408 On the Experimental Relations [ 1 857. 



pure nitric, or hydrochloric, or sulphuric acids, or solutions of 

 potassa or brine. They dissolved in damp chlorine gas, not 

 changing in colour during the solution. I believe them to 

 consist of pure gold. 



When these gold films were heated to dull redness they 

 changed. The reflexion, though not much altered, was a 

 little more metallic and golden than before; more light was 

 transmitted after the heating and the colour had altered from 

 greenish to violet, or from grey-green to ruby or amethyst ; 

 and now two or three films superposed often gave a very ruby 

 colour. This action is like that of heat on the particles 

 separated by electric explosions. If not overheated, the 

 particles were not fused to the glass, but could be easily wiped 

 off. Whenever these heated particles were pressed by the 

 convex agate, they changed in character and transmitted green 

 light. Heat took away this character of the gold, the heat of 

 boiling oil, if continued, being sufficient ; but on applying 

 pressure at the same spot, the power of transmitting green 

 light was restored to the particles. In many cases, where the 

 gold adhered sufficiently to the glass to bear a light drawing 

 touch from the finger or a card, such touch altered the light 

 transmitted from amethystine to green ; so small is the pressure 

 required when the particles are most favourably disposed. 



Heating injured the conducting power for electricity of these 

 films, no doubt by retraction of the particles, though there 

 was no such evident appearance in these cases, as in the un- 

 attached gold-leaf of the particles running up into globules. 



A given film, examined very carefully in the microscope by 

 transmitted lamp-light, with an aperture of 90 and power of 

 700 linear, presented the following appearances. The un- 

 heated part was of a grey colour, and by careful observation 

 was seen to be slightly granular. By very close observation 

 this grey part was often resolvable into a mixture of green and 

 amethystine stria?, it being the compound effect of these which in 

 general produce the grey sensation in the eye. When a part 

 of such a film was heated, the transmitted colour was changed 

 from grey to purple, as before described, and the part thus 

 heated was evidently more granular than before. This dif- 

 ference was confirmed in other cases. That the heated part 

 should thus run up, seems to show that many of the particles 



