And in the sense of hearing, the adequate stimulus of which consists of air vibrations, it is the different vibration rates that account for the qualitative or pitch differences in sound sensations. What characteristics of the stimulating fluids are responsible for the specific sensations sweet, sour, bitter, and salt? The answer to this question has been sought in the chemical constitution of the sapid substances. There is found to be a certain relation between chemical groups and the taste sensations they produce. Kiesow and others have pointed out that acids are sour; that many chemical salts have a salt taste; that many carbohydrates taste sweet, and that most of the alkaloids are bitter. There are too many exceptions to these simple relations between chemical structure and sensation quality to have them serve as an answer to our question. There are chemical salts which taste sweet, there are acids which do not taste sour, and there are chemical substances whose tastes differ according to their concentration and even according to the part of the tongue which they affect. Sternberg, who has made a very extensive study of the relation between chemical structure and sensation quality, has recognized the impossibility of finding a simple relation between chemical groups and sensation quality. His method of study consisted in cataloguing separately all those substances which taste sweet, sour, bitter, and salt, and then looking for similarities of structure within the same sensation group and differences among different groups. This is just the opposite of the customary procedure which was to take the chemical groups as a starting point and examine into the tastes aroused by them. He finds no difference in the molecule of a substance producing a sweet taste and a molecule of another substance producing a bitter taste, and finds similarities among the molecules of substances producing different tastes. He is forced to the conclusion that the tastes are due to the character of the intramolecular vibrations; that the taste mechanism is capable of responding to the relations among the atoms which have escaped the other senses, even when their keenness is increased by all sorts of artificial devices. Nagel, in reviewing this work of Sternberg, says that, in spite of the interesting facts which have been accumulated by him and others, very little has been contributed to the solution of the question of the stimulus for the different taste qualities.