KNOWLEDGE. 



January. 1911. 



and also shows the neck to have taken a cylindrical 

 form : whilst in No. 6 the distortion of the drop has 

 proceeded further, and the c\-lindrical neck, being 

 unstable, is seen "to be breaking up into three 

 separate portions, each of which forms a sphere. 

 These photographs, however, do not give such a 

 complete idea of the process as may be (obtained b\- 

 watching the experiment, as onh' six detached 

 stages are represented. \'ariations occur, also, in 

 the fate of the neck, which sometimes shrinks back 

 into the mass remaining on the surface: sometimes 

 breaks off and forms a single sphere, and often 

 several spheres of varying sizes. The exact method 

 of taking the photographs \\ ill he explained later. 



And now. the detached drop ha\-ing fallen to the 

 hiittoni of the beaker, comes the sur[)rising part of 

 the experiment. The fallen drop is seen gradually 

 to rise to the surface, where it joins the mass from 

 which it previously broke awa\'. M once another 

 drop commences to form, and having become detached, 

 falls and rises in the same manner as the previous 

 drop. So long as the temperature of the water is 

 maintained at 70" C or over, this procedure continues 

 indefinitely. The forces at work on the drop perform 

 a Sisyphean task. 



Here it may be exjilained that the photographs 

 shown do not represent one and the same drop, but 

 that a snap-shot was taken at different phases in 

 the formation of six separate drojjs. The uniformit\- 

 with which the process is repeated is thus exhibited 

 in a striking manner, as the photographs might easih' 

 be taken to represent six stages in tlii_' formation of 

 a single drop. It may be added that the temperature 

 should ne\'er exceed 85 ~' C. nor fall lielow 70" C if 

 photographs are being taken : and that the distorting 

 effect of the glass beaker may be ox'ercome bv jdacing 

 it in a rectangular vessel with glass sides, also 

 (-ontaining hot water. This plan was followed b\- 

 Mr. Abel in securing the photographs shown : it is 

 also suitable for the optical projection of the 

 experiment. 



The explanaticjn of the formation of the drops and 

 their subsequent ascent to the surface is, in the 

 main, simple. .\niline is a liquid which at low 

 temjieratures is denser than cold water, but at high 

 temperatures lighter than hot water, owing to its 



higher degree of expansion. If dropped into water 

 below 60" C. aniline will sink ; but if the temperature 

 of the water exceed 65" C, the aniline, after becoming 

 warmed b}' its surroundings, will rise. Hence, in 

 the experiment under notice, the aniline ascends to 

 the surface of the water o\\ing to its lesser densit\'. 

 By spreading out on the surface, however, the 

 aniline is cooled more than the water beneath, and 

 soon becomes denser than the water in consequence 

 of this cooling. The aniline then tends to sink, and 

 a drop breaks off as shown in the photographs. 

 By passing through the hot water, however, the 

 temperature of the detached drop rises, and if the 

 temperature exceed 65" the drop will again become 

 lighter than its surroundings and will ascend 

 to the surface. The success of the experiment 

 depends upon this remarkably delicate balance 

 in the temperature-densit\' relations of aniline and 

 water. It must be stated, however, that aniline is 

 partially soluble in water, and hence the drop is 

 falling not through [jure water, but a saturated 

 solution of aniline in water. Questions with regard 

 to the surface tensions of the liquids would also have 

 to be entered into in giving a detailed explanation : 

 and these apparently minor details probabh- furnish 

 the reason why certain other liquids, which might be 

 expected to behave in the same manner if substituted 

 for aniline in the experiment, fail to do so. So far 

 as investigations ha\e been conducted, no other 

 liquid has been made to operate automaticalK' in 

 producing and reproducing the drop. 



One other feature of the experiment is deserving 

 of notice. It ma}' be made to represent the principle 

 of elementar\" heat engines. The lower part of the 

 water ser\-es as the source of heat : the atmosphere 

 above the surface acts as the refrigerator to which 

 the rejected heat is given ; and work could be done 

 b\' the mo\'ing drop. It would not be difficult to 

 construct an indicator diagram for this rudi- 

 mentary heat-engine, based on Carnot"s well-known 

 cycle, for the drop passes through a regular and 

 recurrent set of operations. .A beaker of hot water 

 and some aniline nia\- thus be made to teach man}- 

 useful lessons, and furnish a further example 

 of how the [)rofoimdest scientific truths ma}" 

 frequentl}- be deduced from the simplest experiments. 



THE GREAT NEBUL.\ IX oRloX. 



The photograph of the Great Nebula in Orion 

 (reproduced on the opposite page) is made from a 

 negative which we owe to the courtes\- of Sir 

 William Christie, F.R.S., the recent .Astronomer 

 Royal, and is one of the triumphs of celestial 

 photography at the Ro\-al Observatory, Greenwich. 

 No finer photograph of the nebula is in existence. 

 The reproduction is enlarged three diameters, from a 



negative taken bv Mr. Melotte. on December 1st, 

 KS99. with the thirt\-mch reflector, the focal 

 length eleven feet hve inches, and the exposure 

 two and a quarter hours. The nebula seen through 

 a telescope glows like a vast filmy cloud of 

 emerald light. Its brightest portions have edges 

 sharp as in an engraving, and are side by side 

 with regions of the most intense blackness. 



