1857.] of Gold (and other Metals) to Light. 407 



is placed on pure water, it immediately throws out a film 

 which appears to cover the whole of the surface ; in a little 

 while the film thickens around the particle and is easily di- 

 stinguished by its high reflective power. It is this film which 

 reduces the gold in solution, being itself consumed in the 

 action ; the result is a continued extension from the phosphorus 

 outwards, which, after it has covered the solution with a thin 

 film of gold, continues to cause a compression of the parts 

 around the phosphorus and an accumulation there, rendering 

 the gold at a distance of half an inch from the phosphorus so 

 thick, that it is brilliant by reflexion and nearly opake by 

 transmission ; whilst near to the phosphorus the forming film 

 is so thin as to be observed only on careful examination, and 

 is still travelling outwards and compressing the surrounding 

 parts more and more. The phosphorus is very slowly con- 

 sumed ; a particle not weighing ylhyth of a grain will remain for 

 four or five days on the surface of water before it disappears. 



Though the particles of these films adhere together strongly, 

 as may be seen by their stiffness on water, still the films cannot 

 be considered as continuous. If they were, those made by 

 vapour of phosphorus could not thicken during their formation, 

 neither could they dry on glass in the short time found 

 sufficient for that purpose. Experimentally also, I find that 

 vapours and gases can pass through them. Very thin films 

 without folds did not sensibly conduct the electricity of a 

 single pair of Grove's plates ; thicker films did conduct ; yet 

 with these proofs that these films could not be considered as 

 continuous, they acted as thin plates upon light, producing 

 the concentric rings of colours round the phosphorus at their 

 first formation, though their thickness then could scarcely be 

 the i-Jijth, perhaps not the Tnhfth of a wave undulation of light. 

 Platinum, palladium, and rhodium produced films, showing 

 these concentric rings very well. 



Many of these films of gold, both thick and thin, which, 

 being of a grey colour originally, were laid on a solution 

 of cyanide of potassium to dissolve slowly, changed colour as 

 they dissolved and became green ; if change occurred, it 

 was always towards green. On the other hand, when laid on 

 a solution of chlorine, the change during solution was towards 

 an amethyst or ruby tint. The films were not acted upon by 



