1865.] Prof. Guthrie on Bubbles. 23 



9. In the drops 4, 6, and in the bubbles 7, 8, the restraining film is a gas. 

 There is a remarkable inverse analogy between the cases 4 and 9. In 4 a 

 gaseous film hinders a liquid from reaching a liquid ; in 9 a liquid film 

 hinders a gas from reaching a gas. The cases 7 and 10 are also called, 

 hubbies, although their only title to the name is the liquid film in each. 

 Viewed as film-envelopes, 1, 2, 3, 4, 5, 6 are bubbles ; viewed as spheroidal 

 liquid masses they are drops. 



Further, the spurious drops and bubbles differ from the true ones which 

 we have to examine in a very important particular. The spurious ones are 

 essentially statical phenomena, and retain their indefinite size for an inde- 

 finite time. The true drops and bubbles, on the other hand, grow until 

 their exact equilibrium is established, and they acquire their definite size at 

 the instant of the overbalancing of that equilibrium that is, at the instant 

 of their motion. It is, in fact, this overbalancing which determines the 

 definiteness of their size, by withdrawing them from the size-determining 

 effect of the action of the contending forces which accompany and condition 

 their growth. 



All attempts to get a perfectly uniform succession of bubbles of the pure 

 form SGL (corresponding to water dropping from a glass sphere) failed 

 through the impossibility of getting the immersed solid protected by the gas 

 from the adhesion of the liquid. But by a contrivance similar to that 

 described in the case SLjL^, where L x was lighter than L 2 *, it was found 

 possible to get bubbles of uniform size, and to measure them. 



The most obvious manner of doing this is to force a gas at a fixed rate 

 through an ordinary gas-delivery tube, and to collect and measure a given 

 number of the bubbles in a calibrated tube over the pneumatic trough. 

 This plan, however, is open to the objection of requiring a large quantity of 

 liquid medium. 



The apparatus employed is seen in fig. C. The quart bottle A is filled 

 a little above the mark a with water, which is in some experiments 

 covered with a film of oil. Through its cork three tubes, C, D, F, pass 

 absolutely air-tight. The tube C is a simple funnel-tube, open near the 

 bottom of A. The tube D also reaches to the bottom of A, and acts as a 

 siphon : its longer limb is narrowed at the point, and delivers its water into 

 the little flask M, whose neck bears a mark m. The shorter limb of D 

 bears a cock E to regulate its discharge. The third tube F, which opens 

 immediately under the cork of A, is fastened by a caoutchouc joint to the 

 tube B. In this joint, and pressing the ends of both tubes, is a compact 

 mass of cotton-wool. B passes through the cork of the little test-tube G, 

 which is divided into millimetres, and contains the liquid through which the 

 hubbies are to pass. Through the cork of G another tube H is passed, 

 whose lower end h is bent out horizontally, and is beneath the surface of 

 the liquid in G ; H is connected by a caoutchouc joint with I, which 

 passes nearly to the bottom of a second little test-tube J. The tube J con- 

 tains a few drops of the liquid which is in G, and the space between I and 

 * On Drops, p. 478. 



