210 PROTOPLASM 



Methods. — Viscosity values may be specific, that is to say, 

 relative to a standard; or absolute. In the first case, water is the 

 customary standard and is taken as unity (at 0°C.). In the 

 second, the vahies are expressed in dyne seconds per square 

 centimeter, or poises. The absolute value is known as the coeffi- 

 cient of viscosity, with 77 as its symbol. The relative viscosity 

 value of glycerin is 830, which means that glycerin is 830 times 

 as viscous as water (at 20°C.). Several absolute values or 

 coefficients of viscosity are given in the following list : 



Water 0.018 at 0°C. 



Water 0.01 at 20°C. 



Olive oil . 84 at 20°C. 



Glycerin 8 . 30 at 20°C. 



Glucose 27,000.00 at 67°C. 



One may express the values in centipoises (1 centipoise is one 

 one hundredth of a poise). Inasmuch as the absolute viscosity 

 of water in centipoises at 20°C. is 1.005, then viscosity values 

 determined relative to water become absolute values in 

 centipoises. 



An instrument for measuring viscosity is known as a viscosim- 

 eter, or viscometer. There are numerous types, the simplest 

 being a glass capillary tube through which the liquid is allowed 

 to flow. The rate of flow is determined by a number of factors, 

 such as the fineness of the capillary and, naturally, the viscosity 

 of the liquid. These factors are all combined in a formula 

 developed by Poiseuille which will be considered shortly. 



In the nonliving world, ways of measuring viscosity involve 

 most often a capillary viscometer in which the rate of flow of 

 the liquid through a fine capillary is compared to that of a stand- 

 ard solution, usually water. The torsion method (with a revolv- 

 ing drum) of Couette is frequently used in the laboratory. 

 Commercially, the gravity method, in which balls are allowed to 

 fall or air bubbles allowed to rise in the unknown and standard 

 liquids, is common practice. But of greater interest to us are 

 ways of measuring protoplasmic consistency. Should the 

 physicist find the methods crude and the values inexact, let him 

 remember that it is living matter which is being investigated, 

 that it must remain alive, and that it can usually be had only in 

 microscopic droplets. The rheologist, in particular, should also 



