230 PROTOPLASM 



not to be confused with it, even though they may rest upon similar 

 structural features. Such properties are contractility, rigidity, 

 cohesion, adhesion, tensile strength, toughness, stickiness 

 (tackiness), and glutinosity. It should be pointed out that 

 these properties, including elasticity, have no necessary relation- 

 ship to viscosity. Elasticity may interfere with the accurate 

 measurement of viscosity, but it does not rest upon the same 

 mechanism. A change in one may, but need not, involve a 

 corresponding change in the other. Glycerin is highly viscous 

 yet possesses no elastic qualities (other than bulk elasticity). 

 A very thin soap solution, with a viscosity value barely more 

 than that of water, may be quite elastic {i.e., extensile). Such 

 a solution also possesses rigidity — an unusual property to ascribe 

 to a liquid — but it, like elasticity, is a property of (certain) 

 liquids. 



Hatschek has called attention to the fact that if a soap solution 

 is quite thin, its elastic qualities (if it possesses any) may be 

 demonstrated by putting it into a flask which is then given a 

 quick turn, or swish; the solution will swirl around, come to a 

 rather sudden stop, and return a short distance. Had the liquid 

 been inelastic (in the sense of stretch and recovery), as are water 

 and glycerin, it would have come to a standstill slowly, without 

 showing any tendency to return. The elasticity of thin solutions 

 and at the same time their rigidity may be demonstrated in yet 

 another way. 



If a liquid possesses structural features that enable it to keep a 

 metal particle within it in suspension, this quality may be used 

 as a means of measuring the stretching capacity of the liquid. A 

 microscopic (15^) metal (nickel) particle is placed in the liquid, 

 e.g., a soap solution, with the aid of a micromanipulator. The 

 particle, as it remains in suspension, is then attracted by an 

 electromagnet (Fig. 35). If the particle, on release of the 

 magnetic field after having been drawn through the solution, 

 shows any return, whether wholly or in part, the solution is 

 elastic. A particle suspended in glycerin stops precisely where 

 it is when the magnetic field is eliminated, because glycerin is 

 inelastic; but in certain soap and other elastic solutions, the 

 particle returns. 



The elastic and rigid qualities of thin solutions are well illus- 

 trated by the behavior of soap solutions. A 0.1 per cent solution 



