169 
OP GOLD (AND OTHEE METALS) TO LIGHT. 
refused association with the latter fluid, retaining a fllm of alcohol or water, and adhering 
by it to the glass of the vessel ; but when the camphine was removed, a partial diffusion 
of them in fresh alcohol could be effected, and gave the colour as before. All these 
transfers, however, injured the particles as to thefr condition of division. In one case I 
obtained a ruby fllm on a white plate ; on pouring off the water and allowing parts to 
become dry, these became violet, seen by the light going through them to the plate and 
back again to the eye. I could not wet these places with water, a thin feebly reflecting 
sm’face remained between it and them. Using alcohol, the parts already dry remained 
\iolet, when wetted by it ; but wetting other parts with alcohol before they were dry 
from water they remained rosy, became bluish when dry from the alcohol, and became 
rosy again when re-wetted by it. 
It will be necessary to speak briefly of the reduction of gold into a divided state by 
some other chemical agents than those afready described If a drop of solution of proto- 
sulphate of iron be introduced to, and instantly agitated with, a weak neutral solution of 
chloride of gold in such proportion that the latter shall be in excess, the fluid becomes 
of a blue-grey colour by transmission and brown by reflexion ; and a deposit is formed 
of a green colour by transmitted light, greatly resembling the colour of beaten or pressed 
metal. It is not however pure gold, but an association of it and oxide of iron. Hydro- 
chloric or other acids remove the iron and reduce the gold to a dark, dense, insoluble set 
of particles, in very small quantity apparently, yet containing all that was present in the 
bulky green deposit. If the solution of gold be made slightly acid beforehand, then 
the change and precipitation is to appearance much less ; the reflexion by the particles 
is feeble but of a pale brown coloui’, the general transmitted light is amethystine ; in the 
dark tube the tint is blue; the particles are much condensed and settle quickly, but 
occasionally leave a good ruby fllm on the side of the glass, which has all the characters 
of the ruby Aims and particles before described. The loose gold particles quickly adhere 
together. Hence it appears that the green precipitate often obtained by protosulphate 
of iron is not pure gold in a divided state ; and that when care is taken to produce such 
pure divided gold, it presents the appearances of divided gold obtained by other means, 
the gold being competent to produce the ruby, amethystine, and blue colours by trans- 
mission. Usually the gold rapidly contracts and becomes almost insensible, and yet the 
test of protochloride of tin will show that all has been separated from solution ; it then 
forms a striking contrast to the depth of colour presented by the same solution of gold 
precipitated by phosphorus, and most impressively directs attention to the molecular 
condition of the metal in the latter state. 
A very small quantity of 'protochloride of tin, added to a dilute solution of gold, gave, 
first the ruby fluid, showing diffused particles by the cone of rays ; this gradually became 
purple, and if the gold were in sufficient quantity a precipitate soon began to fall, being 
the purple of Cassius. If the chloride of tin were in larger quantity, a more bulky pre- 
* See Gmelin’s ‘ Chemistry,’ vi. p. 219, “ Terchloride of gold,” for numerous references in relation to 
changes of these kinds. 
2 A 
MDCCCLVII. 
