# Popular Science Monthly/Volume 52/January 1898/Correspondence

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A CURIOUS OPTICAL PHENOMENON.
Editor Popular Science Monthly:
SIR: To one interested in observing the action of light under unusual circumstances, a very pretty display of colors can be seen in the amalgam room of the Ojo de Agua Silver Mill at San Luis de la Paz, Guanajuato, Mexico.
This room is twelve feet wide by twenty long, with whitewashed walls. The outside wall faces the northeast and is pierced by two large windows, covered with heavy wire gratings and coarse white muslin. Opposite these windows and against the southeast wall are large iron kettles set two feet above the floor in solid cement. Above these in wooden racks are hung heavy canvas sacks two feet long, to receive and strain the liquid amalgam as it comes from the settlers. This amalgam runs into the sacks, and the excess of quicksilver percolates through the coarse canvas and falls in a shower into the kettles below.
To one standing in front of this shower will appear some of the colors of the spectrum. A transverse section of this cylinder, representing the body of falling metallic particles, would have a diameter of from five and a half to nine inches, according to the rapidity of the discharge of amalgam into the top of the sack. The slower the discharge, the smaller the sectional area and the larger the individual drops, and vice versa.
The colors are repeated six times in the width of the stream, three times on each side of the center line or axis. This is observed at a point halfway between the surface of the "quick" in the kettles and the flow of particles from the sides of the sacks, the colors forming a line coincident with the horizontal plane. Of course, this changes slightly with the position of the observer, in accordance with the law of incidence and reflection.
The refractive action is toward the outside, as the violet appears at the outer edge. This (the violet) is fairly strong, the indigo fainter, and the blue nil. Under the most favorable circumstances only can the green be detected. The yellow is about as strong as indigo, the orange of the same value as the violet, and the red most pronounced of all.
The red of each outer spectrum borders on the violet of the next inner on each side of the center, and this is repeated in the second pair where they connect with the inner or last pair. The third or inner is so faint that only the violet is visible, and that but faintly. There seems to be a gradual decrease of intensity toward the center, which is not apparent from the fact of the stronger end of each spectrum being inside; that is, the stronger end (red) of the first or strongest spectrum coming against the weaker end (violet) of the second or weaker spectrum, etc.
The greater the volume the smaller the individual particles and the stronger the colors, and vice versa. Late in the afternoon the sun shines on the windows, but does not occasion an increase in the value of the colors, except when some opaque object is interposed. In fact, it is always best to stand facing the "quick" and between it and the window opposite. Direct sunlight never enters the room, the windows being covered with coarse white muslin as above stated. Artificial light produces the same effect as sunlight.
The condition of the "quick" is as follows: While in the receiving kettles it is kept under a saturated solution of caustic soda to cut any grease which it may collect in passing through the pans and settlers. It contains a trace of zinc and possibly of silver. If lead, copper, or iron in the form of amalgams are present, they are in such very small proportion that it is impossible to estimate them.
It may be that in falling there is formed a film of varying thickness of zinc oxide on the surface of each globule which would have a decomposing action on the light reflecting from the surface of the globule on which it would form a coating. This I merely advance as a hypothesis.
* Henry M. Stanley.
* Mexico, D. F., November 1, 1897.