Observations on various Liquids. 87 



end into a conical funnel ; each of the inner balls also has been 

 backed by a zone of lead. 



At this stage of the construction, when the small spheres 

 were seen to be symmetrically and securely placed in the 

 tunnel, they were taken out, and very carefully flattened in the 

 turning-lathe, so as to present approximately plane but still 

 well rounded faces to each other ; and they were then electro- 

 plated with a shell of silver as thick as writing-paper. As the 

 balls lie finally in the cell, their least distance from each other 

 is exactly J inch. 



The cell is closed by two panes of the finest plate glass, 

 about -^q inch thick, and 2 inches square, which are simply 

 kept upon the plate faces of the block by pressure. The press 

 is made of two small planks of mahogany, shaped as shown 

 by the dotted lines in the diagram, and connected at the corners 

 by four square-headed screw-nails, each provided with a bat's- 

 tail nut ; but in working the cell, I find two diagonally oppo- 

 site screws quite sufficient in most cases. Between the planks 

 and glass are two thin sheets of india-rubber cloth, these, as 

 well as the planks, being channelled neatly in continuation of 

 the tunnel. 



The whole piece is supported by two fine pillars of glass, 

 which are firmly fastened to the block by coils of silk thread. 

 The pillars terminate below in a solid wooden stand. When 

 the cell is in position on the experimental table, the two faces 

 of the plate of liquid are vertical, and the axis of the electric 

 field, or the straight line joining the centres of the inner balls, 

 is horizontal. 



The supporting pillars, as well as the ends of the cell-block, 

 are covered with a thick shell of lac varnish. 



I think that I have now mentioned every thing essential to 

 the cell, except a small stopper of glass, which, at the beginning 

 of each experiment, is dropped into the mouth of the vertical 

 boring. It prevents the entrance of stray particles of dust, 

 and is of use also in restraining evaporation when the cell is 

 charged with a very volatile liquid. This plate cell is far 

 superior to the old one described in the second of my former 

 papers ; it gives finer optical effects, and is ever so much more 

 easily handled. 



2. Working of the Cell. — To charge the cell with a liquid, I 

 always use a small filtering funnel drawn out into a fine end, 

 which passes along the upper boring, quite through the roof 

 of the tunnel, leaving a narrow surrounding space for escape 

 of air. When the charge of liquid is found to be not suffi- 

 ciently clean, the cell is emptied at once by turning the lower 

 cock, and is then charged again from the bottle as before ; and 



H2 



