14 PHYSICAL CHEMISTRY 



reading but requires a larger sample. One per cent of protein 

 raises the refractive index .001 per cent. 



The viscosity of the solution is affected very little by neutral 

 salts but very much by emulsion colloids. Not only does the 

 concentration of the emulsoid change the viscosity, but (at the 

 same concentration) the colloidal state affects it. Apparently, 

 the greater the hydration of the colloid the greater the viscosity. 

 The viscosity of a solution is measured by the time required for it 

 to flow through a capillary tube divided by the time required by 

 pure water to flow through the tube at the same temperature. 



The rotation of the plane of polarized light may be used to 

 find the concentration of an asymmetric solute in a mixed solu- 

 tion since only molecules having an asymmetric atom, usually 

 carbon, have this property. Rotation is measured with a polari- 

 meter. Colloidal solutions may polarize light dispersed by the 

 particles (Tyndall effect) and gels polarize light when deformed, 

 and are then called doubly refractive or anisotropic. 



The surface tension may be used as an index of the concentra- 

 tion of certain solutes. Neutral inorganic salts have very little 

 effect on the surface tension, but anesthetics have a marked 

 effect. The surface tension is found by dividing the weight of 

 a drop of the solution by the weight of a drop of water dropped 

 from the same pipette. 



The electrical conductivity is much used to find the concentra- 

 tion of ions, but it depends, not only on the number, but also 

 on the speed of the ions, and hence on the viscosity of the solu- 

 tion. The greater the hydration of an ion the less is its speed, 

 hence the speed is supposed by some writers to decrease with 

 decrease in concentration until maximal hydration occurs. When 

 such a dilution is reached, further dilution evolves no heat, and 

 the solution is called "dilute." 



The color of a solution is determined qualitatively with the 

 spectroscope and quantitatively with the colorimeter. A change 

 in the quality of the color is supposed by Stieglitz to be due to 

 change in the positions of electrons within the molecule, follow- 

 ing an upsetting of the equilibrium by addition or removal of 

 atomic groups. It seems probable that a change in the degree 

 of hydration may lead to a change in color. The color of a sol 

 may be due to the size and refractivity of the colloid particles. 



