in THE SENSE OF TASTE 143 



picric acid, as also to sweet solutions, as sugar, glucose, and 

 lactose. 



IV. The relations between the nature of adequate thermal, 

 tactile, auditory, and visual stimuli and the quality of the 

 sensations aroused in consciousness are well known, but the same 

 cannot be said of the chemical senses, taste and smell, since we do 

 not yet know in what chemical property the capacity of certain 

 substances to act as adequate stimuli for taste or smell consists. 

 We only know a few of the conditions that determine the taste 

 and odour of a substance. It is generally admitted that sapid 

 substances are adequate taste stimuli only when in a state of 

 solution. Formerly, however, it was believed that gases were also 

 capable of directly stimulating the organs of taste (Joh. Miiller, 

 Stich). Carbonic acid has a distinctly sour taste; chloroform, 

 sulphuretted hydrogen, and nitrous oxide are sweetish ; aldehyde 

 and ether vapour are slightly bitter ; acetic acid vapour is strongly 

 acid, and on excluding smell cannot be distinguished from some of 

 the mineral acids. These tastes are distinctly perceptible, even 

 when the substances are brought in the form of gas into direct 

 contact with the tongue after drying it carefully, and closing 

 the nostrils. 



The direct action of gases was disproved (Valentin, von 

 Vintschgau) by the fact that gases, before coming into contact 

 with the specific taste-endings, are necessarily dissolved in the 

 fluids which fill the pores of the taste-buds. It is consequently 

 indisputable that substances, whether solid, liquid, or gaseous, 

 must, in order to affect the peripheral organs of taste, be in the 

 form of solutions, or be dissolved in the oral secretions. Solubility 

 in the oral secretions, at least to a minimal extent, is therefore an 

 indispensable condition of gustatory stimuli. 



Compounds of the different metals, at the surface of which 

 there is a difference in electrical potential, and which on contact 

 with the tongue may give rise to electrical excitation of the taste- 

 organs, form an apparent exception to this law. 



On the other hand, solubility alone is not sufficient to enable 

 a substance to excite sensations of taste. Some gases, e.g. oxygen, 

 hydrogen, nitrogen, never arouse sensations of taste, though 

 soluble in the oral secretions and absorbable. The same holds for 

 distilled water, after carefully cleansing the mouth from the food 

 residues and sapid substances mingled with the saliva. It is more 

 surprising that chloride or sublimate of mercury, to which 

 individual tissues are certainly not indifferent, has no taste in 

 certain concentrations (Sternberg). 



According to Graham sapid substances belong to the category 

 of crystalloids, while colloids are tasteless. Although it is not easy 

 to control the absolute value of this statement, owing to the 

 difficulty of obtaining colloids in the pure state, and to the fact 



