Sir William Ramsay prepared an explanation quite analogous to the theories of color vision and called it a dynamic theory. According to him, the real stimulus to the taste organs is molecular vibration, the different taste sensations being due to stimulation by different rates of molecular movement. Just as in the case of the luminiferous ether or of the air there is quite a range of molecular vibration rates, from exceedingly slow to exceedingly rapid. And, just as in the case of vision and hearing, so is the taste mechanism tuned to respond to only the middle range of these molecular vibration rates. Substances may then be insipid, either because their molecular movements are too slow or too fast to affect the receiving mechanism. In vision we have the analogous case of the infra-red and the ultra-violet rays not producing visual sensations, because they are beyond the range of sensitivity of the eye. Yet the effect of these rays can be recorded by other means. The rate of molecular movement depends on the weight of the molecule, so that very heavy or very light molecules would not produce taste sensations. About the same time Richet and Gley performed a series of experiments which seemed to show that the molecular weight of the substance was an important factor in producing taste sensations. They found that the intensity of the effect produced by different salts was in proportion to their molecular weight; that if account was taken of the different molecular weights of the salts used as stimuli the threshold stimulus would be the same for all of the salts. But if solutions were prepared according to the absolute weight of the salts these threshold stimuli appeared to be quite different for the different salts. Later experiments have shown that the same relation does not hold for sour-producing substances, certain sours of very small molecular weight having the sourest taste. Sternberg, whose work was mentioned in chapter VII, recognized only two elementary taste sensations, sweet and bitter, and he found no differences in the molecule of substances producing these two sensations. He concludes that it is the intramolecular vibrations that form the real taste stimuli. By transferring the seat of the activity from molecular vibration to intramolecular vibration the whole matter becomes more difficult of solution and verification. Still, the modification of the theory of Ramsay, made necessary by the work of Sternberg, would leave its essentials, namely, that vibrations of some sort form the stimulus and that the sense organ of taste is tuned to receive only a limited range of vibration rates.