the base; such salts are termed acid salts.  Bicarbonate of soda is one of these acid salts, but so feeble is carbonic acid in its acid properties and practical evidences, that we shall see both monocarbonate or “neutral” carbonate of soda and bicarbonate or “acid” carbonate of soda show evidences of, or, as chemists say, react with alkalinity towards litmus.  However, phenolphthalein, though reacting alkaline with monocarbonate of soda, indicates the acidity of the bicarbonate of soda, a thing which, as I have just said, litmus will not do.  We will take two jars containing solution of monocarbonate of soda, and in the first we will put some phenolphthalein solution, and in the second, some litmus tincture.  The solution in the first jar turns rose coloured, and in the second, blue, indicating in each case that the solution is alkaline.  If now, however, carbonic acid be blown into the two solutions, that in the first jar, containing the phenolphthalein, becomes colourless as soon as the monocarbonate of soda is converted into bicarbonate, and this disappearance of the rose colour indicates acidity; the blue solution in the jar containing litmus, on the other hand, is not altered by blowing in carbonic acid.  Furthermore, if to the colourless solution containing phenolphthalein, and which is acid towards that reagent, a little reddened litmus is added, this is still turned blue, and so still indicates the presence of alkali.  We have, therefore, in bicarbonate of soda a salt which behaves as an acid to phenolphthalein and as an alkali to litmus.  Another extremely sensitive indicator is the coal-tar dyestuff known as “Congo red”; the colour changes produced by it are exactly the inverse of those produced in the case of litmus, that is, it gives a blue colour with acids and a red colour with alkalis.

We have now learned that acids are as the antipodes to alkalis or bases, and that the two may combine to form products which may be neutral or may have a preponderance either of acidity or of basicity—­in short, they may yield neutral, acid, or basic salts.  I must try to give you a yet clearer idea of these three classes of salts.  Now acids in general have, as we have seen, what we may call a “chemical appetite,” and each acid in particular has a “specific chemical appetite” for bases, that is, each acid is capable of combining with a definite quantity of an individual base.  The terms “chemical appetite” and “specific chemical appetite” are names I have coined for your present benefit, but for which chemists would use the words “affinity” and “valency” respectively.  Now some acids have a moderate specific appetite, whilst others possess a large one, and the same may be said of bases, and thus as an example we may have mono-, di-, and tri-acid salts, or mono-, di-, and tri-basic salts.  In a tri-acid salt a certain voracity of the base is indicated, and in a tri-basic salt, of the acid.  Again, with a base capable of absorbing and combining with its compound