274 ANNUAL OF SCIENTIFIC DISCOVERY. 



and to the ordinary observation, transparent ; but when illuminated by 

 a cone of rays the suspended particles show their presence by the opales- 

 cence, which, is the result of their united action. The settling particles, 

 if in a flask, appear at the bottom like a lens of deep-colored fluid, 

 opaque at the middle, but deep ruby at the edges ; when agitated, they 

 may be again diffused through the liquid. These particles tend to aggregate 

 into larger particles, and produce other effects of color. It is found 

 that boiling gives a certain degree of permanence to the ruby state. Many 

 saline and other substances affect this ruby fluid ; thus a few drops of solu- 

 tion of common salt being added, the whole gradually becomes of a violet 

 color ; still the particles are only in suspension, and when illuminated by a 

 lens, are a golden yellow by reflected light : they separate now much more 

 rapidly and perfectly by deposition from the fluid than before. Some speci- 

 mens, however, of the fluid, of a weak purple or violet color, remain for 

 months without any appearance of settling, so that the particles must be ex- 

 ceedingly divided ; still the rays of the sun, or even of a candle in a dark 

 room, when collected by a lens, will manifest their presence. The highest 

 powers of the microscope have not as yet rendered visible either the ruby or 

 the violet particles in any of these fluids. Glass is occasionally colored of a 

 ruby tint by gold ; such glass, when examined by a ray of light and a lens, 

 gives the opalescent effect described above, which indicates the existence of 

 separate particles at least such has been the case with all the specimens Mr. 

 Faraday has examined. It becomes a question whether the constitutions of 

 the glass and the ruby fluids described are not, as regards color, alike. At 

 present, he believes they are ; but whether the gold is in the state of pure 

 metal, or of a compound, he has yet to decide. It would be a point of con- 

 siderable optical importance if they^should prove to be metallic gold ; from 

 the effects presented when gold wires are deflagrated by the Leyden dis- 

 charge over glass, quartz, mica, and vellum, and the deposits subjected to 

 heat, pressure, &c., he inclines to believe that they are pure metal. 



REDUCTION OF METALS BY HYDROGEN. 



St. Claire Deville states that the reduction of volatile metals depends very 

 much upon the rapidity of the current of reducing gas. Thus, for instance, 

 when oxide of zinc is ignited in a rapid current of hydrogen, the metal is 

 reduced, while by ignition in a slow current of the same gas no metallic zinc 

 is obtained, there is formed at another part of the tube, crystallized oxide of 

 zinc. 



He considers that in the latter case reduction does not take place, but that 

 owing to the slowness of the current of the gas, the water vapor formed is 

 not removed soon enough to prevent a further reaction between it and the 

 vapor of zinc, reproducing oxide of zinc. In this way he accounts for the 

 transfer of the oxide of zinc from one part of the tube to another, and is of 

 opinion that the apparent volatilization of oxide of zinc in this experiment 

 and in metallurgical operations, is owing to this reaction which takes place at 

 a lower temperature, than the reduction of the oxide of hydrogen. Ann. de 

 Chim. et de Phys. xliii. 47. 



