PRACTICAL EXERCISES 999 



23. Beats. Cause two tuning-forks of nearly equal pitch to 

 vibrate at the same time. Make out the beats, and count their 

 number per second. 



24. Sympathetic Vibration. Take three tuning-forks mounted on 

 resonators. Let two of them be of the same pitch. Strike one of 

 these ; the other is thrown into sympathetic vibration, and continues 

 to give out a note after the first is quickly stopped by touching 

 it. The third fork is unaffected. 



25. Determine by means of Galton's whistle the pitch of the 

 highest audible tone. 



26. Cranial Conduction of Sound. When a tuning-fork is held 

 between the teeth, a part of the sound passes out of the ear from the 

 vibrating membrana tympani ; if one ear is closed, the sound is 

 heard better in this than in the open ear. If the tuning-fork is held 

 between the teeth, till, with both ears open, it becomes inaudible, it 

 will be heard for a short time if one or both ears be stopped ; and 

 when in this position the sound again becomes inappreciable, it can 

 still be caught if the handle be introduced into the auditory meatus. 



27. Taste. (i) Apply to the tongue by means of a camel's-hair 

 brush a solution of quinine (i to 1,000), sodium chloride (i to 200), 

 cane-sugar (i to 50), and sulphuric acid (i to 1,000). Determine at 

 what part of the tongue the strongest sensations are elicited by each. 

 (2) Prepare a series of solutions of sulphuric acid of gradually in- 

 creasing strength, beginning with a solution (a two-thousandth 



2,000 



gramme-molecular solution) (p. 398). Put into the mouth, after 

 previous rinsing with distilled water, 4 or 5 c.c. of one of the solutions 

 of the acid, beginning with the weakest, and determine at what con- 

 centration of the H ions the acid taste first appears, rinsing out the 

 mouth after each observation . Repeat the experiment with solutions 

 of hydrochloric acid, and determine whether the threshold value is 

 the same. 



A similar comparison of the necessary concentration of the OH 

 ions can be made with solutions of sodium hydroxide and potassium 

 hydroxide. 



(3) Connect two short pieces of platinum wire with the copper wire 

 from the poles of a Daniell or dry cell. Apply one platinum wire to 

 the inner surface of the lip and the other to the tip of the tongue. 

 Reverse the poles. Note the difference in the sensation according to 

 whether the anode or the kathode is on the tongue. 



28. Smell. (i) Pass a current through the olfactory mucous mem- 

 brane by connecting one electrode with the forehead and the other 

 by means of a small piece of sponge or cotton-wool soaked in physio- 

 logical salt solution with one nostril. An odour like that of phos- 

 phorus will be perceived. 



(2) To distinguish between Taste and Smell. Use a solution of 

 clove-oil in water which can just be distinguished from water when 

 it is placed on the tongue by means of a camel's-hair brush. Close 

 the nostrils, and determine whether the clove-oil can now be detected. 



29. Touch and Pressure. (i) Prepare a number of hair aasthesio- 

 meters by fastening hairs of different thicknesses to small wooden 

 handles about 3 inches long by means of sealing-wax. Hairs as 

 straight as possible should be chosei , or straight portions of hairs. 

 The hair is to be fastened on one end of the piece of wood at right 

 angles to the long axis of the handle, so that about an inch of the 

 hair projects to one sid,e. Determine the pressure value of each 



