LABORATORY WORK 175 



stone bridge is used (Fig. 11). When C and D are of such relative 

 lengths that no sound is heard, the resistances in the arms are such 



A C 



that A is to B as C is to D, or = = i.e., if C is in centimeters, and 



B JD 



1000 C /zoo C\ . , 



A =1000 ohms, -^ = -^ orB=iooo( ^ ) in ohms. 



B 100 -C \ C / 



If this apparatus is available, the resistances of molar and deci- 

 molar solutions of both hydrochloric and acetic acids should be 

 compared. If dilution had no effect on the acid, the resistance of 

 the deci-molar acid should be ten times that of the molar. In the 

 case of the strong acid, this will be found to be very nearly what is 

 found. In that of the weak acid, the resistance of the deci-molar 

 solution will not be so much as ten times that of the molar acid, 

 showing that dilution has caused the formation of a greater pro- 

 portion of conducting ions than were present in a portion of the 

 original solution containing the same quantity of acid. 



Take next defibrinated blood and obtain some serum by the 

 use of the centrifuge. Place the serum in the vessel used for the 

 acids and determine its resistance, best by the telephone method, 

 but it may be possible with care to see the effects when the first 

 simple method is used. Replace the serum by an equal volume of 

 the corpuscular deposit. The resistance will be much greater. 

 That the lower conductivity of the corpuscles is not due to the 

 absence of electrolytes from them, but to the fact that they are im- 

 permeable to these electrolytes and cannot therefore conduct an 

 electrical current, can be shown by adding a small quantity of 

 saponin. This, as can be tested, has itself a negligible power of 

 conduction. But the conductivity of the blood corpuscles is 

 greatly increased, because saponin destroys the cell membrane. 



Indicators. That it is the hydrogen-ion concentration of a 

 solution to which these substances react can be made obvious by 

 taking a solution of crystal-violet of about o.i per cent, and adding 

 a drop to each of a series of dilutions of hydrochloric acid, begin- 

 ning with twice molar and going down to one-thousandth molar. 

 The dye is yellow in the strongest, green in molar acid, greenish- 

 blue in o.i m., blue in o.oi m., violet in o.ooi m. 



That different indicators react to different hydrogen-ion con- 

 centrations can be seen by taking a known volume of deci-normal 

 sodium hydroxide (adding phenol-phthalein, a few drops of a o.i 

 per cent, solution in alcohol), and adding from a burette deci- 

 normal phosphoric acid until the colour disappears. Note the 

 amount of acid required. This indicator shows a concentration of 

 hydrogen-ions of o.oooooooi (io~ 8 ) normal as being acid. Add 

 next to the same solution a few drops of o.i per cent, methyl- 

 orange. The colour is yellow, that of the dye in alkaline solution. 



