99 



made a series of over four hundred experiments in detecting acid in the 

 presence and in tlie absence of sugar. 



The experiments Avere conducted as follows : Solutions of hydrochloric 

 acid of known strength were prepared, and equivalent solutions of sodium 

 hydroxide were added gradually, the solution being tasted after each ad- 

 dition until sourness disappeared. In other experiments the acid was 

 added to the alkali until soui'ness was noticeable. Both methods proved 

 about eiiually delicate. As long as the solution was strongly acid or alka- 

 line, only a drop or two was introduced into the mouth, but when the 

 neutral point was almost reached a cubic centimeter of liquid was used 

 and held in the mouth for a few seconds. The graduations of the burettes 

 were hidden during every titration, that the judgment might not be prej- 

 udiced. 



Experiments were made with solutions of acid varying from 0.715 

 normal to 0.0143 norjnal, or solutions containing 0.715 to 0.0143 milligrams 

 of hydrogen ions per cubic centimeter. Sugar was added in quantities 

 ranging from 0.04 to 0.8 grams per cubic centimeter. 



"With the experience gained in about twenty titrations considerable 

 accuracy of taste had been acquired, so that consistent results were then 

 obtained differing only about 1 part in 70 in a 15 cubic centimeter titration 

 with the stronger solutions and in the absence of sugar, from those ob- 

 tained with chemical indicators, the error being in almost all cases in the 

 same direction, as might be expected — sourness disappeared with the ad- 

 dition of less cdkali than the acidity as determined by phenolphthalein, or 

 sourness appeared only on adding slightly more acid than required by the 

 indicator. With the more dilute solutions, however, the absolute results 

 were more exact. This is accounted for by the presence at the end point 

 of less salt (due to the neutralized acid and alkali) in the more dilute solu- 

 tions, the presence of salt reducing the delicacy of the sense of taste for 

 sourness. With the most dilute solutions it was foimd possible to recog- 

 nize with certainty the jireseuce of 0.U07 milligrams of hydrogen ions in 

 the mouth, in 1 cubic centimeter of liquid, although 4 milligrams of salt 

 were also present. In the niost concentrated solution 0.01 milligrams of 

 hydrogen ions was recognizable in the presence of 34 milligrams of salt. 



The presence of sugar had the same effect as that of salt — the more 

 sugar present in the solution the larger was the quantity of acid necessary 

 for detection by taste; even the largest quantities of sugar used (0.8 grams 

 per cubic centimeter) increased the necessary quantity of acid less than 1.5 



