592 THE POPULAR SCIENCE MONTHLY. 



attractive and repulsive forces are at equilibrium ■within the 

 liquid, there is supposed to be in the immediate vicinity of the 

 free surface a tendency to the dispersion of the particles which is 

 constantly opposed by the attractive forces. The condition of the 

 superficial layer may be compared with that of a thin, elastic 

 membrane under stretch, the cohesion of which constantly op- 

 poses itself to a more considerable elongation. The superficial 

 layer of a liquid is thus subject to a contractile force or tension, 

 by virtue of which it tends to become as small as possible. M. 

 Gossart, comparing the relative situation of two molecules, A 

 within the drop, and B at its surface, against the air or another 

 liquid or a solid body, shows that each molecule is attracted by 

 the others only from a certain distance (less than ten thousandth 

 of a millimetre), which is as formidably great to it as it seems 

 little to us. Those molecules which are at a greater distance 

 from A and B will have no more action upon them than the stars 

 have upon our sun, earth, and planets. Regarding these spheres 

 alone, A, equally solicited in all directions by an equal number 

 of molecules, will be free in its movements, and obedient to 

 Pascal's principle ; while B has not the same surrounding in 

 every direction. Hence a kind of rarefaction which extends to 

 only a slight depth in the drop ; and hence also, on the surface, 

 the elastic membranous or resistant quality. 



This property is illustrated in some experiments described by 

 M. Van der Mensbrugghe. Take two pencils, one of which should 

 be of light wood and thinner than the other (Fig. 1) ; place them 

 alongside and in contact ; drop a little clear water in the angle 

 between them, so as to moisten the line of contact. There will be 

 formed a slight liquid mass, adherent to both pencils, of concave 

 outline, the section of which is represented by a b in the corner 

 diagram of Fig. 1. The lighter pencil will hang from the other 

 by virtue of the tension of the concave surfaces a b, that bound 

 either side of the line of contact. With the pencils twelve centi- 

 metres long, a weight of eighteen hundred milligrammes may be 

 sustained in this way. In a second experiment, a ring of copper 

 wire a millimetre thick and three and one quarter inches in 

 diameter, is laid carefully upon the surface of pure water, when 

 — if everything be entirely clean — it will float, as in Fig. 2, section 

 a, and this, notwithstanding copper is 8'8 times heavier than 

 water. This takes place because all the tensions of the liquid 

 that touches upon the ring produce an upward resultant. A ring 

 weighing seventeen hundred and thirty milligrammes may be thus 

 upheld, while the maximum effect of the tensions is three thousand 

 seven hundred and seventy milligrammes, or more than double the 

 weight of the ring. Needles, globules of mercury, a thin ring of 

 platinum, etc., may be similarly made to float on water. 



