TRANSACTIONS OF SECTION A. 529 



The following Papers wore read :— 



1. On Eladic Fatiyue, as shotvu by Metals and Woodn. 

 By Professor A. Gray, F.R.S., J. S. Dunlop, and A. Wood. 



2. The Clearing of Turbid Solutions, and the Movement of Small Sus- 

 pended Particles hy the hijiuence of Light. By Professor G. Quincke, 

 of Heidelberg.— ^e% Reports, p. 60. 



3. On the Relation between TemperatiLve and Internal Viscosities of Solids. 

 By Professor A. Gray, F.R.S. 



4. Note on Hydrostatic Pressure. 

 By W. Ramsay, F.R.S., and G. Senter, B.Sc. 



The problem of hydrostatic pressure has usually been treated as if the 

 liquid, in which the floating solid is immersed, were a co7itimcum. According 

 to the molecular theory, however, all liquids must be regarded as consisting of 

 discrete particles, moving among each other freely. Accepting this view, hydro- 

 static pressure must be attributed to the bombardment of the immersed body by 

 molecules, or perhaps by congeries of molecules ; and the kinetic energy_ of the 

 molecules must be capable of transmission from those parts of the fluid which are 

 not in contact with the solid to those which are in contact, in such a manner that 

 the lower portions of the immersed solid are exposed to greater pressure than the 

 upper, due to the kinetic energy of all portions of fluid at a higher level than the 

 lower portions, and at a lower level than the upper portions. 



Picton and Linder, working in the laboratory of University College, showed 

 that colloidal solutions can be prepared of various degrees of fineness of the sus- 

 pended particles; some solutions were prepared in which the particles were 

 distinctly visible with high microscopic magnification, while others contained 

 particles in such a minute state of subdivision that even under the highest power 

 of a microscope, the colloidal solution appeared homogeneous, and the particles 

 were too fine to polarise a beam of light by reflection. Between these two 

 extremes intermediate grades were successfully made ; while the particles of 

 solid in such 'solutions' as contained visible solid were in rapid pedetic 

 (Brownian) motion, a particular grade of 'solution' was prepared, in which, 

 although the particles were too small to be visible, they reveded their presence by 

 polarising'- light ; and under the microscope an appearance of confused motion 

 impressed itself on the eye ; it seemed as though the particles were in such rapid 

 motion that they did not stay in focus long enough to create a permanent visual 

 impression. 



The questions arose : do such pa.rticles produce hydrostatic pressure ? is that 

 pressure equal to the theoretical pressure which would bo produced by a solution 

 of the same density ? at what stage of subdivision of the solid does such hydro- 

 static pressure become apparent ':! 



An attempt has been made to answer the first two of these questions, and with 

 fair success. The investigation will be continued in the hope of tinduig an answer 

 to the third question. 



The colloidal solution selected was one of arsenious sulphide in pure water. 

 Such a solution is easily prepared by passing a current of sulphuretted hydrogen 

 through an aqueous solution of arsenious acid to saturation, and then expelling 

 excess of hydrogen sulphide by a current of hydrogen for several hours. The 

 density of such a solution was determined in two ways: first by means of a 

 Sprengel's pyknometer ; and second by weighing in the solution a large cylinder of 

 glass (65 c.c), weighted with mercury, so as to make it sink. Corrections were 

 introduced for reduction to weighing ia vacuo, and for temperature. 



