1857.] of Gold (and other Metals] to Light. 425 



as they sink together will lie for months at the bottom of the 

 fluid without uniting or coming nearer to each other, or without 

 being taken up by metallic mercury put into the same vessel. 

 This is consistent with what we know of the manner in which 

 gold and platinum can be thoroughly wetted if cleaned in water, 

 and of the difference which occurs when they are dried and 

 become invested with air. I endeavoured to transfer the gold 

 particles unchanged into other media, for the purpose of noting 

 any alteration in the action on light. By decanting the water 

 very closely, and then carefully adding alcohol with agitation, 

 I could diffuse them through that fluid ; they still possessed a 

 blue colour when looked through in the dark tube, but seemed 

 much condensed or aggregated, for the fluid was obscure, not 

 clear, and the particles soon subsided. I could not transfer 

 them from alcohol to camphine ; they refused association with 

 the latter fluid, retaining a film of alcohol or water, and adhe- 

 ring by it to the glass of the vessel ; but when the camphine 

 was removed, a partial diffusion of them in fresh alcohol could 

 be effected, and gave the colour as before. All these transfers, 

 however, injured the particles as to their condition of division. 

 In one case I obtained a ruby film on a white plate ; on pour- 

 ing off the water and allowing parts to become dry, these be- 

 came violet, seen by the light going through them to the plate 

 and back again to the eye. I could not wet these places with 

 water ; a thin feebly reflecting surface remained between it and 

 them. Using alcohol, the parts already dry remained violet, 

 when wetted by it ; but wetting other parts with alcohol before 

 they were dry from water, they remained rosy, became bluish 

 when dry from the alcohol, and became rosy again when re- 

 wetted by it. 



It will be necessary to speak briefly of the reduction of gold 

 into a divided state by some other chemical agents than those 

 already described*. If a drop of solution of protosulphate of 

 iron be introduced to, and instantly agitated with, a weak 

 neutral solution of chloride of gold in such proportion that 

 the latter shall be in excess, the fluid becomes of a blue-grey 

 colour by transmission and brown by reflexion ; and a deposit is 

 formed of a green colour by transmitted light, greatly resembling 



* See Gmelin's ' Chemistry,' vi. p. 219, " Terchloride of gold," for nume- 

 rous references in relation to changes of these kinds. 



