One of the most interesting attempts to solve the question of the conditions of sapidity is that which makes the only condition necessary, the contact of the substance with the nerve endings within the taste buds. Now, Graham pointed out that all tastable substances belong to the class of crystalloids, while tasteless substances belong to the colloids. It is known, too, that living membranes are permeable by certain solutions and not by others. Colloid membranes, of which all of the mucous membranes of the body are examples, are impervious to colloids in solution, while the passage of crystalloids in solution is rapid. Hence, no colloids, even those in solution, could actually reach the free nerve endings of the taste buds. As Nagel says, however, the truth of this assumption is not easy to settle, since it is very difficult to get pure colloids and to make sure that their chemical constitution is not modified by the saliva before contact with the taste nerves. Other possibilities will be discussed in the chapter dealing with the theories of the taste function. It will be sufficient here to say that the adequate stimulus to taste is a liquid, or a solid, or gas, which may be dissolved in the saliva. Gases such as chloroform vapor, carbonic acid gas, and the like, were at one time thought to act directly upon the taste buds, but a safer view is that the gases are first dissolved in the saliva before acting on the taste mechanism. A few experiments have been reported in which the gases were said to have produced taste sensations when the mucous membrane was dry, but it is practically impossible to produce this condition, since the small glands of the tongue open directly into the fissures containing the taste buds and tend to keep the neighborhood of the taste pores in a moist state. To say that the adequate stimulus for taste is a fluid is to give only an incomplete description of adequate stimuli. In the case of vision the adequate stimuli are ether vibrations; and the different visual sensations, reds, yellows, blues, etc., are attributed to ether vibrations of different rate.