354 Lord Rayleigh on the 



very great pressure, darkened considerably, and at points 

 where the pressure was greatest it became absolutely black, 

 It did not appear, however, that any iodine was set free ; none 

 could be extracted w T ith alcohol. 



Mercuric oxy chloride, 2HgO, HgCl 2 . — Heavy pressure 

 caused much darkening. 



Sodium thiosulphate with a moderate pressure compresses 

 to a hard translucent cake, but does not undergo any decom- 

 position. 



To the foregoing cases are to be added those of the three 

 © © 



silver haloids described in a previous paper. These blackened 

 with a quite moderate pressure, even in the case of the iodide. 



Although in all these instances the darkening is well 

 marked, still the actual proportion of material affected is 

 small ; so that in many cases it is difficult to apply tests 

 to decide as to the precise nature of the substances formed. 

 In some cases, however, this can be done, and we are justified 

 in concluding that many of the salts of easily reducible 

 metals, especially of silver, mercury, and platinum, undergo 

 reduction by pressure. Such reactions are endothermic ; and 

 it therefore follows that mechanical force can bring about 

 reactions which require expenditure of energy, which energy is 

 supplied by mechanical force precisely in the same way that 

 light, heat, and electricity supply energy in the endothermic 

 changes which they bring about. 



In the second part of this paper additional support will be 

 adduced for these conclusions. 



XXXVIII. On the Flow of Viscous Liquids, especially in Tico 

 Dimensions. By Lord Rayleigh, Sec. U.S.* 



THE problems in fluid motion of which solutions have 

 hitherto been given relate for the most part to two 

 extreme conditions. In the first class the viscosity is sup- 

 posed to be sensible, but the motion is assumed to be so slow 

 that the terms involving the squares of the velocities may be 

 omitted ; in the second class the motion is not limited, but 

 viscosity is supposed to be absent or negligible. 



Special problems of the first class have been solved by 

 Stokes and other mathematicians ; and general theorems of 

 importance have been established by v. Helmholtzf and by 

 KortewegJ, relating to the laws of stead}' motion. Thus in 



* Communicated by the Author, 

 t Collected Works, i. p. 223. 

 | Phil. Mag. xvi. p. 112 (1883). 





