C. Barus — Compressibility of Colloids. 297 



The scheme is to be repeated on a smaller relative scale for 

 succeeding ruptures, and to be inverted for relief of pressure. 



11. Looking at these facts as a whole, I conclude that no true 

 compressibility (implying pure hydrostatic stress) has been 

 measured for the coagulated colloid. The results are merely 

 an evidence of compressibility and what is observed is a shear 

 made possible by this compressibility. Hence the change from 

 the very viscous fluid colloid to the coagulated (solid) colloid, 

 looked at from the point of view of viscosity, is also a profound 

 change in relation to compressibility. It is a change from 

 compressibility to incompressibility relatively speaking, a 

 change from the properties of the solvent liquid to those of 

 the colloidal solute, supposing the body to be taken a little 

 distant in temperature, either side of the melting point. Con- 

 tinuous colloid with liquid inclusions may therefore be inferred 

 for the coagulated state, whereas in the liquid state the colloid 

 is discontinuous. 



Carus-Wilson* has ingenuously interpreted the stress-strain 

 diagram of a metal particularly with reference to the yield 

 points which terminate in permanent set, by the aid of 

 Andrews' vapor-pressure diagram. The large abrupt yields of 

 low temperatures gradually vanish as temperature rises and the 

 metal becomes more plastic. Some such comparison might be 

 instituted here. 



32. What has impressed me as specially interesting in these 

 experiments with coagulated colloids is their possible bearing 

 on the dynamic manifestations of the ether. Following the 

 lead of well known great thinkers, the hypothesis which 

 attributes to the ether a jelly-like constitution, dynamically 

 speaking, is familiarly in vogue to-day. 



The phenomenon of the electric spark and the above experi- 

 ments on the breakdown of mechanical strain as evidenced by 

 the motion of the mercury projectiles are closely analogous. In 

 both cases, there is an originally continuous and, I will say 

 bluntly, a solid medium. When breakdown occurs there is in 

 each case motion into the continuous strained medium, through 

 the channel of broken down, discontinuous or triturated 

 medium left in the wake. Finally there is recementation 

 resulting in a new continuous medium. 



Now the point I wish to accentuate is this, that we may dis- 

 tinguish between the same solid jelly-like medium in the con- 

 tinuous and in the discontinuous or triturated state, in the 

 sense pointed out above ; that the former transmits stress like 

 a solid locally, showing rigidity, whereas the latter transmits it 

 like a liquid, and in proportion as the degree of discontinuity 

 is greater, virtually imparts hydrostatic stress. The same ether 



*C. A. Carus-Wilson, Phil. Mag. (5), xxix, p. 200, 1890. 



