1876.] 



Forms assumed by Drops of Liquids. 



263 



axis D D, made o£ a fine sewing-needle working in small triangular holes 

 cut in the copper plates E, E ; underneath the wand and along it is bound 

 to it a platinum wire, one end of which is bent vertically under the axis, 

 and dips into mercury contained in a hollow in the deal board in which 

 E, E are fixed. The other end of the platinum wire rests, when the wand 

 is horizontal, on a strip of platinum foil E, wound round a copper wire 

 Gr Gr. A Bunsen's element (H) sends a current through the coils K, K of 

 a relay, along the wire L L to the mercury in the httle trough, along the 

 platinum wire, platinum foil, and mre Gr Gr back to H. 



The current which passes through the relay comes from 1 or 2 Bun- 

 sen's elements, M, through the inducing spiral of a E-uhmkorff's coil, N, 

 into the relay at 0, down the platinum wire P, which, when the iron bar 

 gg is held down by the magnet, dips into mercury contained in the 

 U tube S E', so that the current passes out of the mercury at E' up a 

 thick immersed copper wire and back to the pile M. 



The plate of thin glass having been placed on B, the balance of the 

 wand is so adjusted, with a small counterpoise at the other end A, that 

 the sHghtest downward pressure, even the breath of the observer, is suf- 

 ficient to raise the end A, and to break the connexion between the 

 platinum wire and foil. Accordingly the moment the drop touches the 

 plate, the current of the pile H is broken at E, the core of K K ceases 

 to be a magnet, and the point of the platinum wire P is pulled out of the 

 mercury at E by the force of the spring S, and the strong primary spark 

 obtained at the surface of the mercury is sufficient to illuminate the drop 

 on the plate. The stage at which it was required to see the drop could 

 be altered at pleasure by altering the depth of immersion of .the platinum 

 wire at E, which was done by plunging the connecting wire at E' more 

 or less deeply into the mercury in that branch of the U tube. This gave 

 a rough adjustment ; a finer was obtained by regulating by means of the 

 screw T the tension of the spring S, and so changing the rate of with- 

 drawal of the platinum point. 



To secure the complete illumination of the drop and plate, the end E 

 of the U tube and the plate were surrounded with a white cardboard 

 box, with slits to allow of the motion of the wand, the wire P, and the 

 admission of the drop, and open in front, so that the plate could be seen. 

 Complete darkness was found by no means necessary for the experiments ; 

 light just sufficient to allow the plate to be seen and the eye easily di- 

 rected on it was found the most convenient. The results I have obtained 

 have been with mercury and milk. Mercury, from its high reflecting 

 power, is easy to see ; water, from its transparency, even when coloured 

 with indigo, very difficult to see ; and I substituted milk, which has 

 the advantages of appearing white on a black ground, of being semi- 

 transparent, and of showing blue or darkish where it is spread thinly 

 over the black plate, and thus allowirg an estimate to be made of the 

 relative thickness of the drop in various places, and especially the advan- 



