AN EXPERIMENT ON HYDROSTATIC PRESSURE 



BY 



WILLIAM EAMSAY. 



In treating of the phenomena of hydrostatic pressure, the problems 

 have generally been handled as if the fluid transmitting the pressure were 

 a continucum, that is a non-grained médium. But the molecular theory 

 of matter is now so firmly established, that there eau be no doubt that 

 hydrostatic pressure must be attributed to the impacts between molécu- 

 les of the fluid, with each other^and with the immersed body. 



Now where the grained-ness of the fluid is very fine, as with ordinary 

 water, this hypothesis does not admit of proof. But if matter be placed 

 in the fluid so coarse-grained as to be visible under high microscopic 

 power, yet so fine-grained as to be in incessant pedetic or Brownian 

 motion, it is an open question whether such fine particles should exert 

 hydrostatic pressure. The rapidity of the excursion of each particle, as 

 long ago shown by the author, is a function of its size, as also, up to a 

 certain point, of the température : and a colloidal solution, containing 

 such visible particles, may be left for months on the stage of a micros- 

 cope, in a cellar, without suffering any altération in the character or 

 amount of the motion of the suspended particles. The présence 

 of any substance which will render the liquid an electrolyte, as shown 

 by experiments conducted by Pïcton and Linder in the laboratory of 

 University Collège, at once causes coagulation of the particles, and préci- 

 pitation; Brownian motion then ceases. 



The simplest way of attacking the problem was to détermine the 

 density of such a colloidal solution by two methods, one of which involves 

 hydrostatic pressure; a colloidal solution of arsenious sulphide in water, 



