VOL. LXXXVII.] PHILOSOPHICAL TRANSACTIONS. J 3Q 



XL On the Action of Nitre on Gold and Platina. By Smithson Tennant, Esq., 



F.R.S. p. 21Q. 



Gold, which cannot be calcined by exposure to heat and air, has been also con- 

 sidered as incapable of being affected by nitre. But in the course of some experi- 

 ments on the diamond, an account of which has been communicated to the r. s., 

 I observed, that when nitre was heated in a tube of gold, and the diamond was not 

 in sufficient quantity to supply the alkali of the nitre with fixed air, a part of the 

 gold was dissolved. From this observation I was induced to examine more particu- 

 larly the action of nitre on gold, as well as to inquire whether it would produce any 

 effect on silver and platina. 



With this intention I put some thin pieces of gold into the tube together with 

 nitre, and exposed them to a strong red heat for 2 or 3 hours. After the tube was 

 taken from the fire the part of the nitre which remained, consisting of caustic 

 alkali, and of nitre partially decomposed, weighed 140 grs.; and fJOgrs. of the 

 gold were found to have been dissolved. On the addition of water about 50 grs. of 

 the gold were precipitated, in the form of a black powder. The gold which was 

 thus precipitated was principally in its metallic state, the greater portion of it 

 being insoluble in marine acid. The remaining gold, about 10 grs. in weight, 

 communicated to the alkaline solution, in which it was retained, a light yellow 

 colour. By dropping into this solution diluted vitriolic or nitrous acid, it became 

 at first of a deeper yellow, but if viewed by the transmitted light, it soon appeared 

 green, and afterwards blue. This alteration of the colour from yellow to blue 

 arises from the gradual precipitation of the gold in its metallic form, which by the 

 transmitted light is of a blue colour. Though the gold is precipitated from this 

 solution in its metallic form, yet there seems to be no doubt that while it remains 

 dissolved it is entirely in the state of calx. Its precipitation in the metallic state i& 

 occasioned by the nitre contained in the solution, which having lost part of its 

 oxygen by heat, appears to be capable of attracting it from the calx of gold; for I 

 found that if the calx of gold is dissolved by being boiled in caustic alkali, and a 

 sufficient quantity of nitre which has lost some of its air by heat is mixed with it, 

 the gold is precipitated by an acid in its metallic state.* 



* As the precipitation of gold in its metallic form, by nitre which has lost some of its oxygen has 

 not, I believe, been noticed, it may not be improper to mention some of those facts relating to it which 

 seem most entitled to attention. Nitre, which has been heated some time, precipitates gold in its 

 metallic state from a solution in aqua regia, if it be diluted with water. If a solution of gold in nitrous 

 acid be dropped into pure water, the calx of gold is separated, which is of a yellow colour ; but if the 

 water contain a very small proportion of nitre which has lost some of its air by heat, as 1 gr. in 6 oz., 

 the gold is deprived of its oxygen, and becomes blue. The alkali of the nitre does not assist in pro- 

 ducing this effect. Nitrous acid alone, which does not contain its full proportion of oxygen, occasions 

 the same precipitation, unless it is very strong ; and if a mixture of such strong nitrous acid, and of a 

 solution of gold in nitrous acid, be dropped into water, the gold is deprived of its oxygen, and is preci- 

 pitated of a blue colour. Two causes contribute to produce this effect on the addition of water. The 

 adhesion of the calx of gold to nitrous acid is by that means weakened, and the oxygen is attracted 



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