292 



C. Barus — Compressibility of Colloids. 



When pressure continually increases (very gradually), the 

 meniscus in 1 passes into the conoidal form 2, thence into the 

 unstable figure 3, from which presently a drop is shot off, 

 upward. This exceedingly minute projectile may penetrate 

 the whole column of coagulated colloid, VY An or even 20 cm long. 

 The phenomenon repeats itself at consecutive intervals even at 

 constant pressures. 



Figs. 1, 2, 3, 4. Deformations observed on compression of a coagulated 

 colloidal thread, by a mercury meniscus advancing from below. 



With the 10 per cent gelatine coagulum, on one occasion, I 

 counted twelve of these little projectiles (on another thirteen, 

 etc.), each less than T \ mm. in diameter and disposed at regular 

 intervals, i. e. nearly equidistant in the axis of the column of 

 coagulated (solid) colloid. The top one rose 12 cm through the 

 medium and against gravity, the lowest about l cm above the 

 meniscus. On removing pressure five of them gradually dropped 

 back upon the meniscus, which in the telescope soon resembled 

 a bunch of silver grapes. When the tube is allowed to stand 

 vertically over night with pressure removed, all drops often 

 fall out of the colloidal column before morning. This motion 

 of droplets, up or down, is equally evident to the naked eye. 



At other times and particularly with older or abused col- 

 umns, balloon-shaped projectiles even -5 mra high break off and 

 walk up leisurely through the colloid, say at a rate of 2 cm/8ec . 

 The motion on close inspection is apt to be jerky. In such 

 cases, a trail of exceedingly small droplets, scarcely •02 mm in 

 diameter, is apt to be seen in the axis of the tube, where there 

 was no such trail before, or even in advance of the large drop. 



The phenomenon is best seen after slow cooling of the 

 thread. On change of pressure slow creeping of drops across 

 the crosshairs of the telescope is a frequent occurrence. Irregu- 

 lar dispositions of the drops were also often observed. 



To further elucidate these phenomena additional experi- 



