682 



SCIENCE. 



[N. S. Vol. VIII. No. 203. 



the relatively enormous viscous resistance 

 of the coagulated colloid. The motion of 

 the projectile is extremely rapid at first 

 (say several met./ sec, or more), but grad- 

 ually slows down, until after 5-10 minutes it 

 has been reduced to the merest creeping 

 visible in the telescope. This phenomenon 



is repeated on increasing the pressure, but 

 even at the same pressure many projectiles 

 may be successively shot off to be distrib- 

 uted along the axis of the column. Later 

 projectiles frequently actuate the earlier 

 ones to renewed motion (5-10 cm.) without 

 touching them and even after the latter 

 have come to rest. 



An interesting case is the drop of mer- 

 cury purposely broken off ahead of the 

 meniscus, as shown in Fig. 4, while the col- 

 loid was yet liquid. After thorough co- 

 agulation the effect of pressure is shown in 

 Fig. 5. The drop soon takes a conoidal 

 form,* and thereafter shoots off projectiles 

 of mercury from its apex, being continually 

 replenished by bombardment from the me- 

 niscus below, changing form but remaining 



*The curvature due to stress is, of course, superim- 

 posed on the curvatiire of the globule. 



in place. On another occasion, however, 

 the drop deliberately exploded, being there- 

 after represented by some dozen small pro- 

 jectiles distributed through the lower 10 cm. 

 of the colloidal column. The top meniscus 

 neither moves nor changes form. 



On gradually removing pressure the ex- 

 periment is reversed, i. e., the projectiles 

 move in a somewhat similar manner back 

 toward the (lower) meniscus out of which 

 they originated. But the march back is 

 not complete, showing that much energy' ia 

 ■wasted in virtue of viscosity. 



Experiment explained. — My explanation of 

 these occurrences is as follows : The phe- 

 nomenon throughout is elastic in character. 

 The lower end of the colloidal column, 

 loaded with a uniformly distributed pres- 

 sure, yields like an elastic disc — least at the 

 edges where it is sustained by the walls of 

 the capillary tube and most near the center ; 

 for the jelly is not quite incompressible 

 (compressibility of the order of about 10"" 

 per atmosphere). The column is telescopic- 

 ally sheared, so to speak, and gives way 

 symmetrically with respect of the axis. 

 When stress exceeds the limits of rupture 

 the strain breaks down, as indicated by the 

 motion of the mercury projectile. 



Consider the intrusion of the mercury 

 globule for a moment ; ahead of it there is 

 always the continuous overstrained solid 

 colloid ; behind it discontinuous or triturated 

 colloid, the debris of the original continu- 

 ous column. The former transmits stress 

 like a solid, locally showing definite rigidity; 

 the latter transmits hydrostatic stress. So 

 the projectile is pushed forward hy rear-end 

 pressure communicated by the mercury, but 

 pushed forward at a gradually retarded rate; 

 for, though the intensely viscous quasi- 

 liquid not exactly 



" • -drags with each remove a lengthening chain," 



it must certainly make its way through an 

 ever-lengthening channel, which eventually, 



