of Gold and other Metals to Light. 521 
precipitate through the fluid in a time which is as nothing to 
that required by the particles of the ruby fluid from which they 
are obtained. But that the blue particles are always merely 
larger particles does not seem admissible for a moment, inasmuch 
as violet or blue fluids may be obtained in which the particles 
will remain in suspension as long as in the ruby fluids; there is 
probably some physical change in the condition of the particles, 
caused by the presence of the salt and such affecting media, 
which is not a change of the gold as gold, but rather a change 
of the relation of the surface of the particles to the surrounding 
medium. 
When salt is added in such quantity as to produce its effect 
in a short time, it is seen that the gold reflexion of the particles 
is quickly diminished, so that either as a general turbidness or 
by the cone of rays it becomes less visible; at last the metal 
contracts into masses, which are comparatively so few and sepa- 
rate, that when shaken up in the fluid, they confer little or no 
colour or character, either by reflected or transmitted light. In 
these cases no re-solution of the metal is effected, for neither 
the salt nor hydrochloric acid, when used in like manner, have 
any power to redissolve the gold. The same aggregating effect is 
shown with all the fluids whatever their colour, and also with 
the deposits that settle down from them. When salt is added to 
the solution of gold before the phosphorus, and therefore before 
the reduction of the gold, the fluid first produced is always ruby ; 
but it becomes violet, purple, or blue, with a facility in propor- 
tion to the quantity of salt present. If that be but small, the 
ruby will remain for many days unchanged in colour, and the 
violet-ruby for many weeks, before the gold will be deposited, 
the degree of dilution or concentration always having its own 
particular effect, as before described ; the more finely divided pre- 
parations, 7. e. the ruby and amethystine, appear to be more perma- 
nent than when the salt is added after the separation of the gold. 
Many other bodies besides salt have like action on the par- 
ticles of gold. A ruby fluid is changed to or towards blue by 
solutions of chlorides of calcium, strontium, manganese ; sul- 
phates of magnesia, manganese, lime; nitrates of potassa, soda, 
baryta, magnesia, manganese ; acetates of potassa, soda and lime; 
these effect the change freely: the sulphate of soda, phosphates 
of soda and potassa, chlorate of potassa, and acetate of ammonia 
acted feebly. Sulphuric and hydrochloric acids produce the 
change, but show no tendency to dissolve the gold. Nitric acid 
acts in the same manner, but not so strongly: it often causes 
re-solution of the gold after some time, because of the hydro- 
chloric acid which remains in the fluid. 
Amongst the alkalies, potash produces a similar action in a 
