Isometrics relative to Viscosity. 



89 



which the results are obtained ; for one is only too apt to at- 

 tribute an absence of flow to the effect of pressure on viscosity, 

 when the real cause is to be found 

 in the geometry of the apparatus 

 employed. I have therefore availed 

 myself of transpiration methods, 

 since the theory of the experi- 

 ments is in this case very fully 

 given. 



The marine glue, §2, was forced 

 out of a sufficiently large reservoir, 

 through tubes of steel about 10 cm 

 long, and 0*5 to l cm in diameter, cf. 

 figure 1. Pressures as high as 

 2,000 atm. were applied at the 

 reservoir, by aid of my screw com- 

 pressor.* Temperatures between 

 10° and 30° were kept constant by 

 a suitable water bath. Through- 

 out the work the flow was so ex- 

 cessively slow (amounting to an 

 advance of only a few millimeters 

 per hour), that Poiseuille's law was 

 at once applicable. The only con- 

 siderable source of error in the 

 work is the occurrence of more or 

 less incidental slipping. However, 

 inasmuch as the outflow of marine 

 glue is capped by a rounded sur- 

 face, it follows that the flow is 



f 1 rl f t"K ' J? i]. Figure 1. — Sectional elevation of 



most marKeCt at tne axis OI tne the lhgh pressure transpiration appa- 

 tllbe compatibly with the theory of ratus. Scale $,E, steel reservoir; D, 

 the experiment. Methods of ch'arg- steel transpiration tube ; C, charge ; 



W Tnaninnlfltinn ptr» must Wp R movable solid disc; 4, compressed 

 ing, manipulation, etc., must neie oil foreed in through the tube F. 



be omitted. 



6. Volume Viscosity. — At the end of stated intervals of 



time (usually hours), the small cylinders of marine glue which 



had exuded were cut off in the plane of the top of the tube 



with a sharp knife, and weighed. Now it was curious to note 



that these cylinders, left to themselves for about a day, showed 



a gradual and marked deformation, such that the originally 



plane bottom or surface of section eventually expanded into a 



symmetrical projecting conoid, with an acute apex angle of 



less than 45°. I take this to be an example of volume viscosity, 



inasmuch as an expansion gradually increasing at a retarded 



••Phil. Mag., V, xxx, p. 338, 1890. 



