1857.] of Gold (and other Metals) to Light. 435 



any length of time. It is perfectly neutral ; gives no signs of 

 dissolved gold by any of the tests of the metal ; is not changed 

 by sulphuretted hydrogen, gallic acid, pyrogallic acid, dilute 

 caustic alkalies, or carbonated alkalies or lime-water ; or by 

 dilute sulphuric, hydrochloric or nitric acids, the actions being 

 continued for fourteen days : being boiled with zinc filings, it 

 does not change ; and even when dilute sulphuric or hydro- 

 chloric acid is added to evolve nascent hydrogen, still the ruby 

 character undergoes no alteration. Strong sulphuric, or nitric, 

 or hydrochloric acid does not alter it whilst cold ; but when 

 warmed, the first causes the gold to separate as dark aggre- 

 gated metallic particles, and the two latter gradually cause the 

 change to amethyst and blue formerly described. Chlorine, or 

 a mixture of hydrochloric and nitric acids, dissolves the gold, 

 the ruby colour disappears, and the ordinary solution of gold 

 is produced. In all these cases the ruby gold behaves exactly 

 as metallic gold would do with the same agents, and quite unlike 

 whatwould be expected from any possible combination of oxygen 

 and gold. 



In some of these jellies the ruby particles are so determinate 

 as to give the brown reflexion by common observation ; in others 

 they are so fine as to look like ruby solutions, unless a strong- 

 sunlight and a lens be employed ; and the impression again 

 arises, that gold may exist in particles so minute as to have 

 little or no power of reflecting light. Ruby particles of extreme 

 fineness, when present in small amount in water, appear to re- 

 main equally diffused for any length of time ; if in larger amount, 

 that which settles to the bottom will remain for weeks and 

 months as a dense ruby fluid, but without coming together : both 

 circumstances seem to imply an association of the particles of 

 gold with envelopes of water. Many circumstances about the 

 ruby jellies imply a like association with that animal substance, 

 and many of the stains of gold upon organic substances pro- 

 bably include an affinity of the metal of the like kind. 



Relations of Gold (and other metals) to polarised Light. 



It has been already stated, that when a ray of common light 

 passes through a piece of gold-leaf inclined to the ray, the light 

 is polarized. When the angle between the leaf and the ray is 

 small, about 15, nearly all the light that passes is polarized; 



