412 



POPULAR SCIENCE MONTHLY 



©orrcsirotttteuxe. 



A CURIOUS OPTICAL PHENOMENON. 



Editor Popular Science Monthly : 



SIR : To one interested in observing the 

 action of light under unusual circum- 

 stances, 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, Gua- 

 najuato, 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 quick- 

 silver 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 spec- 

 trum. A transverse section of this cvlinder, 

 representing the body of falling metallic par- 

 ticles, would have a diameter of from five 

 and a half to nine inches, according to 

 rapidity of the discharge of amalgam 

 the top of the sack. The slower the 



the 

 into 

 dis- 

 and 



vice 



charge, the smaller the sectional area 

 the larger the individual drops, and 

 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 out- 

 side, 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 fol- 

 lows : While in the receiving kettles it is kept 

 under a saturated solution of caustic soda to 

 cut any grease which it may collect in pass- 

 ing through the pans and settlers. It con- 

 tains a trace of zinc and possibly of silver. If 

 lead, copper, or iron in the form of amal- 

 gams 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 

 vance as a hypothesis 



coating. 



This I merely ad- 



Henrv M. Stanley. 



Mexico, D. F., November 1, 1897. 



%aitox T s gaMje. 



EDUCATION IN BONDS. 



THE warfare of science with the- 

 ology has been amply and im- 

 pressively related by such writers as 



Buckle, Draper, and President An- 

 drew White; and many have sup- 

 posed that science, having accom- 

 plished this warfare and come out 



