74 



SCIENCE 



[N. S. Vol. XLIX. No. 1255 



density (dense enough, to float glass). Prob- 

 ably since tlie internal friction of liquids 

 vanishes with the relative velocity of layers, 

 and since the apparent motions are magnified, 

 there are eventually no frictional torques left 

 to absorb whatever angular momenta may be 

 renewed. The occurrence of direct and retro- 

 grade motions at the same time, separated 

 sharply by a plane of demarcation suggests 

 vortices. Above this plane particles move with 

 about the same speed in one direction, below 

 the plane with a very different speed in the 

 opposite direction. A particle which happens 

 to be in the plane in question does not move 

 at all. After a long interval the direction of 

 the motions above and below a plane of demar- 

 cation may be found to have reversed, respec- 

 tively. If a solution is cleared of particles by 

 the lapse of a sufficient time for subsidence, 

 they may be restored by brisk rotation. The 

 number, size, density of color and speed of the 

 particle naturally increase with the violence 

 of rotation. 



To conclude: After the cessation of the 

 initial disturbances, the liquid, left to itself and 

 owing to the presence of motes, shows a per- 

 sistent motion of its middle layers in the gen- 

 eral direction of the impinging beam of light, 

 while the motion of the relatively thin layers 

 at the top or bottom (one or both) is usually 

 persistently retrograde, but slow in compari- 

 son. This continues, imitil after the lapse of 

 hours the particles have practically subsided, 

 when the retrograde motion seems to be equally 

 prominent. Even when the liquid is manually 

 rotated clockwise with violence, this impressed 

 motion ceases in a few minutes whereupon the 

 counterclockwise, red-blue motion, in the di- 

 rection of the impinging beam, sets in vigor- 

 ously. 



It suffices to add a few statistical remarks. 

 The telescope may be adapted for small dis- 

 tance by placing 3 diopter spectacle lenses in 

 front of it. Its external focal plane is then 

 only about a foot off and within the liquid. 

 The rays seen in the ocular of the spectro- 

 telescope may be regarded as coming from a 

 virtual slit within the cylinder ; or else, on nar- 

 rowing the incident beam L to within a centi- 



meter (in case of a cylinder 10 centimeter in 

 diameter), the diffuse internal caustic has al- 

 ready been similarly narrowed down to a short 

 internal spectrum rh in the figure. Hence if 

 the solution rotates slowly about the axis a, 

 particles enter the red (r), and leave the blue 

 (b) end, and are therefore seen sharply in the 

 spectrum travelling from red to violet. The re- 

 verse is the case if G rotates in the clockwise 

 sense. The small distance rh is the virtually 

 magnified by the immense dispersion of the 

 grating (15,000 lines to inch) O. Since the 

 rays cross within the cylinder, G, the motion 

 from red to blue will also characterize all par- 

 ticles distinctly seen and rotating counter- 

 clockwise. Finally, this rotation corresponds 

 in a general way with the direction of advance 

 of the light transmitted through the cylinder. 

 It would be simplest to refer the cause of 

 persistence to a case of voi'tical motion in the 

 wake of the beam of light traversing the solu- 

 tion. But the invariable occurrence, in the 

 lapse of time, of motion in the middle layers 

 of the liquid in the direction of the imping- 

 ing light, no matter how the liquid is arti- 

 ficially rotated in the beginning, leaves this 

 explanation unsatisfactory. Such ■ vortices 

 would not be orderly and persistently equiva- 

 lent to the effect of a pressure in the direction 

 and of the beam of light. In case of a black 

 body and a solar constant of 3 gram-colories 

 per minute, the energy per unit of volume or 

 the light pressure in question may be roughly 

 estimated at 7 X 10"^ dynes per square cm. 

 Even if but a part of the energy is absorbed 

 by the liquid, this is by no means an insignifi- 

 cant pressure in a medium whose internal 

 friction vanishes with its motion. In fact if 

 the given estimate be treated as a tangential 

 force relative to the surrounding dark liquid, 

 of about .01 viscosity, a speed of 7 X 10~^ 

 cm,/sec. (under normal conditions) would cor- 

 respond to the shear. One may therefore infer 

 that speeds within a tenth millimeter per sec- 

 ond, about of the order observed, are not im- 

 possible. The very slow but persistent regres- 

 sive movement at the top and bottom of the 

 layer of liquid remains unexplained. Further- 

 more I was unable to find any adequate corre- 



