566 



NA TURE 



\_April 17, 1890 



The chapter on "Eleccro- Motors and their Uses" is 

 good considering how much may be said on this subject 

 and how short a space is 14 pages to say it in. By what 

 means, however, Messrs. Immisch have succeeded in 

 making the dogcart for the Sultan of Turkey go "ten 

 miles an hour for about five hours " by means of " twenty- 

 four small accumulators which weigh about seven 

 hundredweight" we are at a loss to conceive, since the 

 weight of accumulators, according to our calculation, must 

 be much greater than this in order that they may have 

 anything like a reasonably long life. 



Chapter XVII., on" Electro-Metallurgy," is interesting 

 although very brief, but the descriptions of the electrical 

 circuit-closers for torpedoes in the next chapter, on 

 " Electricity in Warfare," we find too short to be in- 

 telligible. A chapter of 5 pages then follows on 

 " Medical Electricity," and another chapter of the same 

 length on " Miscellaneous Applications of Electricity," in 

 which a very interesting account is given of the electrical 

 method employed in America for protecting furnished 

 dwelling-houses that have been left locked up during the 

 absence of the tenants. 



On closing this book one certainly cannot deny that one 

 has had one's money's worth, even if the entertainment 

 has been of the " variety order " so characteristic of the 

 amusements of the present day. If a member of the 

 general public will read the book right through, as we 

 have done, he may perhaps feel with exultation that he 

 has mastered the whole subject of electrical engineering ; 

 indeed, even a well-trained electrician can learn from it 

 many things that he did not know before, concerning 

 those branches of the subject to which he has not given 

 special attention. But we fear that, if even a general 

 reader were to turn up any particular subject to study in 

 detail, he would probably wish he had been told a good 

 deal more about what was most important, and not so 

 much about everything electrical whether important or 

 not. The best features of "Electricity in Modern Life" 

 are the many interesting scientific narratives, in the 

 writing of which Mr. dc Tunzelmann appears to excel ; 

 the worst are the mistakes in the science, which more 

 knowledge, or more care, ought to have eliminated. 



ON THE TENSION OF RECENTL V FORMED 

 LIQUID SURFACES} 



TT has long been a mystery why a few liquids, such as 

 ■*• solutions of soap and saponine, should stand so far 

 in advance of others in regard to their capability of 

 extension into large and tolerably durable lamina;. The 

 subject was specially considered by Plateau in his valuable 

 researches, but with results which cannot be regarded as 

 wholly satisfactory. In his view the question is one of 

 the ratio between capillary tension and superficial viscosity. 

 Some of the facts adduced certainly favour a connection 

 between the phenomena attributed to the latter property 

 and capability of extension ; but the " superficial viscosity " 

 is not clearly defined, and itself stands in need of 

 explanation. 



It appears to me that there is much to be said in favour 

 of the suggestion of Marangoni ('' Nuovo Cimento," vols, 

 v.-vi., 1871, p. 239), to the effect th,at both capability of 

 extension and so-called superficial viscosity are due to 

 the presence upon the body of the liquid of a coating or 

 pellicle composed of matter whose inherent capillary 

 force is less than that of the mass. By means of varia- 

 tions in this coating, Marangoni explains the indisputable 

 fact that in vertical soap films the effective tension is 

 different at various levels. Were the tension rigorously 

 constant, as it is sometimes inadvertently stated to be, 

 gravity would inevitably assert itself, and the central 

 parts would fall 16 feet in the first second of time. 



« A l^aperreatl by I.orJ Rayleigh, SiC.R.S., before the Rjyal Sjciety, 

 on March 6. 



By a self-acting adjustment the coating will everywhere 

 assume such thickness as to afford the necessary tension, 

 and thus any part of the film, considered without dis- 

 tinction of its various layers, is in equilibrium. There is 

 nothing, however, to prevent the interior layers of a 

 moderately thick film from draining down. But this 

 motion, taking place as it were between two fixed walls^ 

 is comparatively slow, being much impeded by ordinary 

 fluid viscosity. 



In the case of soap, the formation of the pellicle is 

 attributed by Marangoni to the action of atmospheric 

 carbonic acid, liberating the fatty acid from its combina- 

 tion with alkali. On the other hand, Sondhauss {Po^gen- 

 dorff's Annaleti, Ergiinzungsband viii., 1878, p. 266) found 

 that the properties of the liquid, and the films themselves, 

 are better conserved when the atmosphere is excluded by 

 hydrogen ; and I have myself observed a rapid deteriora- 

 tion of very dilute solutions of oleate of soda when 

 exposed to the air. In this case a remedy may be found 

 in the addition of caustic potash. It is to be observed, 

 moreover, that, as has long been known, the capillary 

 forces are themselves quite capable of overcoming weak 

 chemical affinities, and will operate in the direction 

 required. 



A strong argument in favour of Marangoni's theo'ry is 

 afforded by his observation,^ that within very wide limits 

 the superficial tension of soap solutions, as determined 

 by capillary tubes, is almost independent of the strength. 

 My purpose in this note is to put forward some new 

 facts tending strongly to the same conclusion. 



It occurred to me that, if the low tension of soap 

 solutions as compared with pure water was due to a 

 coating, the formation of this coating would be a matter 

 of time, and that a test might be found in the examination 

 of the properties of the liquid surface immediately after 

 its formation. The experimental problem here suggested 

 may seem difficult or impossible ; but it was, in fact, 

 solved some years ago in the course of researches upon 

 thecapillary phenomena of jets (Roy. Soc. Proc, May 15, 

 1879). A jet of liquid issuing under moderate pressure from 

 an elongated, e.g. elliptical, aperture perforated in a thin 

 plate, assumes "a chain-like appearance, the complete 

 period, X, corresponding to two hnks of the chain, being 

 the distance travelled over by a given part of the liquid in 

 the time occupied by a complete transverse vibration of 

 the column about its cylindrical configuration of equi- 

 librium. Since the phase of vibration depends upon the 

 time elapsed, it is always the same at the same point in 

 space, and thus the motion is steady in the hydrodynamical 

 sense, and the boundary of the jet is a fixed surface. 

 Measurements of X under a given head, or velocity, 

 determine the time of vibration, and from this, when the 

 density of the liquid and the diameter of the column are 

 known, follows in its turn the value of the capillary 

 tension (T) to which the vibrations are due. Cceieris 

 Paribus,Toz\-';a.nd\.h\s relation, which is very easily 

 proved, is all that is needed for our purpose. If we wish 

 to see whether a moderate addition of soap alters the 

 capillary tension of water, we have only to compare the 

 wave-lengths X in the two cases, using the same aperture 

 and head. By this method the liquid surface may be 

 tested before it is ^Jg second old. 



Since it was necessary to be able to work with moderate 

 quantities of liquid, the elliptical aperture had to be 

 rather fine, about 2 mm. by l mm. The reservoir was 

 an ordinary flask, 8 cm. in diameter, to which was sealed 

 below as a prolongation a (i cm.) tube bent at right 

 angles (Figs, i, 2). The aperture was perforated in thin 

 sheet brass, attached to the tube by cement. It was 

 about 15 cm. below the mark, near the middle of the 

 flask, which defined the position of the free surface at the 

 time of observation. 



' Poggendorff'i Annalcn, vol. cxliii., 1871, p. 342. The original pamphlet 

 dates from 1865. 



