294 



C. Barus — Compressibility of Colloids. 



9. At first sight these phenomena seem to be of a capillary 

 nature. Drops of oil break off in Plateau's well known experi- 

 ments more slowly but otherwise in much the same way. The 

 tendency to jerky motion and much else suggests the action of 

 surface tension. 



A moment's reflection shows, however, that surface tension 

 cannot be the primary motive force,* since droplets of mercury 

 often less than *01 cm in diameter are to be projected through a 

 coagulated (solid) colloid, a distance of 10 cm or more. Again 

 these projectiles often start afresh in their motion (cf. Table 6), 

 when many centimeters above the meniscus. In these fine bore 

 tubes (diameter '031 c,u ) capillarity necessarily plays an impor- 

 tant part; but it is of entirely subordinate interest and I shall 

 not further mention it. 



The real phenomena is elastic in character. The original 

 meniscus in figure 1 loaded with a uniform pressure upward 

 from below, is deformed in accordance with a shear symmetri- 

 cally around the axis of the tube. The colloidal meniscus 

 yields very much as any elastic disc secured at the edges and 

 uniformly loaded would do. It is strongest at the edges, which 

 are sustained by the glass walls of the capillary throughout the 

 length of the colloidal column, and weakest at its axial points. 

 As pressure increases the strain gradually reaches the limit of 



narrow dissepiment of colloid into the drop, replenishing it in substance but giv- 

 ing it the appearance of a grape cluster with apex upward. 



(5) Further projectiles shoot off from the drop. They induce the 4 cm group to 

 penetrate farther upward. The top group is stationary. 



(6) Meniscus shouting into the drop at its base : the drop discharging many 

 projectiles from its apex. They induce motion in the preceding group (without 

 reaching them), which in their turn actuate the earlier group until finally the 

 upper (original) group moves nearly to the top of the column. Various projec- 

 tiles have coalesced. Distribution of projectiles from the meniscus upward is 

 now as follows : 



Meniscus -00 cm Projectile., 5'73 cm Projectile 1341 cm 



"Drop," clustered.. -21 '• . 6 93 " 14-38 



Projectile -38 " .. 7'91 <; 14-56 



" ■« " - ™ gsgs. \ "■« 



3-29 " .. 8-52 



3.33 " ..1013 Top . !■ 16-13 



meniscus ) 



_ _ 3-73 " ..11-32 



" 3 90 " ..12-40 



(7) Further projectiles shot off from drop, which is now smaller but still in place. 

 Top projectiles stationary. 



(8) After the tube breaks retrograde motion of the projectiles is observed, 

 often covering 4 cm . The experiment has been spoiled by the accident, however, 

 release of pressure being too sudden. Next day the projectiles are found to have 

 aggregated in clusters of 2-6, irregularly along the unbroken column. Some 

 have reached the meniscus. The drop is gone. 



* The relevant formulas are given in my paper in this Journal, III, xxxvii, p. 

 339, 1889. 



