1879.] 



the Capillary Phenomena of Jets. 



89 



manner that the eye sees only the single point of each, which is prin- 

 cipally illuminated. Hence, when the stream is illuminated by the 

 electric spark, the swellings appear like a string of pearls ; but their 

 form cannot be recognised, because the intensity of the light reflected 

 from the remaining portions of the masses is too small to allow this, 

 on account of the velocity with which the impression is lost."* The 

 electric spark had, however, been used successfully for this purpose 

 some years before by Buff,f who observed the shadow of the jet on a 

 white screen. Preferable to an opaque screen in my experience is a 

 piece of ground glass, which allows the shadow to be examined from the 

 further side. I have found also that the jet may be very well ob- 

 served directly, if the illumination is properly managed. For this 

 purpose it is necessary to place the jet between the source of light 

 and the eye. The best effect is obtained when the light of the spark 

 is somewhat diffused by being passed (for example) through a piece 

 of ground glass. 



The spark ' may be obtained from the secondary of an induction 

 coil, whose terminals are in connexion with the coatings of a Leyden 

 jar. By adjustment of the contact breaker the series of sparks may 

 be made to fit more or less perfectly with the formation of the drops. 

 A still greater improvement may be effected by using an electri- 

 cally maintained fork, which performs the double office of controlling 

 the resolution of the jet and of interrupting the primary current of 

 the induction coil. In this form the experiment is one of remarkable 

 beauty. The jet, illuminated only in one phase of transformation, 

 appears almost perfectly steady, and may be examined at leisure. 

 The fork that I used had a frequency of 128, and communicated its 

 vibration to the reservoir through the table on which both were 

 placed without any special provision for the purpose. The only weak 

 point in the arrangement was the rather feeble character of the 

 sparks, depending probably upon the use of an induction coil too 

 large for the rate of intermittence. A change in the phase under 

 observation could be effected by pressing slightly upon the reservoir, 

 whereby the vibration communicated was rendered more or less 

 intense. 



The jet issued horizontally from an orifice of about half a centi- 

 metre in diameter, and almost immediately assumed a rippled out- 

 line. The gradually increasing amplitude of the disturbance, the 

 formation of the elongated ligament, and the subsequent trans- 

 formation of the ligament into a spherule, could be examined 

 with ease. In consequence of the transformation being in a more 

 advanced stage at the forward than at the hinder end, the liga- 

 ment remains for a moment connected with the mass behind, when 



* "Phil. Mag.," xviii, 1859, p. 172. 

 f " Liebig's Ann.," lxxviii, 1851. 



