of Gold and other Metals to Light. 529 
film has every character of lustre, colour, &c., in the parts dif- 
fering in thickness, that a film formed at the surface has. Whilst 
the particles are diffused through the fluid it is difficult to deal 
with them by tests and reagents; for their absolute quantity is 
very small, and their physical characters are very changeable, 
chiefly as I believe by aggregation; still there are some expe- 
dients which enable one to submit even the finest of them to 
proof. In several cases particles from ruby and amethystine fluids 
adhered to the sides of the bottles or flasks in which the fluids 
had been preserved, and the process of boiling seemed to favour 
such a result; the adhesion was so strong, that when the fluid 
contents were removed and the bottles well washed, the glass 
remained tinged of a ruby or a violet colour. These films, in 
which the fine particles were fixed mechanically apart and in 
place, were then submitted to the action of various chemical 
agents. Drying and access of air did not cause any marked 
alterations in them. Strong nitric acid produced no change, 
nor hydrochloric acid, nor sulphuric acid. Neither did a solu- 
tion of chloride of sodium, even up to brine, cause any alteration 
in the colour or any other character of the deposit. A little 
solution of chlorine or of nitromuriatic acid dissolved them at 
once, producing the ordinary solutions of gold. I can see no 
other mode of accounting for these effects (which are in strong 
contrast with what happens when ruby fluid is acted on by these 
agents), than to suppose that the gold particles, being in a high 
state of division, were retained in that state for the time by their 
adhesion to the glass. Of course chemical change was free to 
occur, but not a change dependent upon their mutual aggrega- 
tion ; yet they were not held by any special chemical attraction 
to, or combination with, the glass; for a touch with a card, a 
feather, or the finger, was sufficient to remove them at once, and 
if rubbed off with a point of wood, they coated it with brilliant 
metallic gold. 
Again, though these particles are so finely divided that they 
pass easily through ordinary filters, still a close filter catches 
some; and if a ruby fluid be passed through again and again, 
the paper at last becomes of a rosy hue, because of the gold 
which adheres to it; being then well washed, and, if needful, 
dried, the gold is again ready for experiment. Such gold paper, 
placed across the middle of the dark tube and examined by trans- 
mitted light, was of the same ruby tint as when looked through 
in the open air. It was unaffected by salt or brine, though 
these, added to the rosy fluid which had passed the filter, in- 
stantly changed it to violet-blue. Portions of the paper were 
put into separate glasses with brine, solutions of hydrochloric, 
nitric and sulphuric acids, ammonia, potassa, soda and sulphu- 
Phil. Mag. 8. 4, No. 96. Suppl. Vol. 14. 
