514 Prof. Faraday on the Experimental Relations 
acids and hydrochloric acid, by the further action of the 
water. 
The fluids produced may easily be tested for any gold yet 
remaining unreduced, by trial of a portion with solution of proto- 
chloride of tin. If any be found, it is easily reduced by the 
addition of a little more of the phosphorus in solution. After 
all the gold is separated as solid particles, the fluid may be con- 
sidered in its perfected state. Occasionally it may smell of 
- phosphorus in excess, even after it has been poured off from the 
deposited particles of it and the sulphide. In that case it is easy 
to deprive it of this excess by agitation in a bottle with air. 
When kept in closed vessels mouldiness often occurs. If this 
be in groups, it is collected with facility at the end of a splinter 
of wood and removed, or the whole fluid may be poured through 
a wet plug of cotton in the neck of a funnel, the reduced gold 
passing freely. All the vessels used in these operations must be 
very clean; though of glass they should not be supposed in 
proper condition after wiping, but should be soaked in water, and 
after that rinsed with distilled water. A glass supposed to be 
clean, and even a new bottle, is quite able to change the cha- 
racter of a given gold fluid. 
Fluids thus prepared may differ much in appearance. Those 
from the basins, or from the stronger solutions of gold, are often 
evidently turbid, looking brown or violet in different lights. 
Those prepared with weaker solutions and in bottles, are fre- 
quently more amethystine or ruby in colour and apparently clear. 
The latter, when in their finest state, often remain unchanged 
for many months, and have all the appearance of solutions. 
But they never are such, containing in fact no dissolved, but 
only diffused gold. The particles are easily rendered evident, by 
gathering the rays of the sun (or a lamp) into a cone by a lens, 
and sending the part of the cone near the focus into the fluid ; 
the cone becomes visible, and though the illuminated particles 
cannot be distinguished because of their minuteness, yet the 
light they reflect is golden in character, and seen to be abun- 
dant in proportion to the quantity of solid gold present. Por- 
tions of fluid so dilute as to show no trace of gold, by colour or 
appearance, can have the presence of the diffused solid particles 
rendered evident by the sun in this way. When the preparation 
is deep in tint, then common observation by reflected light shows 
the suspended particles, for they produce a turbidness and degree 
of opacity which is sufficiently evident. Such a preparation 
contained in a pint bottle will seem of a dull pale-brown colour, 
and nearly opake by reflexion, and yet by transmission appear to 
be a fine ruby, either clear or only slightly opalescent. 
That the ruby and amethystine fluids hold the particles in sus- 
