216 PROTOPLASM 



The magnetic force (as measured in terms of amperes of current 

 applied to the electromagnet) necessary to set the metal particle 

 in motion and the rate of movement of the particle while it is 

 being pulled through the protoplasm by the magnet are measures 

 of the viscosity of protoplasm. The method substitutes an 

 electromagnetic force for that of gravity in the fall method and 

 that of a centrifugal force in the centrifuge method. The metal 

 (iron or nickel) particles, if very small (7 m), must be inserted 

 with the aid of a micromanipulator (Fig. 37). The results 

 are only fairly accurate, for the experimental error is great. The 

 method is of interest chiefly because of the novelty involved in 

 getting a minute metal particle freely suspended in living proto- 

 plasm and then attracting it with an electromagnet. 



The Microdissection Method. — The determination of the 

 consistency of protoplasm by the manipulation of glass dissecting 

 needles is comparable to judging the consistency of a liquid by 

 watching some one stir it with a stick. In a sense, no method 

 could be more crude, yet it is possible thus readily to distinguish 

 differences in consistency between, for example, water, a thin 

 oil, bread dough, and butter. It requires but little practice, by 

 actual experimental test with solutions of differing concentrations 

 of gelatin, to distinguish in the same way but with the use of a 

 very fine glass needle, 10 viscosity values, from that of water 

 to that of a firm jelly. Such a scale of values, determined by 

 microdissection, has been established for protoplasm. The 

 criteria by which one judges are the distance from a moving 

 needle at which particles are disturbed and the length of time 

 that deformations maintain their shape. For example, a furrow 

 made across an Amoeba may close slowly, indicating high vis- 

 cosity; or quickly, indicating low viscosity. 



The simpler and more direct way of doing a thing, even though 

 crude, may be the more exact one in the end. The application 

 of physical laws and mathematical formulas leads to greater 

 accuracy -provided the laws and formulas hold for the intricate 

 system being investigated, as they do for the simpler systems 

 on which they were founded. The great difficulty with proto- 

 plasm, which so often upsets measurements of its physical 

 properties, is its lack of homogeneity. Most methods for deter- 

 mining viscosity are based on the assumption that the material 

 is homogeneous, which is never true of protoplasm. In this 



