136 ON THE SENSES. 



of the auditory nerves. Both these latter senses we propose to examine as exactly 

 as possible hereafter, when the advantages which we possess for their analytical 

 study in comparison with the sense of taste, and which are here only intimated, 

 will be more thoroughly considered, and, we hope, more clearly evinced. 



However unknown to us the properties which give to substances the power of 

 producing sensations of taste and the way in which they operate on the nerves 

 of that sense, it is, at least, some compensation that we know to a certain ex- 

 tent the conditions under which a substance empirically recognized as an object 

 of taste exerts its specific power, and the circumstances on which the intensity 

 of the impression depends. First stands the fact that a substance can produce 

 no such impression unless it reaches the seat of this sense in a fluid form ; if of 

 a solid nature it must at least be first dissolved in the saliva. This condition 

 restilts from the necessity, which was above provisionally assumed, that the 

 sapid substance shall penetrate through the coat of epithelium to the mucous 

 membrane. A solid body, even reduced to the finest powder, could not make 

 Its way through the closely compacted cells of which the covering of the tongue 

 consists; but in solution it will be imbibed first by one and then by another 

 stratum of these cells, until it finally reaches the underlying and sensitive sur- 

 face. Now these successive imbibitions may be supposed to imply the lapse of 

 a sensible interval of time, and yet experience teaches us that 'no interval can 

 be appreciated between the moment of contact with the tongue and that of the 

 ensuing sensation. Still, and in spite of this apparent contradiction, we must 

 insist that no alternative remains for us but to accept this theory of a gradual 

 transmission, seeing that it is supported by the admitted necessity of solution 

 for producing sensations of taste, and no other supposition is possible. More- 

 over, we have no knowledge of the rapidity with which such transmission is 

 accomplished, so that there is at least no proof that it may not be sufficiently 

 great to render the interval in question quite inappreciable. 



From what has been said it results that a body which is insoluble in water, 

 and cannot therefore fulfil the above condition, is incapable of yielding a sensa- 

 tion of taste. We must take care, however, not to reason conversely that solu- 

 bility in water is the actual quality which in itself fits a substance for the ex- 

 citation of the gustatory nerves. That this is not the case, obviously follows 

 from the fact that many substances readily soluble in water are tasteless— nay, 

 that water itself is so. Were solubility as directly the condition of taste as the 

 undulations of the luminous ether aru the condition of sight, a substance to be 

 more sapid — that is, capable in smaller quantity of" conveying a more intense sen- 

 sation — should be the more soluble ; but this common experience refutes. Ex- 

 tremely soluble bodies sometimes convey a very faint taste; others, soluble' only 

 with difficulty, an intense one. The degree of concentration of the solution re- 

 quisite m general to produce a sensation of taste is very different with different sub- 

 stances ; while some act in a very diluted stale, others require a concentration 

 nearly equivalent to saturation. It has been elsewhere stated that as regards 

 odorous substances, a scale has been established by experiment showing the de- 

 gree of impregnation of inhaled air, requisite in the case of different articles 

 for the production of a sensation of smell ; and in like manner it has been at- 

 tempted experimentally to assign values to the different degrees of dilution 

 which different objects of taste will bear, without thereby losing their property 

 of gustatory excitation: The numbers thus found afford of course no absolutely 

 exact valuation, but as a general appreciation they furnish in this respect a suf- 

 ficiently accurate comparison of the different substances submitted to trial. We 

 will not withhold from our readers some of the results. It would perhaps be 

 scarcely supposed that sugar rates unfiivorably in this scale of values— indeed, 

 tne most untavorably of the compared substances as regards their relative ca- 

 pacity for dilution without loss of taste. Solutions of sugar which contain less 

 than one part of sugar to 99 parts of water, no longer excite any perceptible 



