166 
DE. FAEADAY ON THE EXPEEBIENTAE EELATIONS 
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 suiTounding 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 con- 
tracts into masses, which are comparatively so few and separate, 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 hydro- 
chloric 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. 'V^Tien 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 \iolet, purple, or blue, nith a facihty in pro- 
portion to the quantity of salt present. If that be but small, the ruby wiU 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 parti- 
cular effect, as before described ; the more finely divided preparations, i. e. the ruby and 
amethystine, appear to be more permanent than when the salt is added after the sepa- 
ration of the gold. 
Many other bodies besides salt have like action on the particles of gold. A ruby 
fluid is changed to or towards blue by solutions of chlorides of calcium, strontium, man- 
ganese ; sulphates of magnesia, manganese, lime ; nitrates of potassa, soda, baryta, 
magnesia, manganese ; acetates of potassa, soda, and lime ; these effect the change ffeely : 
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 m 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 weak degree. So also 
does soda. Lime-water produces a change in the same du’ection, but the gold quickly 
precipitates associated with the lime. 
Ammonia causes the ruby fluid to assume a violet tint; the deposit is slow of formation 
and often ruby in colour ; the alkali apparently retards the action of common salt. 
Chlorine or nitromuriatic acid turns the ruby fluid blue or violet-blue before they 
dissolve the gold. 
Solution of sulphuretted hydrogen changes the ruby slowly to purple, and finally to 
deep blue. Ether, alcohol, camphine, sulphide of carbon, gum, sugar and glycerine 
cause little or no change in the fluids ; but glycerine added to the dense deposits causes 
serious condensation and alteration of them, so that it could not be employed as a me- 
dium for the suspension of particles in the microscope. 
