146 PHYSIOLOGY CHAP. 



e(|uimolecular solutions of a. great number of acids with tlie object 

 of determining whether the intensity of the respective tastes is in 

 any relation with their acidity in a chemical sense, that is with 

 the amount of soda they are capable of neutralising. By patient 

 research he arrived at the conclusion that the acidity of different 

 equimolecular solutions of acids is so much the stronger in pro- 

 portion as their molecular weight is lower, and that, generally 

 speaking, the intensity of the acid taste of a molecule of any acid 

 depends on the relation between the weight of the acid hydrogen 

 (i.e. that which can be replaced by a metal) contained in tin- 

 molecule and the weight of that molecule. 



Another important question was attacked by Hb'ber and 

 Kiesow (1898) : do substances soluble in water, or of which the 

 molecules are capable of ionisation (electrolytes), owe their taste 

 to the free ions or to the molecules that are not dissociated, 

 i.e. are electrically inactive? Can free ions of different kinds 

 give rise to different sensations of taste when they act on the 

 gustatory organs? All these possibilities seem probable in view 

 of the fact discovered by Hober and Kiesow, that a single salt 

 solution may give rise to a whole scrie> of gustatory sensations 

 which differ not only in strength but also in quality. 



We know by the laws that govern the dissolution or dissocia- 

 tion of salts that in solutions of minimal to medium concentra- 

 tion the cleavage of the molecules into kations and anions, 

 respectively, is complete, and that it is only in more concentrated 

 solutions that non- dissociated elect rie.-illy inactive molecules 

 are present. Hober and Kiesow studied the taste of certain 

 saline solutions in relation to the different degrees of their 

 ionisation, and recognised that the taste sensation aroused as a 

 whole by the solution of an electrolyte (or ionisable salt) is the 

 resultant of a certain number of different elementary taste 

 sensations, which are severally excited by the ions. 



Highly dilute solutions of alkalis which are completely or 

 almost completely dissociated have a sweet taste ; in stronger 

 concentration they have a characteristic soapy taste, which is 

 probably due to the unsplit molecules. 



The salt taste of a series of salts (NaCl, KC1, MgCl , 

 (CH 3 )NH ? C1, (C 2 H,)NH 3 C1, NaBr,, Nal, K 2 S0 4 , Na 2 S0 4 ) is due 

 to the anions set i'ree by the dissociation of the molecules. In 

 other salts, on the contrary, the effect of the kations predomi- 

 nates, and there is no salt taste. This is the case with the salts 

 of magnesium, which are bitter, and the salts of beryllium, which 

 are sweet. 



The work of Hober and Kiesow thus shows that three of 

 the primitive qualities of taste, salt, sweet, and bitter, can be 

 produced by the free ions. As Richard also showed that acidity 

 is excited by hydrogen ions, we may conclude that all four 



