162 
DE. EAEAEAT ON THE EXPEEBIENTAL EELATIONS 
or the apex of the cone, which give the nearly pure effect of reflexion. In order to 
observe the transmitted ray in an unmingled state, a glass tube closed at one end was 
surrounded with a tube of black paper longer than itself, and with the black surface 
inwards. When a fluid (or the particles in it) was to be examined, it was put into 
this tube, and a surface of white paper illuminated by daylight or the sun, regarded 
through it, other light being excluded from the eye ; or the tube was sometimes inter- 
posed between the eye and the sky, and sometimes the rays of the sun itself were 
reflected up to the eye through it. In speaking hereafter of the tints of the hght trans- 
mitted by the particles (which will of course vary with the proportion of different rays 
in the original beam of light), a pure white original light is to be imderstood. but 
occasionally differently-tinted papers were employed -with this tube as sources of different 
coloured lights. 
The very oblique angle at which reflected light comes to the eye from the diffused 
particles, is well seen when the lens cone, or a direct ray of the sun, is passed into the 
fluid and observed from different positions ; it is only when the eye is behind and nearly 
in the line of the ray, that the unmixed transmitted ray is obserwed. In the dark tube I 
think that no reflected light arrives at the eye : for if half an inch in depth of water be 
introduced, white light passes ; if a drop of the washed deposit, to be hereafter described, 
be introduced, the light transmitted is either blue or ruby, or of other intermediate tint, 
according to the character of the deposit ; but if water be then added until the column 
is six inches or more in length, the quantity of light transmitted does not sensibly alter, 
nor its tint ; a fact which I think excludes the idea of any light being reflected from par- 
ticle to particle, and Anally to the eye. 
If a given ruby-tinted fluid, containing no gold in solution, be allowed to stand for a 
few days, a deposit will fall from which the fluid may be removed by a siphon ; being 
now allowed to stand for a week, a second deposit Avill be produced ; if the fluid be again 
removed and allowed to stand for some months, another deposit AA'ill be obtained, and 
the fluid will probably be of a bright ruby ; if it be noAv alloAved to stand for several 
months, it will still yield a deposit, looking however more like a ruby fluid than a collec- 
tion of fine particles at the bottom of the fluid, whilst traces of yet finer particles of gold 
in suspension may be obtained by the lens. All these deposits may be Avashed AAith 
Avater and Avill settle again ; the coarser are not much affected, but the finer are, and 
tend to aggregate ; nevertheless specimens often occur, especially after boiling, AAliich tend 
to preserve their fine character after Avashing, if the Avater be A’ery clean and pure. 
The colour of these particles Avhilst under, or diffused through Avater, is by common 
reflected light brown, paler and richer, sometimes tending to yelloAA', and sometimes to 
red. The same difference is shoAvn when illuminated by sunlight. EA'erything tends 
to show that the light reflected is very bright considering the size of the particles, and 
therefore of the reflecting surfaces ; yet comparing by the cone of light a ruby fluid 
when first prepared and before it has become very sensibly turbid, AAith the same fluid 
after the evident turbidity is produced, in both of Avhich cases I belieAe the gold to 
