666 



NA TURE 



[OCTOliER 29, 1908 



DROPS AND Sl'LASIIES.'^ 



THE few who have access to the Traiisaclions of 

 the Roval Society, and who remember the first 

 presentation of Prof. Worthington's beautiful photo- 

 graphs illustrating the successive movements that 

 occur in the phenomenon of the splash of a drop, 

 and some proportion of the many who may have seen 

 his two articles on the subject in Pearson's Magazine, 

 will welcome the appearance of the fascinating' quarto 

 volume entitled " A Study of Splashes." Not only 

 will their recollection of an interesting research be 

 revived, but the more perfectly executed and more 

 numerous and complete series of photographs here 

 presented will show the phenomena in all their 

 original beautv as displayed on the lantern screens 

 at the Royal Institution and elsewhere. 



Besides showing the results and explaining the 

 interesting cooperation of the forces of dynamics and 

 of surface tension which have given rise to the 

 phenomena, Prof. Worthington has given very full 

 details of his method so that many who can ex- 

 temporise physical apparatus will be able to follow 

 him, and so to investigate the same or analogous 

 movements. 



.\s a scries of twelve or twenty successive photo- 

 graphs illustrating the movements of the liquid which 

 occur in a small fraction of a second cannot at 

 present be taken from a single falling drop or splash, 

 but each requires a new drop to be photographed at 

 a different stage, predetermined in time within one 

 or two thousands of a second, it is essential that so 

 far as is possible the phenomena should be exactly 

 repeated, and that the initial conditions should be 

 identical. The method by which liquid is allowed to 

 drop or balls to fall with sufficient exactness is illus- 

 trated and described, and to this no further reference 

 need be made in the present notice. The more in- 

 teresting part of the apparatus is that in which, the 

 ball or drop having been liberated, an illuminating 

 electric spark is formed as many thousandths of a 

 second after the first contact of the splash as may 

 have been determined, as also is the means of utilising 

 the lisrht of the spark for the purpose of obtainin;; a 

 shaded picture. This is illustrated on p. 7. The 

 action depends upon the equality of speed of two 

 fallinij bodies, one the drop or ball which will make 

 the splash, and the other a conducting ball which in 

 its fall will pass close to two nobs forming part of an 

 electric circuit. This circuit comorises the two Leyden 

 jars of an electric influence machine, the nobs in ques- 

 tion and the illuminatinfj spark frap, the two nobs 

 bein.gf connected with the charged insides, and the 

 terminals of the illuminatini? soark-g-ap with the 

 uncharged outsides of the jars. The conducting;- ball 

 as it passes close to the nobs discharjjes the circuit, 

 and a spark is formed at the illuminating spark-gap. 

 .According as an electromagnetic trigger which 

 liberates the conducting ball is set higher or lower, 

 the time which will elapse after the breaking of the 

 matrnetic circuit up to the time of the formation of the 

 spark mav be made greater or less at will. The drop 

 or ball which makes the splash is liberated in a 

 similar manner, and so it is merely necessary to in- 

 crease the height of fall of the conducting ball little 

 bv little as compared with that of the drop or splashing 

 ball to illuminate the splash at any desired period 

 of its existence; perhaps not nierelv necessary, as it is 

 also important that the jars should be charged every 

 time to the same potential, otherwise the spark would 



1 "A .Study of Spla-hes." By Prof. A. M. Worthinglon, C.B., F R.S. 

 Pri. xii+iag; illustrated. (London: Longmans, Green and Co., 1908.) 

 Pr'.ce 6s. td. net. 



NO. :^",.T, VOL. 78] 



not be liberated when the conducting ball had in suc- 

 cessive falls reached the same place. The spark is in 

 the focus of an illuminating reflector which gives a 

 large field of light as well as a central bright point, 

 and an ordinary camera with a lens to throw an image 

 of the splash upon the plate completes the means by 

 w^hich the phenomenon which exists at the moment 

 of the spark is jiliotographed with all the perfection 

 of light and shade which make the results so beau- 

 tiful. 



Even with all these precautions successive drops 

 are not necessarily exactly alike at their initiation, as 

 very small changes in the electrical conditions of the 

 two circuits and in the form of a liquid drop, if that 

 is used, at its start will make initial differences which 

 the instabilitv of the liquid forms subsequently de- 

 veloped rapidlv accentuate. .Still, even with these dif- 

 ferences in detail the essential characteristics of a 

 series of drops at different stages are, with the pre- 

 cautions mentioned, so nearly identical that the ' 

 curiously different characters of splashes made in 

 different circumstances can be well followed through- 

 out their existence. 



When the under-water phenomena are photographed 

 the field reflector mentioned above is replaced by a 

 lantern condenser, close to which a fine ground sheet 

 of glass is placed so as to give a uniform field of 

 illumination. 



It mav be interesting to mention that with the 

 apparatus used bv Prof. \\'orfhington he was able 

 to obtain sparks the effective duration of which was 

 not more than three millionths (3/1,000,000) of a 

 second. When it is remembered with what e.xtreme 

 rapiditv the edge of a liquid film changes its form 

 under capillary forces, it will be seen that a spark cf 

 very short duration is essential, and judging by results 

 one sufficiently short has been obtained. 



The book is illustrated by eighteen series of photo- 

 graphs, each series representing' a succession of events 

 in one type of splash equivalent as nearly as possible 

 to a succession of events in a single splash. The first 

 series is represented by twenty-four successive photo- 

 graphs of a drop of water falling through a space of 

 40 centimetres into still milk and water, and there is an 

 alternative smaller series. The second shows the fall 

 of water into water, and incidentally the importance of 

 keeping the water into which the drop falls perfectly 

 free from contamination. Two corresponding photo- 

 graphs are shown side by side; in one the water had 

 been skimmed to purify it from the small trace of 

 grease and smoke which the previous falling drop had 

 brought down from the smoked spoon in which it 

 had been supported ; in the other the surface was 

 completely renewed by keeping up a gentle supnly 

 of pure water so that contamination could be com- 

 pletely removed by gentle overflow. The absence of a 

 multitude of fine ripples in the first contrasts with the 

 closely furrowed surface of the second, as does a 

 patch on the sea into which a sardine tin has been 

 emptied of its oil with the surrounding portions 

 on a day when the surface is black with small 

 ripples. 



Succeeding series show the formation of bubbles, 

 the contrast in the splashes formed by smooth and 

 rough spheres, and other phenomena which it is ditifi- 

 cult to describe adequately in a notice and without 

 the photographs to refer to. Some of these are illu- 

 minated from above onlv, and some are illuminated 

 bv light on a level with the liquid surface, a trans- 

 parent cell being used in this case so that the under- 

 water phenomena may be seen as well as those above 

 the surface. 



The last series illustrates the effect of the fall of a 



