416 On the Experimental Relations [1857. 



light transmitted is either blue or ruby, or of other inter- 

 mediate tint, according to the character of the deposit ; but 

 if water be then added until the column is six inches or 

 more in length, the quantity of light transmitted does not 

 sensibly alter, nor its tint; a fact, which I think excludes the 

 idea of any light being reflected from particle to particle, and 

 finally to the eye. 



If a given ruby-tinted fluid, containing no gold in solution, 

 be allowed to stand for a few days, a deposit will fall from 

 which the fluid may be removed by a siphon ; being now 

 allowed to stand for a week, a second deposit will be produced ; 

 if the fluid be again removed and allowed to stand for some 

 months, another deposit will be obtained, and the fluid will 

 probably be of a bright ruby ; if it be now allowed to stand for 

 several months, it will still yield a deposit, looking, however, 

 more like a ruby fluid than a collection of fine particles at the 

 bottom of the fluid, whilst traces of yet finer particles of gold 

 in suspension may be obtained by the lens. All these deposits 

 may be washed with water and will settle again ; the coarser 

 are not much affected, but the finer are, and tend to aggregate ; 

 nevertheless specimens often occur, especially after boiling, 

 which tend to preserve their fine character after washing, if 

 the water be very clean and pure. 



The colour of these particles whilst under, or diffused 

 through water, is by common reflected light brown, paler and 

 richer, sometimes tending to yellow, and sometimes to red. 

 The same difference is shown when illuminated by sunlight. 

 Everything tends to show that the light reflected is very 

 bright considering the size of the particles, and therefore of 

 the reflecting surfaces ; yet comparing by the cone of light a 

 ruby fluid when first prepared and before it has become very 

 sensibly turbid, with the same fluid after the evident turbidity 

 is produced, in both of which cases I believe the gold to be in 

 solid metallic particles, though of different sizes, it would seem 

 that more light is transmitted and absorbed and less reflected 

 by the finer particles than by the coarser set, the same quantity 

 of gold being in the same space. I believe that there may be 

 particles so fine as to reflect very little light indeed, that 

 function being almost gone. Occasionally some of the fluids 

 containing the very finest particles in suspension, when illu- 



