June 28, 1877] 



NATURE 



16: 



ON DROPS 



A MONG the many ways in which electricity is called 

 -^*- in to give assistance in various physical investiga- 

 tions, one of the most elegant and interesting is the ap- 

 plication of the electric spark to render momentarily 

 visible a body that is rapidly moving or changing its 

 form. The duration of the electric spark is so short — 

 probably not more than .T^Jno of a second — that a body, 

 such as a rotating wheel or oscillating rod, moving in a 

 dark room wiih extreme rapidity, will, if illumined by an 

 electric spark, seem stationary, since the wheel or rod 

 has not tmie to change its position appreciably during 

 the short instant for which it is visible. If the spark be 

 bright, the impression is left on the eye long enough for 

 the attention to be directed to it, and for a clear idea to 

 be formed of what has been seen. 



The writer of this article has recently applied this 

 ineihod to watchii g the changes of form in drops of 

 vaiious liquids fallmg vertically on a horizontal plate. 

 As u-ually seen, a drop of water falling from a height of 

 ten or twelve inches on a smooth solid substance, such as 

 glass or wood, seems to make an indiscriminate splash. 

 The whole splash takes place so quickly that the eye can- 

 not follow the changes of form ; the impression made by 

 the last part of the splash succeeding that of the first 

 part so quickly as to confuse it. 



A little careful observation, however, shows that the 

 drop passes through very definite symmetrical forms, and 

 that a splash is by no means an irregular hap-hazard 

 phenomenon. 



Let the reader let fall a few drops of milk, about \ inch 

 in diameter, on a smooth dark surface of wood or paper, 

 from a height of, say, six inches (milk is better than water, 

 as it is easier to see, especially on a dark ground) ; he 

 will observe that the liquid makes a blot with a more or 

 less regular undulated edge, but the splash is too quick 

 to follow with the eye. 



Let him now substitute a drop of mercury for the milk. 

 By watching the splash very intently he will be able to 

 catch a glimpse of the mercury spread out in the symmetri- 

 cal starlike form of Fig. Ila of Set 2. After the drop has 

 been thus spread out it recovers its globular form, since 

 the mercury does not wet the plate. On increasing the 

 height of fall a few inches, it will be noticed that small 

 drops split off in a more or less complete circle, and are 

 left lying on the plate, while the rest of the drop gathers 

 itself together in the middle of the circle. 



The chief reason why these appearances could not be 

 seen with milk is that the milk wets the glass or wood 

 and sticks to it, while the mercury does not. But by 

 smoking a slip of glass or card tolerably thickly in the 

 flame ot a candle we get a finely-divided suiface of lamp- 

 black to which the milk does not adhere any more than 

 the mercury, and by very careful watching we may notice 

 that the same radial star is formed by the milk, but it is 

 much more difficult to catch sight of than the mercury 

 star. But if the mark on the lamp-black be examined 

 after the drop of milk or mercury has rolled away it will 

 be found to consist of delicate concentric rings with num- 

 berless fine radial striae where the smoke has been swept 

 away. These may be seen very well by holding the glass \ 

 plate up to the light if it has not been too thickly smoked, j 



The marks thus made are very beautiful and symme- 

 trical, and it will be found, if the glass be uniformly ! 

 smoked, that the same-sized drops of the same liquid I 

 falling from the same height will produce almost exactly 

 similar marks : while if the height be changed the mark 

 on the lamp-black will be somewhat changed ; and it 

 is a fair inference, if each drop makes almost exactly 

 the same complicated, symmetrical mark, that the splash 

 of each drop takes place in almost exactly the same 

 way. 

 The glimpse that may be caught of the drop in the way . 



described is obtained when the drop is really almost 

 stationary, having flattened itself out on the plate and 

 being on the point of contracting again to its original 

 form. 

 ! That a drop if so flattened out will recover itself is seen 

 on pressing down a drop of mercury with the finger or a 

 drop of water with a piece of black-lead or other substance 

 tc which it does not adhere. On removing the pressure 

 the drop springs back to its old form ; the force which 

 causes this being exerted by the curved surface of the 

 liquid at the edge of the flattened drop, on the liquid 

 within. The flatter the drop becomes the greater is the 

 curvature of the edge and the greater the corresponding 

 pressure tending to restore it to its original gljbular form. 

 , The extent to which a drop that has fallen on a plate will 

 spread out depends on the velocity with which it strikes 

 the plate, i.e., on the height of fall ; so that as long as the 

 drop returns to the globular form the whole phenomenon 

 of the splash may be regarded as an OjCillation similar to 

 that of a pendulum ; the velocity of the liquid outwards 

 being checked, overcome, and finally reversed by the 

 ever-increasing pressure of the curved edge, just as a 

 pendulum has its velocity checked, overcome, and finally 

 reversed by the action of gravity. 



It is only when the height of fall is very great that the 

 liquid flies off in all directions and the splash ceases to be 

 an oscillation ; this case corresponds to that of a simple 

 pendulum started with a blow so violent as to break the 

 string. 



But the liquid star and the complicated pattern on the 

 smoked glass show that the splash is not a simple spread- 

 ing out of the drop equally in all directions to return 

 again. 



In order to observe the form of the drop at any given 

 instant during the splash, it is necessary to make use of 

 the electric spark and to take advantage of the fact that 

 drops of the same size falling from the same height will 

 all behave in the same way. 



It will be necessary to let a drop, say of mercury, fall 

 on a plate in comparative darkness, and to produce a 

 strong spark at the instant the bottom of the drop comes 

 in contact with the plate, and so illumine it ; the observer 

 will then see the drop in the form it has at that instant. 



A second drop must be let fall in the same way, and be 

 illumined by the spark not at the first mom.ent of contact, 

 but a shade later, say ,u second later, when the drop 

 will have spread itself out slightly on the plate, and simi- 

 larly we must illuminate a third drop a shade later than 

 the second, and so on. The observer can, after a little 

 practice draw from memory on each occasion the drop in 

 the form in which he has seen it. It will be seen that the 

 process consists in isolating consecutive phases of the 

 splash from those that precede and follow, and which 

 take place in darkness and so do not confuse what has 

 been seen as they would do in continuous daylight. 



The device adopted by the writer for so timing the 

 appearance of the spark as to illumine the drop at any 

 desired phase of the splash consisted essentially in break- 

 ing the current of an electro-magnet at the instant the 

 drop began to fall ; the magnet thus ceasing to act, re- 

 leases a spring which immediately begins to pull the 

 terminal wire of a strong electric current out of the other 

 terminal, which is a cup of mercury, and the strength ot 

 the spring and the depth of immersion of the wire in the 

 mercury are so adjusted that the wire leaves the surface 

 of the mercury, and the required spark is produced at the 

 instant the drop reaches the plate. 



For the next drop the spark is made to appear a shade 

 later, either by slackening the spring or increasing the 

 depth to which the terminal wire is immersed in the 

 mercury. 



The following figures have been drawn in the way 

 described, and show the behaviour of a drop of mercury 

 about -} in. in diameter, falling from a height of about 



