One method of classifying sense qualities that has often been advocated uses as its basis the varieties of objects, agencies, or stimuli[2] by the application of which the sense qualities are produced or aroused. Thus the whole field of sense experience might be divided into thermal, electrical, mechanical, photic (produced by light), etc. But such sensations as are aroused by electrical stimuli, for example, may be auditory, visual, cutaneous, gustatory (having to do with taste), etc., while these same varieties of sensory experiences may be aroused in some cases by mechanical stimulation. Hence the classification of stimuli does not yield an adequate analysis of modes of sensation. In the field of taste this method, although it has been seriously attempted, is equally futile. Thus various writers have attempted to group taste sensations according to the species of plants and animals whose tissues possessed sapid (taste-producing) qualities. It is obvious that this method is unsatisfactory, since it is by no means true that all specimens of a given vegetable taste alike. Even different parts of the same plant have widely different tastes, and indeed the taste of a given part varies widely with time and circumstances. Footnote 2: By a “stimulus,” in this connection, is meant any object, force, or agent that acts upon a sense organ. Even Chevreul, a famous early student of the sense of taste, adopted a chemical classification, on the basis of the composition of the substance tasted. Here again it is true that substances chemically very dissimilar may possess tastes which are strikingly alike. Thus acetate of lead, chloroform, and cane sugar, which, chemically considered, are very dissimilar, may easily be mistaken for each other if their taste alone is relied on; while starch, which is chemically closely related to sugar, has no taste. It is also true that such different tastes as sweet and bitter may characterize substances which are chemically very closely related. It is, however, true that certain broad lines of chemical classification may be drawn. Thus those substances belonging to the colloid[3] group are tasteless, the crystalloids all being sapid. When substances are arranged according to the “periodic law” of chemistry, the elements present in sweet-tasting substances are in general neither extremely positive nor extremely negative. While it is the general rule that soluble alkaloids are bitter, acids sour, sugars sweet, and salts salty, there are many curious exceptions in every case. “It is true that we get the taste of salt only from chemical salt; but there are chemical salts that taste sweet, others that taste bitter, others again that have no taste at all.” Similarly, while it is true that sour tastes belong to acids, it is by no means true that all acids taste sour. Moreover, sugar of lead, which is a salt, tastes sweet; while sulphate of magnesia and other salts taste bitter. Footnote 3: