350 



WILLTAM 11AMSAY. 



of which the particles were too small to be visible under a microscope, 

 yet so large as to polarise a beam of light, was weighed in a pyknometer; 

 its density was also determined by weighing in it a large glass bulb of 

 60 c. es. capacity, made to sink by containing mercury. It was found 

 that the density of a solution of barium chloride, determined by both 

 thèse methods, gave practically the same results, as shown by the fol- 

 lowing table : 



Hydrostatic method. Pyknometer. A. 



I ], 02677 1,02683 0,00006 



II 1,02928 1,02931 0,00003 



The higher density is given by the pyknometer, but it amonnts to 

 only 4.5 parts in 100,000; and the différence is nndoubtedly to be 

 acconnted for by unavoidable expérimental error, such as the weight of 

 the immersed hair by which the sinker was suspended. The différence, 

 on substituting a colloidal solution of arsenious sulphide for the barium 

 chloride, is of the same order: 





Hydrostatic method. 



Pyknometer. 



A. 



I 



1,01187 



1,01192 



0,00005 



II 



1,02323 



1,02330 



0,00007 



III 



1,02272 



1,02276 



0,00004 



IY 



1,01129 



1,01134 



0,00005 



Hère A = 0,00005, again in favour of the pyknometer. It must hence 

 be concluded that such minute particles of arsenious sulphide, which are 

 in rapid pedetic motion, by tbeir impacts with the sinker are capable 

 of transmitting hydrostatic pressure. 



The question lias yet to be investigated ; at what stage of coarse- 

 grainedness do particles cease to transmit hydrostatic pressure ? It is 

 clear that such pressure dépends on pedetic motion, and that the problem 

 is quite différent from one in which the particles are large, and arediscen- 

 ding through the liquid, impelledby gravitational force. Such particles as 

 have been mentioned, which are présent in colloidal solutions, do not 

 settle, although as Picton and Linder have shown, so me varieties are re- 

 pelled from the négative pôle of a battery, provided it is placed at the 

 upper end of a tube containing the colloidal solution, and provided the 



