of Gold and other Metals to Light. 523 
blue, which by washing with water, and especially with a little 
nitric acid, was much restored towards ruby. These changes 
may be due to an affection of the surface, or that which may be 
considered the surface of the particles. 
The state of division of these particles must be extreme; 
they have not as yet been seen by any power of the microscope. 
Whether those that are ruby have their colour dependent upon 
a particular degree of division, or generally upon their being 
under a certain size, or whether it is consequent in part upon 
some other condition of the particles, is doubtful; for judging 
of their magnitude by the time occupied in their descent 
through the fluid, it would appear that violet and blue fluids 
occur giving violet deposits, which still consist of particles so 
small as to require a time equally long with the ruby particles 
for their deposition, and indeed in some specimens to remain 
undeposited in any time which has yet occurred since their for- 
mation. These deposits, when they occur, look like clear solu- 
tions in the fluid, even under the highest power of the micro- 
scope. 
I endeavoured to obtain an idea of the quantity of gold in a 
given ruby fluid, and for this purpose selected a plate of gold 
ruby glass, of good full colour, to serve as a standard, and 
compared different fluids with it, varying their depth, until the 
light from white paper, transmitted through them, was ap- 
parently equal to that transmitted by the standard glass. Then 
known quantities of these ruby fluids were evaporated to dry- 
ness, the gold converted into chloride, and compared by reduc- 
tion on glass and otherwise with solutions of gold of known 
strengths. <A portion of chloride of gold, contaming 0°7 of a 
grain of metal, was made up to 70 cubic inches by the addition 
of distilled water, and converted into ruby fluid: on the sixth 
day it was compared with the ruby glass standard, and with a 
depth of 1:4 inch was found equal to it; there was just one 
hundredth of a gram of gold diffused through a cubic inch of 
fluid. In another comparison, some gold leaves were dissolved 
and converted into ruby fluid, and compared ; the result was a © 
fluid, of which 1°5 inch in depth equalled the standard, a leaf 
of gold being contained in 27 cubic inches of the fluid. Hence 
looking through a depth of 2°7 inches, the quantity of gold 
interposed between the light and the eye would equal that con- 
tained in the thickness of a leaf of gold. Though the leaf is 
green and the fluid ruby, yet it is easy to perceive that more 
light is transmitted by the latter than the former; but inas- 
much as it appears that ruby fluids may exist containing particles 
of very different sizes (or that settle at least with very different 
degrees of rapidity), so it is probable that the degree of colour, 
