218 Messrs. W. B. Hardy and H. W. Harvey. [June 21, 



since water is usually positive to glass and to particles suspended in it, the 

 apparent velocity is commonly the sum of two velocities of opposite sign. 

 Both movements, that of the water and that of the particle, are remarkably 

 dead heat in thin films. If the bodily flux of fluid throughout the whole 

 thickness under one field of the microscope were zero, and the stream lines 

 were constant, the average velocity of particles taken through this entire 

 thickness would, of course, give the mean velocity relative to the fluid. 



When a suspension of yeast cells and red corpuscles in isotonic sugar is 

 observed in a U-tube wide enough to reduce the flux of fluid practically to 

 zero, both migrate to the anode, but the corpuscles travel much the faster. 

 In Thornton's experiment, therefore, the yeast cells move past the observer 

 towards the cathode because they are unable to stem trre current of water 

 which is travelling in that direction. Yeast and blood cells under the 

 conditions of the experiment are not oppositely electrified. Both are negative 

 to the fluid, but the yeast cells migrate more slowly. 



When the depth of the fluid is increased to 2*5 mm., the yeast and blood 

 cells are seen to move in the same direction in the middle regions and 

 in opposite directions in the film of fluid next the glass floor of the cell, and 

 also in that next the surface. 



Troughs of various shapes were used by us to observe these movements. 

 Good results were got with one 2 cm. long, 3 mm. wide, and 2 - 5 mm. deep, 

 with parallel glass sides, which opened at each end into a wide portion, 

 divided into two compartments by a porous plate. The outermost com- 

 partment at each end was filled with a saturated solution of zinc sulphate, 

 into which dipped electrodes of amalgamated zinc. The fluid was not 

 covered in any way. 



The current was between O'OOOl and O002 ampere, and was not allowed 

 to run in the same direction for more than a few seconds at a time. With a 

 current of more than 0*01 ampere, the vapour density rose to a point at 

 which it deposited on the front of the objective — a remarkable result, which 

 can be attributed only partly to heating. The image was, as a rule,, 

 completely " fogged " by dew when the current had run 4 seconds, the 

 front glass of the objective being about 4 mm. above the surface of the fluid. 



The difference in the apparent movement of red corpuscles and yeast cells 

 in the layer next the glass, and in the middle of a stratum of liquid 2 - 5 or 

 more millimetres deep, is not due to a difference in the interface between 

 the cells and the fluid, for the velocity of one kind of cell with respect to the- 

 other was the same in both regions. 



Countings of the number of divisions of a micrometer scale which 

 contiguous yeast and blood separated from each other during a run of the 



