of Gold and other Metals to Light. 519 
which determines the aptitude to transmit in preference this or 
that ray of light. 
Considering the fluids as owing their properties to diffused 
particles, it may be observed, that many of them which in small 
quantities in the dark tube transmit an amethystine light, send 
forward a ruby light when the quantity is increased; and this 
appears to be the general progression. I have not found any 
which by increase in quantity tended to transmit the blue rays 
in preference to the red. 
Elevation of temperature had an effect upon these fluids which 
is advantageous in their preparation. On boiling an apparently 
clear ruby fluid for some time, its colour passed a little towards 
amethystine, and on boiling a like amethystine fluid, its tint 
passed towards blue. The separation of the gold particles was 
also facilitated, for now they would settle in three or four days 
from a fluid which, prior to this operation, would not have depo- 
sited them in an equal degree for weeks. In the case of the 
ruby fluids the colour often became more rosy and luminous, and 
by reflected light the fluid seemed to have become more turbid, 
as if the particles had gained in reflective power; in fact, the 
boiling often appeared to confer a sort of permanency on the 
particles in their new state. When settled, they formed collec- 
tions looking like little lenses of a deep ruby or violet colour, 
at the bottom of the flasks containing the fluid; when all was 
shaken up, the original fluid was reproduced, and then, by rest, 
the gold re-settled. This effect could be obtained repeatedly. 
The particles could fall together within a certain limit, but many 
weeks did not bring them nearer or into contact; for they re- 
mained free to be diffused by agitation. The space they oc- 
cupied in this lens-like form must have been a hundredfold, or 
even a thousandfold, more than that which they would have 
filled as solid gold. Whether the particles be considered as 
mutually repulsive, or else as molecules of gold with associated 
envelopes of water, they evidently differ in their physical con- 
dition, for the time, from those particles which by the applica- 
tion of salt or other substances are rendered mutually adhesive, 
and so fall and clot together. 
In preparing some of these fluids, | made the solution of gold 
hot and boiling before adding the solution of phosphorus. The 
pbeenomena were the same in kind as before: but when the 
phosphorus was dissolved in sulphide of carbon, the gold soon 
fellas a dark flocculent deposit ; when it was dissolved in ether, 
a more permanent turbid ruby fluid was obtained, which, if it 
does not go on changing in aggregation, may give a good ruby 
deposit. 
The particles in these fluids are remarkable for a set of phy- 
