these characteristics. Thus it has come to pass that the term “acid” has in a measure dropped out from scientific nomenclature, and salt of hydrogen has been substituted by chemists. For popular exposition, however, the term is retained. ALKALIS are bases distinguished by an alkaline taste. The derivation is from Arabic, _al-kali_. They are characterized by certain properties, and they change vegetable blues to green, and will restore the blue to a substance which has been reddened by acid. They are soluble in water, and the solutions are caustic in their effects. Potash, soda, and ammonia are alkalis, or chemically, the oxides of potassium, sodium, ammonium, lithium, and cæsium are all alkalis. Potash is sometimes called “caustic” potash. There are alkaline earths, such as oxides of barium, strontium, etc. _Bases_ may be defined as the converse of acids. Acids and alkalis are then evidently opposite in character, and yet they readily combine, and in chemistry we shall find that unlike bodies are very fond of combining (just as opposite electricities attract each other), and the body made by this combination differs in its properties from its constituents. SALTS are composed of acids and bases, and are considered neutral compounds, but there are other bodies not salts, which likewise come under that definition—sugar, for instance. As a rule, when acids and alkalis combine _salts_ are found.