SCIENCE. 



299 



If the outer layer of the liquid should resist the move- 

 ment of the magnet from any viscosity of its own, it would 

 pucker perceptibly ; moreover, viscous bodies propagate 

 motion with difficulty from any distance. 



The coating of dirt imitates closely an elastic body, in- 

 asmuch as it tends to return to its primitive state when 

 broken ; however, it substitutes, in place of superficial 

 vsicosity, superficial elasticity. 



In regard to those liquids of strong expansive force 

 which do not produce bubbles, such as water, the 

 greater p :>rtion of briny solutions, etc., liquids upon whose 

 surface' a layer of dirt cannot easily be attested, the re- 

 tarded movement of the magnet upon the exterior is 

 hardly due to the changes which occur in the cavities of 

 the meniscus, terminating the magnet at each end, partly, 

 also, to the beginning of a laver of dirt, M. Hagen having 

 discovered that the surface of water undergoes modifica- 

 tions bv exposing the liquid to the air. But the principal 

 cause may reasonably be said to be the fact which M. 

 Van der Mengsburgghe has so well described, namely, 

 that when the surface of any liquid is augmented, or, in 

 other words, when any diffusion of the pure exterior takes 

 place, a sudden cooling, followed by an increased tension, 

 ensues, and, reciprocally, a warmth and decrease of 

 tension correspond to any diminution or contraction of 

 the surface. 



This then is the main substance of the theory pro- 

 posed by M. Marangoni in compensation for mine. Let 

 us endeavor to examine it. First of all, it would seem, 

 according to h ; s doctrine, that it is merely necessary to 

 add to any liquid of strong expansive force which 

 does not bubble, a drop of another liqu'd of weak tension 

 in order to produce large bubbles from the former. Now 

 if a drop of olive oil or spirits of turpentine be placed 

 upon distilled water, the liquid will rebel strongly against 

 the formation of bubbles. Should the water be covered 

 with a thin coating of either of the above mentioned 

 liquids, you will find that it bursts in the bowl of the pipe 

 before you have even commenced to blow the bubble. 

 We must admit therefore, in the first place, that the sup- 

 posed coating of dirt must have close connection with the 

 liquid beneath it. The author also assigns an additional 

 and indispensible cause for the produc'ion of bubbles 

 which he describes as the superficial elasticity, or in 

 other words, the facility with which the dirt spreads itself 

 over the liquid, so that the latter is always covered. No- 

 thing however, goes to show us that a thin coating of 

 olive oil or turpentine does not possess the same elasticity. 

 The author, in fact, describes two circumstances in 

 which foreign substances produce a coating upon distilled 

 water which is more or less effectual. First of all, if the 

 pollen of flowers is spread upon the surface and air blown 

 from above within an hour or two, the little apertures 

 formed will remain for a longtime; but the liquid refuses 

 to form bubbles when blown, from a pipe or tube. In 

 the second place, they can be produced, nevertheless, by 

 means of pure distilled water, if the tube is partially filled 

 with small particles of camphor. The diameter of these 

 bubbles may reach an inch and more. But we can read- 

 ily see that these facts are only the beginning of success. 

 However, they are in no wise opposed to the theory of 

 superficial viscidity, since in both cases the outer layer of 

 the water undergoes modifications. 



According to the author, the superficial elasticity is es- 

 timated by the difference which exists between the ten- 

 sion of the pure surface and that of the dirty surface, and 

 he determines this by means of a small apparatus which 

 he calls a capillary balance. In his opinion, as we have 

 already seen, when a bubble is blown the coating of dirt 

 prevents its being broken. In his statement he gives no 

 reason for this but in a preceding work he explains him- 

 self clearly on this point. H. says that if the coating of 

 dirt should become disunited, the excess of tension upon 

 the under layer, or in other words, the superficial elas- 

 ticity, would instantly close the aperture. Hence the 



facility for the formation of bubbles, or as the author calls 

 it, the pompholygenic power, should decrease with the 

 superficial elasticity. Now M. Marangoni is led to the 

 conclusion that all causes which tend to diminish this 

 elasticity without removing the dirt, render the develop- 

 ment of bubbles much easier. Further on, he returns to 

 this proposition and says that " all those conditions which 

 diminish this elasticity to the advantage of the plasticity 

 increase the pompholygenic force." If we examine 

 closely his ideas, we can understand that an increase of 

 plasticity favors considerably the generation of bubbles ; 

 but how is it possible that a diminution of elasticity can 

 lead to the same result ? 



Let us return to the first of the two facts quoted above. 

 The author finds, by means of his capillary balance, that 

 the superfine elasticity of the distilled water, covered 

 with pollen, may become doubly as great as that found 

 in the saponaceous solution. Now, inasmuch as the 

 latter produces large bubbles while the former gives none 

 at all, it is necessary, according to M. Marangoni's prop- 

 osition that the plasticity of the saponaceous solution 

 should be much superior to that of the distilled water, 

 which is rather di f.cult to admit owing to the peculiar 

 rigidity of the surface of the former ; indeed there are two 

 totally different liquids in question ; nevertheless, the 

 author's statement seems to apply equally to both in 

 this case. 



In order to show that the layer of dirt can be accumu- 

 lated upon one portion of the surface of a liquid and 

 diminished upon another, M. Marangoni describes the 

 the following curious experiment. 



He plunges, into a soapy solution, a ring made of iron 

 wire about seven and a half inches in diameter and fas- 

 tened to the end of a fork which serves as a handle ; 

 when the ring is immersed he draws it out again, holding 

 it in a horizontal position ; he then raises it until the cate- 

 noid wave, which unites it to the surface of the liquid, 

 separates into two portions, one of which forms an even 

 layer within the ring, while the other produces a spherical 

 cavity upon the liquid ; now, if the temperature is low 

 enough (from 12 to 14 degrees), this cavity is very hol- 

 low, the radius of the base measuring 48 millimetres, 

 while the height is only 27. M. Marangoni began this 

 experiment four times, always breaking the cavity before 

 again immersing the ring, and by this means he obtained 

 the maximum of depression in which the depth was ex- 

 actly half of the ra "i he base. While the ring is 

 being raised the cifOtmiference, in accordance with which 

 the catenoid lamina unites with the surface of the liquid, 

 contracts, and as M. Marangoni affirms, condenses the 

 coating of dirt on the interior and dilates it on the ex- 

 terior. Hence, when the cavity is once formed a diminu- 

 tion of tension takes place in the space limited by its 

 base, and an increase of tension occurs on the outside; 

 this excess of tension co risequently aids the basis of the 

 cavity to enlarge, and results in the depih being 

 diminished. 



According to my theory, the superficial layer of the 

 liquid contracts, as above stated, on the interior of the 

 opening, and dilates on the exterior; but its consistency 

 does not undergo any modification. The portion which 

 contracts forces a part of its molecules into the mass 

 beneath, and the dilated portions attract these atoms. 

 Now, according to M. Van der Mensbrugghe's theory 

 which I have mentioned already, these effects cannot be 

 produced unless a diminution of tension takes place 

 within the contracted portion and an augmentation of the 

 same in the dilated part. This phenomenon, however, 

 can only occur in a very low temperature, and when, in 

 consequence, the cavities manifest a certain viscidity. 

 When the temperature is notably higher the cavities are 

 smaller and their depression less. At 26 degrees hardly 

 any effect is visible. The radius of the base at this tem- 

 perature was 23 millemetres, and the height 20 ; but I 

 have shown that all cavities formed upon the surface of 



