292 DAVID H. BRIGGS 



This results in the formation of sharp boundaries between the protein solu- 

 tion and the overlying buffer at the top of the right hand leg and at the bottom 

 of the left hand leg of the U tube. Gravitational equilibrium is attained 

 between the solutions because the heavier protein solution occupies the bot- 

 tom portion of the tube. The boundaries are displaced into view by slowly 

 pumping buffer into the right hand half of the apparatus through the stop- 

 cock of part y. After bringing the boundaries by this method into a conven- 

 ient position in each leg, the riglit hand side of the apparatus is again closed, 

 the current source is connected to the electrodes (making the left hand elec- 

 trode that of opposite sign of charge to the sign of charge carried by the pro- 

 tein), and electrophoresis is allowed to proceed. 



Thus requirement h is neatly accomplished with this apparatus. 



In order to reduce convection currents due to wattage dissipation 

 to a minimum, Tiselius recommended that the legs of the U tube be 

 made thin in one dimension and that the temperature at which the 

 electrophoresis is carried out be below the temperature of the maxi- 

 mum density of the solutions involved. For pure water this tempera- 

 ture is about 4°C. and for most salt solutions (buffers) it will be 

 above 0.5°C. up to concentrations of 0.2 M. A convenient tempera- 

 ture to keep the water bath is 0.5° C. With this arrangement and 

 for a U tube of approximately 2.5 mm. diameter in the thin dimen- 

 sion, a wattage dissipation of about 0.2 watt per centimeter of tube 

 length per square centimeter of tube cross section has been found to 

 cause no convection disturbance when the density difference at the 

 boundary is that due to a change in protein concentration of 0.5%. 

 Requirement c is thus adequately fulfilled by this apparatus for 

 aqueous systems of ionic strengths below approximately 0.2. 



The method introduced by Tiselius for observing and characteriz- 

 ing the boundaries is the most novel feature of the apparatus. While 

 a boundary may be detected visually in the case of colored sols, such 

 as gold sol or AS2S3 sol, there are many colorless colloid systems. By 

 using U tubes made of quartz, under ultraviolet light, the boundaries 

 in such systems as those containing proteins may be rendered visible 

 by fluorescence or detectable photographically through absorption 

 by the protein of the ultraviolet light. Often the boundary can be 

 made visible by differences in the Tyndall effect. None of these 

 methods has proved as versatile, however, as the method based on 

 the refractive index changes that occur at the boundary between an 

 overlying buffer solution and a protein solution, for example, equili- 

 brated through a dialyzing membrane against the buffer. When light 



