402 On the Experimental Relations [1857. 



compare with air ; but the general effects, the colours produced, 

 and the order of the colours, were precisely the same in all the 

 cases. These deposits were insoluble in nitric acid and in 

 hydrochloric acid, but in the mixed acids or in chlorine solution 

 were soluble, exactly in the manner of gold. There is no reason 

 to doubt that they consisted of metallic gold in a state of ex- 

 treme division. 



Now as to the effects on light, i. e. as to the coloured rays 

 reflected or transmitted by these deposited particles, and first, 

 of those in the line of the discharge where the wire had been. 

 Here the mica was found abraded much, the glass less, and 

 the rock-crystal and topaz least. Where abraded, the gold 

 adhered ; in all the other parts it could be removed with the 

 slightest touch. The gold deposited in this central place was 

 metallic and golden by reflected light, and of a fine ruby colour 

 by transmitted light. On each side of this line the deposit had 

 a dark colour, but when particularly examined gave a strong 

 golden metallic reflexion, and by transmission a fine violet colour, 

 partaking of green and ruby in different parts, and sometimes 

 passing altogether into green. Beyond this, on each side, 

 where the tints became paler and where the particles appeared 

 to be finer, the transmitted tint became ruby or violet-ruby, 

 and this tint was especially seen when the deposit was caught 

 on a card. As to the reflected light, even at these faintest 

 parts it is golden and metallic. This is easily observed by 

 wiping off a sharp line across the deposit on glass in the very 

 faintest part, and then causing the sun's rays collected in the 

 focus of a small lens to travel to and fro across that edge ; the 

 presence of the metallic gold on the unwiped part is at once 

 evident by the high illumination produced there. It is evident 

 that all the colours described are produced by one and the 

 same substance, namely gold, the only apparent difference being 

 the state of division and different degrees of the application of 

 heat. The thickest parts of these deposits are so discontinuous, 

 that they cannot conduct the electricity of a battery of two or 

 three pairs of plates, i. e. of a battery unable to produce a spark 

 among the particles. 



When any of these deposits of divided gold are heated to 

 dull redness, a remarkable change occurs. The portions which 

 before were violet, blue, or green by transmitted light, now 



