374 Dr. N. Ernest Dorsey on the Surface- Tension of 



dilated values for the wave-length. This applies to all 

 measurements made by them. Most of their observations 

 were made on the interference-waves produced on the surface 

 of a liquid when a vibrating fork is supported vertically above 

 it so that two needles, one attached to either prong, may dip 

 into the liquid. This gives two series of circular waves, and 

 so the second condition mentioned above is not fulfilled. I do 

 not know how much this would affect the results ; but until 

 it is proved to have no effect the results must be considered 

 nugatory. Riess "* says : — " Uer Schliissel fiir die definitive 

 Losung ist in einer minutiosen Messung der Amplituden zu 

 suchen ; " and he goes on to say that the only sure method is 

 an optical or photographic one. Matthiessen found that the 

 formula gives results in accord with his observations except 

 in the neighbourhood of the minimum value of the velocity. 



C. M. Smith f employed this method for determining the 

 surface-tension of mercury, but obtained very erratic results. 

 Lord Rayleigh J thinks he obtained waves due to the sub- 

 octave of his fork, and these are almost sure to have too large 

 an amplitude. 



W. Ochse § attempted to determine the surface-tension of 

 solutions by means of ripples. He measured the interference- 

 pattern of two series of circular waves, as Riess and Matthiessen 

 had done ; his waves had a large amplitude, he assumed that 

 the density did not enter into the equation, and he used rather 

 concentrated solutions. His measurements of the wave-lengths 

 are quite rough, so that the results have but little value even 

 when corrected for the density. 



From this it is seen that the formula has not been disproved ; 

 and the fact that Lord Rayleigh obtained the same value for 

 the surface-tension of water with two forks of different 

 frequencies, and that the result obtained for water in this 

 work agrees within his experimental error with Lord Ray- 

 leigh's, seems to justify the application of Lord Kelvin's 

 formula to these two cases. 



All the work so far described has been with waves easily 

 visible, and these very probably have an amplitude too large 

 to allow of the application of Lord Kelvin's formula. Lord 

 Rayleigh was the first to use waves invisible under ordinary 

 conditions. His method was to have the waves generated by 

 means of a plate of glass which was attached to the lower 



* Exner's Rep. der Phys. xxvi. p. 131 (1890). 



f Proe. Roy. Soc. Edinb. xvii. p. 115. 



\ Phil. Mag. [5] xxx. p. 386 (1890). 



§ Exner's Rep. der Phys. xxvi. p. 641 (1890). 



