406 On the Experimental Relations [1857. 



state. If a plate of glass large enough to cover the basin 

 have six or eight drops of a strong neutral solution of chloride 

 of gold placed on it, and this be spread about by a glass 

 stirrer, so as to form a flowing layer on the surface, the glass 

 may then be inverted and placed over the dish. So arranged, 

 the gold solution will keep its place, but will have a film of 

 metal reduced on its under surface. The plate being taken off 

 after twenty, thirty, or forty minutes, and turned with the gold 

 solution upwards, may then gradually be depressed in an in- 

 clined position into a large basin of pure water, one edge 

 entering first, and the gold film will be left floating. After 

 sufficient washing it may be taken up in portions on smaller 

 plates of glass, dried, and kept for use. Mr. Warren De la 

 Rue taught me how to make and deal with these films : they 

 may by attention be obtained very uniform, of very different 

 degrees of thickness, from almost perfect transparency to 

 complete opacity, and by successive application of the same 

 collecting glass plate may be superposed with great facility. 



These films may be examined either on the water or on the 

 glass. When thick, their reflective power is as a gold plate, 

 full and metallic; as they are thinner, they lose reflective 

 power, and they may be obtained so thin as to present no 

 metallic appearance, all the coloured rays of light then passing 

 freely through them. As to the transmitted light, the thinner 

 films generally present one kind of colour ; it appears as a 

 feeble grey-violet, which increases in character as the film 

 becomes thicker and sometimes approaches a violet ; a greenish 

 violet also appears ; and the likeness of the grey-violet tint of 

 these films to the stains produced by a solution of gold on the 

 skin or other organic reducing substance, or the stain produced 

 on common pottery, cannot be mistaken. Superposition of 

 several grey-violet films does not produce a green tint, but 

 only a diminution of light without change of colour. In those 

 specimens made by particles of phosphorus floating on the 

 solution of gold, very fine green tints occur at the thicker and 

 golden parts of the film. The colour of the gold here may 

 depend in some degree on the manner in which these films are 

 formed : the thicker parts are not produced altogether by the 

 successive addition of reduced gold from the portion of fluid 

 immediately beneath them. When a particle of phosphorus 



