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SCIENCE- G OSSIP. 



was termed the cohesion figures, which were formed 

 when they were allowed to fall upon the surface 

 of water. Thus olive oil first forms a disc with 

 recurved edges, but spaces soon appear round 

 these edges, and portions detach themselves from 

 the main body to become united again in some 

 places, enclosing polygonal spaces, over which is 

 an almost invisible film of oil. In the case of 

 sesame oil the film contracts, and eventually a 

 figure somewhat resembling a spider's web, with 

 well-defined circular edge, is produced. Colza 

 oil, again, forms a circle surrounded by a large 

 number of minute drops, and poppy-seed oil gives 

 a circle with serrated edge. In the case of 

 mixtures the form of the cohesion figure varies 

 with the proportion of the different oils, but little 

 reliance can be placed upon the phenomenon as a 

 test of the purity of any particular oil. — C. A. 

 Mitchell, London. 



[The lucid account of cohesion figures given above 

 by Mr. Mitchell is of more than passing interest. 

 The writer has repeated some of the experiments 

 with different oils, and obtained different shaped 

 films for each kind of oil that had been dropped 

 on the surface of water. The figures closely re- 

 sembled those described. Applying the principle 

 of the conservation of energy it will be seen that 

 the potential energy of the system depends upon 

 — (1) the action of gravity; (2) the mutual action 

 of the liquid particles ; (3) the action between the 

 particles of water and the oil in contact with it. 

 The condition of equilibrium is, that the potential 

 energy shall be a minimum. M. Plateau got rid 

 of gravity by forming a mixture of alcohol and 

 water of the same density as olive oil, and then 

 introducing a small quantity of oil into the 

 mixture. The oil then assumed the form of 

 a sphere under the action of surface tension 

 alone. On altering the form of certain parts 

 of the surface of the oil, the free j)ortions of 

 the surface assumed new forms depending 

 upon the equilibrium of surface tension. In 

 the case, however, of an oil-film on water one 

 must consider three different fluids, and conse- 

 quently three surfaces of contact. In the instance 

 of three fluids which do not mix the three surfaces 

 of separation meet in a line, straight or curved. 

 Denoting the tensions of the three surfaces by 

 T, 2 , T 23 , and T 13 , it is clear that there will be 

 equilibrium if the sum of the components T 13 and 

 T^ equal T ]2 . When one force is greater than 

 the sum of the other two, the condition of equili- 

 brium no longer holds, and the wedge formed' by 

 the surfaces of separation of the oil with air and 

 water becomes very narrow, causing the film to 

 get thinner and thinner and spread out in the 

 manner oil does on the surface of water, as in fig. 2, 

 where T ]2 is greater than the sum of T )3 and T 23 , 

 so that the oil spreads rapidly over the surface of 

 the water, covering it with a thin film of oil, almost 

 invisible. The values of the surface tension vary with 

 time, so that it is probable a slight diffusion takes 

 place. I consider that a film of a third liquid is 

 formed from the oil and water by hydrolytic dis- 

 sociation. The fatty oils, such as olive and linseed 

 oils, are ethereal salts formed by the combination 

 of the fatty acids with glycerine, which acts as a 

 hydroxide or weak base. Hence, since weak acids 

 and weak bases are in question, the chemical 

 nature of water must be taken into consideration. 

 It is usual to speak of water as a perfectly neutral 

 substance, but this is far from being the case. 



Even distilled water must contain ions in moderate 

 quantity, as shown by its electrical conductivity. 

 If the conductivity is not due to dissolved im- 

 purity, the ions with which the electricity travels 

 must come from the water itself. The chemical 

 and electrochemical behaviour of water is accounted 

 for by regarding these ions as hydrogen ions and 

 hydroxyl ions. Hydrolysis can be completely ex- 

 plained by assuming that in water an equilibrium 

 subsists between undecomposed molecules and 

 ions. Since the conductivity of water is very small 

 the amounts of ions present are extremely minute, 

 but are sufficient to confer on water the properties 

 of a weak acid on account of the hydrogen ions, 

 and of a weak base on account of the hydroxyl 

 ions. In the case of oil films on water, a decom- 

 position of these ethereal salts by solution in 

 water is almost certain to occur, and since both 



Wo/*- 



the acid and base of the salts are weak the hydro- 

 lysis will be independent of the dilution. The 

 reader will thus see that the subject of oil films 

 on water presents several fascinating problems on 

 chemical equilibrium in addition to surface tension 

 and optical phenomena. — Ed. Physics.~\ 



Surface Tension and Grease Spots. — The 

 surface of a liquid is in a state of tension similar 

 to that of a membrane stretched equally in all 

 directions. This property of surface tension is 

 useful in removing grease spots from cloth. If a 

 drop of oil is placecLon a glass plate, and a drop of 

 alcohol be brought near it, the alcohol drives the 

 oil along in front and appears to chase it. If a 

 ring of alcohol is made all round the oil the drop 

 stands up in a little compact mass like a lens. 

 Hence to remove a grease spot from cloth, wet one 

 side of the cloth with alcohol or benzol in a ring 

 all round the spot, and then place a piece of 

 blotting-paper near the centre to absorb the little 

 mass of grease driven there. Another method is 

 to heat the grease spot on one side, when the rise 

 of temperature diminishes the surface tension, and 

 so drives all the grease spot to one side, where it 

 can be gathered up as before. 



The National Physical Laboratory.— The 

 formal opening of the new National Physical 

 Laboratory at Bushey House, Teddington, is 

 announced to take place on March 19th, when 

 H.R.H. the Prince of Wales will perform the 

 ceremony. We understand that the institution 

 will be one of the most perfect of its kind. 



