296 



T. Brailsford Robertson: 



thereby rendered visible. In the second place let us suppose 



,, , ,, --, . ^ i. 1. ^ .1 .. concentration of lipoid layer 



tnat the coefficient of distnbution, ; — ~ --^ — 



concentration of watery layer 



is 100 : 1 for the yellow modification of neutral red and zero 



for the red modification. Then at any given concentration 6 



of hydroxyl ions, ii y he the concentration of the red 



modification and x that of the yellow modificatipn: 



x = kf(h)y 



where ä; is a constant. Now let this Solution be shaken up 

 with Ethyl Acetate and let the concentration of the yellow 

 modification in the watery layer now be x^, while that of the 

 red modification is y, and that of the yellow modification in 

 the lipoid layer is x^, then we have: — 



x^ = kf(h)y^ 



a;,==100xi 



x^=:100kf(b)y^ 



that is, the concentration of the yeUow modification in the 

 lipoid layer is 100 times its concentration in the watery layer 

 and, provided / (6) were a linear function, it is the same as 

 would be produced in the watery layer by 100 times the con- 

 centration of hydroxyl ions. In other words, what we may 

 define as the ,,8ensitiveness" of the indicator, namely, the con- 

 centration of the new modification of the indicator which is 

 produced by a given excess of hydrogen or hydroxyl ions is, 

 if we use another solvent immiscible with water and compare 

 the two layers, multipHed by the distribution-coefficient between 

 the two solvents of the new modification of the indicator. In 

 addition to this, as I have said, we have an apparent or 

 „physiological" increase in the sensitiveness of the indicator, 

 due to the physical Separation of the two colours. 



Two frogs were taken, A und B. A was left unstimulated 

 while B was stimulated for half an hour with the induced current 

 supplied by two Edison-Leland cells, the secondary coil being 

 pushed home over the primary. The region chosen for Stimulation 

 was the epidermis surrounding the anus ; the electrodes were about 

 4 to 5 mm apart. After the Stimulation of B had been com- 

 pleted both brains were extracted rapidly, with as little injury 



