224 



NATURE 



[July 5, 1894 



On the other hand, a drop of common Provence oil placed 

 upon the saturaied surface spreads, while the surrounding 

 sjrface diminishes its area and grows turbid. From this we see 

 tiiat the tension of equilibrium of the oil containing free 

 sebac c acid is lower than its saturation tension, and this is also 

 the reason why it is not prevented from spreading by its own 

 solution-current. 



I have repeated these experiments with various kinds of oil, 

 and in each case found that by shaking up with alcohol 

 the tension of equilibrium rose, and the tendency to spread was 

 diminished. 



In the case of ordinary olive oil there was but little difference 

 between the purified and unpurified oil, although it was six 

 times shaken up with fresh alcohol. With rape-seed oil and 

 poppy oil I was more successful. Their ten<i )n of equilibrium 

 was still somewhat inferior to the minimum tension, which was 

 with rape-seed oil 0S5, and with poppy oil 082, but the 

 spreading on a saturated surface was very slow, and upon a 

 large clean surface the oils covered but a comparatively small 

 area. The best success I had with almond oil, which behaved 

 quite like Provence oil. 



On the other hand, the tendency to spread, not only of pure 

 oil, but also of benzol and petroleum, increased when oleic, 

 palmitic, or stearic acid was dissolved in them. 



Pure benzol can rest on water but in a rather thick layer. 

 When the thickness is diminished to a certain degree, the layer 

 breaks into drops, for which the following explanation seems to 

 me most probable. The water-surface surrounding benzol, 

 as in the case of oil, never is in the normal condition, the 

 tension being diminished by the vapour streaming over the water. 

 This vapour current ceases at the tension o"88, which, as il 

 appears, is somewhat lower than the equilibrium tension of 

 benzol. Therefore a thin layer of benzol is broken by the 

 vapour current. 



When a floaling fluid layer is not very thin, the tension of 

 the surrounding anomalous. water-surface, just balanced by it, is 

 no longer equal to the sum of interfacial and surface tension. 

 In the case of benzol it assumes a lower value than the 

 minimum tension of the vapour-current, and therefore a 

 sufficiently thick layer is allowed to spread coherently. 



Benzol which is contaminated, for instance, by stearic acid or 

 resin, behaves quite differently. The tension of equilibrium 

 being lowered by those substances, it spreads so (ar as to show 

 colours of thin plates. 



Pure petroleum seems to be the only liquid which does not 

 spread upon a normal surface. The vapour-current going out 

 from petroleum ceases already at a relative contamination - I, 

 and therefore cannot produce a sensible decrease of tension. 

 Nevertheless it may possibly prevent the floating drop from 

 spreading. When sel>acic acid is dissolved in petroleum, the 

 latter shows a much grciter tendency to spread. 



In order to examine whether the effect of sebacic acids upon 

 the tension of equilibrium be due to a decrease of the cohesion 

 of the solvent, I have compared the surface-tensions of pure and 

 contaminated benzol or petroleum, and those of the purified 

 and unpurified oils. In no case have I found the surface- 

 tension to be diminished by the sebacic acid, hence I came to 

 the conclusion, that it is the interfacial tension which is 

 altered. 



Now let us consider the behaviour of common oil. Il 

 spreads in a coherent film to a certain thickness, which is 

 different with various sorts of oil. Then small holes appear in the 

 interior of the film, whilst the circumference of the latter is still 

 increaiin,;, and by the gradual increase in size of the holes the 

 layer at last it broken and dissolved in Mnall drops. 



Why does the oil thus withdraw from the surface while its 

 circumference is still increasing ? The reason is, no doubt, that 

 the oil <preadi at the minimum tension of pure oil, but not at 

 that of the free sebacic acid contained in it. The solution- 

 current of the latter drives it back from the surface. Outside 

 the oil-film spreading upon a large water-surface the tension of 

 the latter docs not link below the minimum tension of the pure 

 oil ; in the interior of the holes, however, a newly-formed 

 •urface would lie instantaneously saturated with oil, and here 

 the tension, therefore, can be further diminished by the sebacic 

 acid. 



The minimum tension of oleic acid, at which the latter also 

 does not apread, is in relative measure 0'52, and that of palmitic 

 acid about 055. 



