140 'A /:'. Bttrbank — Lecture Experiment on 



Art. XII. — Lecture Experiment on Surface Tension and 

 Superficial Viscosity ; by John E. Burbank. 



While carrying on some experimental work in regard to the 

 motion of solids in water, the author made a very interesting 

 observation on the elastic properties of liquid surfaces, and as 

 no mention of this particular form of the experiment is to be 

 found in the text-books or magazines at hand, there is 

 presented here a brief description of the experiment for the 

 benefit of those who might wish to use it as a lecture-room 

 demonstration of the properties of liquid surfaces. No attempt 

 was made to take any quantitative measurements of any kind 

 as the experiment was considered merely qualitative. And no 

 figures are given, since any one can readily understand the 

 conditions without them. 



A ball of paraffin wax, 4'3 cms diameter, was made as nearly 

 spherical as possible, and a small mass of lead inserted in it so 

 as to keep it in a fixed position and prevent rotation as it 

 moved up or down in a mass of water ; by using different 

 amounts of lead it is possible to so adjust the weight of the ball 

 that it will move very slowly. If the ball and counterweight 

 be such that the mean specific gravity of the mass be a very 

 little less than that of water, it will rise with a velocity of from 

 two to ten centimeters per second. If the adjustment be 

 made so that the ball will have an upward velocity of about 

 4 cms per second, and then the ball submerged to a depth of about 

 40 cms in a jar of water and then allowed to rise, on reaching 

 the surface film of the water it raises the latter through a small 

 distance causing it to stretch ; the action of the film is to contract 

 and force the ball down into the water to a depth of about 10 cms . 

 In many cases the ball would rebound the second, and even 

 the third time, but generally the contamination of the water 

 film by the paraffin caused the surface to break after the second 

 rebound. If the ball be tried repeatedly in the same vessel of 

 water the surface soon becomes so contaminated that only 

 one rebound is observed. 



The force of gravity is not the whole cause of this rebound, 

 as we shall see when we consider the rebound of liquid spheres 

 under the same conditions. In order to show that the rebound 

 was chiefly due to the action of the film, the two following 

 tests were made : 



1. The height to which the ball rose at the instant of first 

 rebound was carefully noted, then the height to which the ball 

 rose after finally breaking the surface was found to be practi- 



