OP ARTS AND SCIENCES. 15 



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 experiments is in this case very fully given.* 



The marine glue, § 2, was forced out of a sufficiently large reser- 

 voir, through tubes of steel about 10 cm. long, and 0.5 to 1cm. in 

 diameter. Pressures as high as 2,000 atm. were applied at the 

 reservoir, by aid of my screw compressor.! Temperatures between 

 10° and 30° were kept constant by a suitable water bath. Through- 

 out the work the flow was so excessively slow (amounting to an 

 advance of only a few millimeters per hour), that Poiseuille's law 

 was at once applicable. The only considerable 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 surface, it follows that the flow is most marked at the axis 

 of the tube compatibly with the theory of the experiment. 



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 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. 

 The restitution of volume is greatest in the axis of the cylinder 

 where the flow is a maximum, and where the matter has been 

 crowded into the smallest space. As a whole, the experiment is 

 somewhat puzzling, for it points to the occurrence of a notable 

 amount of slowly reacting elasticity even in this truly viscous 

 solid. Indeed, as time went on, a re-entrant conoid, correspond- 

 ing to the projecting cone just described, gradually dimpled the 

 second of the two surfaces of section. What is here indicated, 

 therefore, is probably a surface of flow. 



7. Viscosity and Pressure. Isotherms. — Table I. gives a sum- 

 mary of my chief results. The table is one of double entry, and 

 the data contained show the absolute viscosity (77) of marine glue 

 at the stated temperatures and pressures, in terms of one billion 



* In addition to the well known work of Poiseuille, of. Hagenbach, Pogg. 

 Ann., Vol. CIX. p. 385, 1860; Osborne Reynolds, Phil. Trans., Vol. III., 1883, 

 p. 935 ; Wilberforce, Phil. Mag. (5), Vol. XXXI. p. 407, 1891. 



t These Proceedings, Vol. XXV. p. 93, 1890. 



