Solutions of Saponin, Peptone, and Albumin. 563 



as to allow the levelling-screws of the base to rest indepen- 

 dently on the bench. The front of the box consisted of a 

 glass door, which did not extend down to the bottom of the 

 box, a small space being left through which projected the 

 radial arm attached to the turntable. A few small vessels 

 containing calcium chloride were placed on shelves inside 

 the box in order to prevent the condensation of moisture on 

 the surface. The oscillations were observed by means of a 

 telescope placed just outside the box, and focussed on the 

 reflexion of the filament of an incandescent electric lamp in 

 the mirror attached to the suspended system. 



The accuracy of the measurements was limited by the fact 

 that the oscillations damped down rapidly. Within limits, 

 the larger the initial amplitude, the more oscillations could 

 be counted. If, however, the initial amplitude was too big 

 the elasticity of the film was impaired and the disk would 

 scarcely oscillate at all. Considerable practice was necessary 

 before one could always obtain as large an initial amplitude 

 as possible without injury to the surface-film. By having as 

 large initial amplitudes as possible, much greater accuracy 

 was obtained than in my previous experiments. 



A reading from which a value of the surface-elasticity was 

 calculated generally consisted of ten observations of as many 

 oscillations as could be obtained, the times being added up 

 on the stop-watch used (which was of the progressive type — 

 starting again from the point at which it was previously 

 stopped). In many cases the time of the first five oscillations 

 was noted, at first as a check on the accuracy of the observa- 

 tions, and later because it was found that sometimes the 

 period of oscillation increased during the taking of a reading. 

 The significance of this decrease of surface-elasticity will be 

 discussed later. 



As each experiment lasted several weeks six sets of 

 apparatus were used in order to save time. They were all 

 made as nearly as possible similar in every respect — the 

 approximate dimensions of the different parts being as 

 follows : — 



diameter of vessel=15'5 cm. 

 diameter of disk = 7*1 cm. 

 moment of inertia of suspended system = 1950 gr. cm. 2 



More accurate measurements gave for the different sets of 

 apparatus values of the apparatus constant varying from 

 380 to 385 gr. 



2Q2 



