286 C. Bar us — Compressibility of Colloids. 



large part of the present paper will be devoted to them. I 

 say astonishing, as J have been able to force little mercury pro- 

 jectiles, often less than T ^ m,n in diameter, though a solid wall 

 of coagulated jelly over 15 cm thick, by the directed action of 

 hydrostatic pressure applied on the outside of the wall. I 

 know of no other case of well-defined persistent motion, per- 

 formed by bodies in a highly viscous medium, and resulting 

 from the breakdown of mechanical stress within it, wholly 

 without the action of external forces " acting at a distance." 



2. The earlier literature of the subject is meagre and I may 

 refer regarding it to a paper by G-. de'Metz,* to whom some of 

 the best data are due. He accentuates the variable character 

 of the constants observed and certain changes (gelatine) in the 

 lapse of time. Papers like those of Fraas,f Maurer,J and 

 others quoted by the former, refer to the elastics and to the 

 viscosity of colloids, in a way differing from that here con- 

 sidered. 



I do not in the present paper aim at reaching more than an 

 estimate as to how far the elastic properties of a solvent are 

 modified, when it is made thoroughly viscous by the addition 

 of a suitable quantity of colloid. I have therefore subjected 

 the bodies to pressure in capillary tubes, a method which 

 though it does not admit of the application of very high pres- 

 sure, has the undeniable advantage of exhibiting the progress 

 of the experiment to the eye throughout. I have already 

 described the precaution necessary in a former paper§ in con- 

 nection with an extensive survey of the compressibility of 

 liquids, and need therefore only siate here that the body to be 

 examined is introduced into a well annealed capillary tube of 

 fine bore between two terminal threads of mercury. One of 

 these (the upper) is sealed in place ; the lower is movable and 

 transmits the applied pressure. The lower meniscus of the 

 upper and the upper meniscus of the lower thread are observed 

 by aid of a cathetometer, through a clear glass boiling tube, of 

 the kind frequently described in my high temperature work.| 



3. It is sufficient for the present purposes to examine two 

 classes of colloids, representing extremes of compressibility. 

 In the first case of low compressibility water is to be the sol- 

 vent and gelatine and albumen were selected. In the second 

 case (high compressibility) ether is the preferable solvent and 

 solutions of pure india rubber are thus available. 



A solution of 10 per cent by weight of clear gelatine in 

 water sets quite firmly at ordinary temperatures and is not too 



* G. de Metz, Wied. Ann., xli, p. 663, 1890. 



f Fraas, Wied. Ann., liii, p. 1074, 1894. 



% Maurer, Wied. Ann., xxviii, p. 628, 1886. 



§ This Journal, III, xxxix, p. 478, 1890. 



|| Cf. Bull. 54, p. 88, U. S. Geolog. Survey, 1889. 



