atom or molecule several compound atoms or molecules of an acid, we have the possibility of partial saturation, and, perhaps, of several degrees of it, and also of full saturation, which means combination to the full extent of the powers of the base in question.  Also, with an acid capable of, or possessing a similar large absorptive faculty for bases, we have possibilities of the formation of salts of various degrees of basicity, according to the smaller or larger degree of satisfaction given to the molecule of such acid by the addition of a base.  We will now take as a simple case that of hydrochloric acid (spirits of salt), which is a monobasic acid, that is, its molecule is capable of combining with only one molecule of a monoacid base.  Hydrochloric acid may be written, as its name would indicate, HCl, and an addition even of excess of such a base as caustic soda (written NaOH) would only yield what is known as common salt or chloride of sodium (NaCl), in which the metal sodium (Na) has replaced the hydrogen (H) of the hydrochloric acid.  Now chloride of sodium when dissolved in water will turn litmus neither blue nor red; it is therefore neutral.  Such simple, neutral, monobasic salts are mostly very stable.  By “stable” we mean they possess considerable resistance to agencies, that, in the case of other salts, effect decompositions of those salts.  Such other salts which are decomposed more or less readily are termed “unstable,” but the terms are of course only comparative.

Now let us consider a di- or bi-basic acid.  Such an one is vitriol or sulphuric acid (H_{2}SO_{4}).  The hydrogen atoms are in this case an index of the basicity of the acid, and accordingly the fully saturated sodium salt is Na_{2}SO_{4} or neutral, or better normal, sulphate of soda.  In like manner the fully saturated salt of the dibasic acid, carbonic acid (H_{2}CO_{3}), is Na_{2}CO_{3}, ordinary or normal carbonate of soda.  But we must observe that with these dibasic acids it is possible, by adding insufficient alkali to completely saturate them, to obtain salts in which only one hydrogen atom of the acid is replaced by the metal of the base.  Thus sulphuric and carbonic acids yield NaHSO_{4}, acid sulphate or bisulphate of soda, and NaHCO_{3}, bicarbonate of soda, respectively.  An example of a tribasic acid is phosphoric acid, H_{3}PO_{4}, and here we may have three different classes of salts of three various degrees of basicity or base-saturation.  We may have the first step of basicity due to combination with soda, NaH_{2}PO_{4}, or monosodium phosphate, the second step, Na_{3}HPO_{4}, or disodium phosphate, and the third, and final step, Na_{3}PO_{4}, or trisodium phosphate.  Now let us turn to the varying degrees of acidity, or rather the proportions of acid radicals in salts, due to the varying appetites or combining powers of bases.  Sodium only forms simple monoacid salts, as sodium chloride (NaCl), sodium sulphate (Na_{2}SO_{4}); calcium forms diacid salts, e.g.