lULY 5, 1894] 



NA TURE 



or if not so, wilh a very short and practically constant 

 interval between them. 



The discharge of the inner coatings of the jars by the timing 

 sphere reaching the gap between them is accompanied by a 

 simultaneous discharge of the outer coatings across the spark- 

 gap in the dark room, and it is this that illuminates the splash, 

 the stage of the splash that is illuminated depending on the 

 height of fall of timing sphere, which can be adjusted at 

 pleasure. 



The duration of this discharge, if we may argue from Prof. 

 Boys' experiments, probably did not exceed one-hundredth- 

 thousandth of a second. 



Great difficulty was at first experienced in getting enough 

 illumination, and finally the spark gap was placed in the focus 



^^^^^t 



Fig. 2. — Splash of mercury on xylonite. 



of a small silvered watch-glass, which enclosed an angle of nearly 

 180", and this wa> placed to illuminate the splash from one side, 

 at an inclination of about 30' to the horizontal at a distance of 

 6 or 7 cm. ; it is to this that most of the detail obtained 

 was due. 



The camera was inclined at an angle of about 30' to the 

 horizontal, lookingdownward?, and wa^ fitted with asingle pebble 

 spectacle leni to avoid the loii of the ultra-violet rays which 

 occurs with glass; a^the lens wis far from achromatic, the proper 

 adjustment of the distances of object an I plate had to be fo«nd 

 by preliminary experiments. The most rapid plates obtainable 

 (not isjchro nitic) were u;ed, and w;re developel for thirty or 



Fig. 3. — Spla-ih of :i drop of water into milk, early st.tge. 



forty minutes with eikoiiogen, the developer being made as 

 strong as possible in eikonogen. To avoid all chance of fogging, 

 the operations were performed in the dark. 



The mercury splashes with which we began turned out to be 

 the most difficult to photograph, owing to the halation pro- 

 duced by the very bright reflection at some points, and the 

 comparative darkness of the remainder (Fig. 2) We htd to 

 try various surfaces for the drop to fall on to find out how to 

 obtain the best contrast ; we finally adopted a piece of polished 

 while xylonite. 



Of many liquids tried, the easiest to photograph was milk, 

 and with this there was plenty of detail (Fig. 3 and 4). 



These photographs are, as far as we know, the first really 

 detailed objective "views," as opposed to shadows, that have 

 been taken with such a very short illumination. 



NO. 1288, VOL. 50] 



Besides ihe^e we also took a number of shadow photographs, 

 in much the same way as that in which Prof. Boys photographed 

 a rifle bullet, by letting a drop of mercury fall on Ihe clean side 

 of the sensitive plaie itself, and producing a spark between two 



FiG 4.— Sf.Ia>h of a drop of wattr inio milk. 1.4le stage. 



magnesium knobs vertically above the splash. No difticulty 

 was found in this case in getting enough exposure. 



It will be observed that the method requires that different 

 stages should be photographed from different splashes. We 

 hope, however, to succeed in the more difficult task of photo- 

 graphing many stages of the same individual splash. 



R. S. Cole. 



On the Spreading of Oil upon Water. 



In a paper entitled "Die Lehre von der Wellenheruhigurg," 

 by M. M. Richier (see Nature, vol. xlix. p. 4SS), the opinion is 

 expressed that the tendency of oil to spread itself on water is only 

 due to the free oleic acid contained in it, and that if it were 

 possible to completely purify the oil from oleic acid, it would 

 not .spread at all. 



This I found to be actually the ca^e with olive oil, and 

 though I agree by no means with the theoretical views of the 

 author, I will mention the fact, for I may suppose it to be not 

 }et generally known. 



The Provence oil used in my experiment was shaken up 

 twice with pure alcohol, and the rest of the latter being 

 carefully removed, a drop of the oil was placed upon the freshly 

 (ormed water-surface in a small dish by means of a brass wire 

 previously cleaned by ignition. The oil did not really spread, 

 but after a momentary centrifugal movement, during which 

 several small drops were separated from i% it contracted itself 

 ill the middle of the surface, and a second drop deposited on 

 the ?ame vessel remained absolutely motionless. 



Of cour>e the surrounding water-surface proved to be in the 

 anomalous state, the tension deteimined by the method of 

 separating weights being =0^2 of the normal value. It has 

 been diminished by the " solution-current " of the oil (as I have 

 called the contaminating current, issuing from a body in contact 

 with a clean water-surface), which may be observed if the 

 surface be dusted over wilh sulphur or lycopodium before 

 placing the oil upon it. The observation of the solution- 

 current, preceding unpurified oil, is more diflficult, because the 

 oil itself covers rapidly the whole surface. 



.\s soon as the relative tension 082 is attained, the slightest 

 trace of a solution current ceases whilst ordinary oil still shows 

 solution-currents at much lower tensions. The surface-tension 

 0'82 is the lowest possible that can be produced on water by 

 pure Provence oil ; the surface then may be considered as 

 saturated with oil. 



This can be seen most clearly if the drop be deposited upon 

 the adjustable trough filled with water, which was employed 

 in my former experiments (N.VTUKE, March 12, 1891, p. 437). 

 The tension then remains constant on either expansion or 

 contraction of the surface ; on considerable contraction, how- 

 ever, one can perceive a slight precipitate of oil, which gives 

 to Ihe surface a tuibid appearance. 



Evidently the pure oil does not spread over a surface of the 

 minimum tension attainable by its contact with water, because 

 the sum of its surface-tension and the interfacial tension of oil 

 and water, which we may call "tension of equilibrium," is 

 greater iKan the minimum tension. Therefore upon a clean 

 watersurface the oil is repulsed by its own solution current. 



