THE DIRECT ATTACHMENT OF DYES TO TISSUES 189 



experiments of this sort have been carried out especially by 

 Seki,*^2 a Japanese investigator who has played a particularly 

 important role in the scientific study of microtechnical dyeing. 

 An apparatus resembling Seki's is shown in fig. 23; practical 

 instructions for setting it up are given in the Appendix (p. 321). 



The main part of the apparatus is the large U-tube in the middle 

 of the photograph. This contains an aqueous agar gel. It fills the 

 tube up to the level marked 'O' on the cardboard scale fixed beside 

 the tube: the level is also marked by a line on a label stuck on to 

 each limb of the tube. The gel is bufi"ered at a known pH. The dye- 

 solution, buffered at the same pH, is poured into both limbs of the 

 U-tube to the same height. In principle, one might now put the 

 positive wire from an accumulator into the dye solution in one 

 limb and the negative wire into the dye solution in the other, and 

 the experiment would begin. If this were done, however, the 

 electrolysis of water would take place, gas would be formed at the 

 electrodes, and the latter would become depolarized. The rest of 

 the apparatus exists solely to prevent this. It consists of two beakers 

 into which dip electric wires (marked -f and — ), and two small 

 U-tubes, each of which dips on one side into a beaker and on the 

 other into the dye in the big U-tube. The current passes through 

 agar gel in the small U-tubes. The contents of the beakers and 

 U-tubes are given in the Appendix. 



The electric current should be switched on as soon as the dye 

 has been poured into both sides of the big U-tube. The current 

 now tends to make the dye move towards either the negative or the 

 positive pole: that is to say, to make it descend in one or the other 

 of the two limbs of the U-tube. Simple diff"usion also occurs, how- 

 ever, and this causes some descent on both sides. When basic dyes 

 are used, there is also an attraction between the dye and the agar, 

 which causes some descent on both sides. The much greater 

 descent on one side than on the other shows that the dye moves in 

 response to the current. In the figure the dye in question (methyl- 

 ene green) has moved in 42 hours about 6 cm towards the negative 

 pole and only about 2 cm towards the positive. The dye ions are 

 clearly positively charged or cationic, and indeed methylene green 

 is a cationic or basic dye. 



This apparatus enables us to find whether any dye is basic or 

 acid at any particular pH. In general, any dye that is shown by its 

 chemical formula to be basic will behave like methylene green, 

 while any acid dye will move in the opposite direction. The speed 



