ELECTRICAL PROPERTIES OF FLUORESCENT SOLUTIONS. 1 55 



A glass tube of 5 mm. internal diameter was drawn down at its middle 

 point to a capillary about a millimeter in outside diameter and 1.5 cm. in 

 length. Into the larger parts of the tube at each end were led platinum 

 wires in the form of spirals to serve as electrodes. The upper end of the 

 tube was attached to a tubulated bottle to serve as a reservoir for the liquid. 

 To the lower end was attached a second capillary tube to hold the liquid back 

 and keep the first capillary full. With this apparatus the liquid could be 

 illuminated while flowing through the capillary at any rate desired. Numer- 

 ous tests were made with the apparatus, and in all cases the results were 

 entirely negative. The form of the cell was such, however, that its resist- 

 ance was much larger than that of any of the cells previously tried, and 

 as the sensitiveness of the bridge was much less under these conditions, the 

 results were looked upon with some suspicion. 



Another form of circulation cell was therefore devised which was free 

 from the above objection. 



A piece of glass tubing was obtained whose internal cross-section was 

 almost that of a figure 8, except that where the two loops of the 8 would 

 cross one another there was an opening between them about 0.3 mm. in 

 width. In a short length of this tubing were placed two No. 16 platinum 

 wires, each having a length of 5 cm. The wires were parallel to one another 

 in the two loops of the 8, and from the end of each a small platinum wire 

 was led out through the walls of the tube. There was thus left (see Fig. 



' 11 



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3E 



Fig. 153- Fig. 154. Fig. 155- 



153, which shows an enlarged cross-section of the cell) a narrow space 

 between the electrodes through which the liquid could flow, the electrodes 

 themselves almost filling the loops of the 8. Below the electrodes the tube 

 was drawn down to a coarse capillary to make it possible to keep the space 

 between the electrodes filled with liquid. The upper end of the tube was 

 attached to a tubulated bottle as in the last experiment. By illuminating 

 the contracted part of the tube between the electrodes a moving strip of 

 liquid about 0.3 mm. wide and o. 1 mm. from front to back could be excited 

 to fluorescence, while the current flow was from side to side of the strip at 

 right angles to its length. 



Upon connecting the cell to the bridge its resistance reached a steady 

 value almost immediately after the current was turned on, and retained 

 the same value as long as the motion of the liquid continued. 



The movement of the liquid was quite rapid, the time required to empty 

 the 400 c.c. reservoir being about 10 minutes. As the whole space between 

 the electrodes held not more than o. 1 c.c. a complete change of liquid 

 between the electrodes could not have occupied more than a fifth of a 

 second. 



Numerous tests were made with this apparatus, but in no case was there 

 any evidence of a change of conductivity while the liquid was in motion. 

 When, however, the lower end of the tube was stopped the effects reappeared, 

 though they were not as marked as in the original type of cell. The sen- 