The 'leproiion of surface-tension which can be attained by 

 unpurified oil is not so great, but much greater than that 



NO. 1288. VOL. 50] 



produced by pure oil, and depends upon the quantity of oil 

 applied. 



If the quantity be such as to cover the whole surface before 

 breaking, the surface afterwards is not coniaminated with oil 

 at all, but only with sebacic acid ; and the tension is still 

 sinking slowly by the effect of the continued solution-currents of 

 the single drops. 



When less oil is employed, the free sebacic acid contained in 

 it is often not sufficient to produce the lowest possible tension. 

 Then one may observe that freshly added drops of oil still give 

 solution-currents, whilst those of the older drops have alrcidy 

 ceased. 



When a water-surface, on which minute drops of oil which 

 have not yet dissolved are present, is expanded, the tension 

 rises to the minimum value for pure oil, and then remains 

 constant till the whole oil is dissolved, where it begins to rise 

 again in the same manner as the curve given in Nati;re, June 

 IS, 1S93, p. 152. 



The value of surface-tension, at which the linear fall of the 

 curve ceases, being identical with the minimum tension of pure 

 oil, it is evident that the sudden change of direction at the 

 relative contamination 1-3 means saturation of the water-surface 

 with oil. Agnes Pockels. 



Prof. Ostwald on English Chemists. 



"To see ourselves as others see us" is so difficult of attain- 

 ment, that no mirror, however imperfect, should be passed by 

 without a glance bestowed upon it. The image of us which 

 Prof. Ostwald displayed to the electiicians assembled in con- 

 clave at the second anniversary meeting of the C.erman elec- 

 tricians on June 7, is the less pleasant by reason of the con- 

 sciousness that the reflector is a good one. The opening words 

 of the Professors address were virtually as follows ; — 



" It is a positive fact that every year there are imported into 

 (jermany from Kngland so many thousand centners of benzene, 

 amounting to nearly the whole of the production of this material 

 in the latter country. Now benzene is an inlcrmediatc product, 

 destined to be converted into dye-stuffs, medicaments, and other 

 commodities, so that we have the remarkable situation that the 

 country of all the world, in which industry has flourished longest, 

 relegates the most important and profitable part of one of her 

 manuLictures to a foreign country. The reason is of the 

 plainest : England cannot undertake the conversion of its raw 

 material into the finished product, and why? Because of the 

 insufficient training of the English chemist. The would-be 

 practical Englishman with the intention of entering a dye-factory, 

 studies, not general chemistry, but the chemistry of dye-stufls. 

 The (icrman studies chemistry, lock, slock and barrel, never 

 wrecking what his calling is to be. Only when he has a really 

 scientific foundation will he begin to build up his special know- 

 ledge. 15y and by there comes a change over the face of the 

 industry in which these competing chemists are employed. The 

 German — he is ready ; without difficulty he adapts himself, and 

 follows up the novel course. But the Englishman — he cannot 

 imagine at what position he has arrived ; he must begin, so to 

 say, over again." 



Thus spoke one of Germany's — nay, the world's — greatest 

 thinkers. Let our manufacturers, who despise the college-bred 

 youth, meditate thereupon. .'\. G. BtoXAM. 



Goldsmiths Institute. 



" Testacella Haliotidea." 



Tate, in his " Molluscs of Great Britain," gives a list of coun- 

 ties in which this mollusc may be found. In this list Worcester 

 is not included. Hence it may be of interest to note that 

 specimens are not infrequently collected in asparagus-beds 

 here, as also are those of the much rarer T. anlitlnm. A good 

 specimen of the latter was recently given by me to Mason 

 College, Birmingham. 



.Vrmalns i^rosmlarin. — Here the gooseberry plantations arc 

 often devast.atcd by the larvae of this saw-fly, in the extirpation 

 of which pest the insectivorous value of the cuckoo to planters 

 may be appreciated through the following incident. Kecenlly 

 the attention of a resident of Crowlc, a village near Worcester, 

 was dirccleil to his gooseberry plant.ition, close by a window 

 of his house. A cuckoo was in one of the grub-infested bushes, 

 fluttering its wings, and so causing numbers of the ])ests to fall 

 on the ground, whence they were quickly gathered by the bird. 



