30 ABion of Nitre vpm Gold, ^c. 



valves be made to open mechanically, the maximum pf exhau^ion v?ill in tlneory be in 

 the duplicate ratio of the fmaller fpace to the larger, afluming both ftrokes to be eqqal in 

 the barrels through which the air fuccefiivcly pafles. And in Sadler's pump with oil 

 (Ibid, plate xix. fig. i.) the maximum will be indefinitely great, bccaufe every llrpkc muft 

 take out a like part of the refidue of air from the receiver. 



VIII. 



On the Aaion of Nitre upon Gold and Platina. By SmjtHSON TenNANT, Efq.'F. R. S.* 



vJOLD, which cannot be calcined by expofure to heat and air, has been alfo confidered as 

 incapable of being afFefted by nitre. But in the courfe of fome experiments on the dia- 

 mond, an account of which has been communicated to this fociety, I obfcrved that when 

 nitre was heated in a tube of gold, and the diamond was not in fufficient quantity to fupply 

 the alkali of the nitre with fixed air, a part of the gold was diflblved. From this obier- 

 vaiion I was induced to examine more particularly" the a£lion of nitre upon gold, as well 

 as to enquire whether it would produce any efFe£t upon filver and platina. 



"With this intention I put fome thin pieces of gold into the tube, together with nitre, and 

 cspofed them to a ftrong red heat for two or three hours. After the ttib^ was taken from 

 the fire, the part of the nitre which remained, confifting of cauftic alkali, and of nitre par- 

 tially decompofed, weighed 140 grains ; and 60 grains of the gold were found to have been 

 diflblved. Upon the addition of water, about 50 grains of the gold were precipitated in the 

 form of a black powder. The gold which was thus precipitated was principally in its 

 metallic ftatp, the greater portion of it being infolublc in marine acid. The remaining 

 gold, about ten grains in weight, communicated to the alkaline folution in which it was' 

 retained, a light yellow colour. By dropping into this folution diluted vitriolic or nitrous 

 acid, it became at firftof a deeper yellow, but, if viewed by the tranfmitted light, it foon 

 appeared green, and afterwards blue'. The alteration of the colour from yellow to blue, 

 arifes from the gradual precipitation of the gold in its metallic form, which by the tranf- 

 mitted light is of a blue colour : though the gold is precipitated from this folution in its 

 metallic form, yet there feems to be no doubt that, while it remains diflblVed, it is entirely 

 in the ftate of calx. Its precipitation in the metallic flate is occafioned by the nitre con- 

 tained in the folution, which, having loft part of its oxygen by heat, appears to be capable 

 of attra£ting it from the calx of gold ; for I found, that if the calx of gold is diflblved by 

 being boiled in cauftic alkali, and a fuflScient quantity of nitre, which has loft fome of its air 

 by heat, is mixed with it, the gold is precipitated by an acid in its metallic ftate f . 



Having 



* Pliilofophical Tranfaftions, M,DCC,xcvir. 



,f As the precipitation of gold in its metallic form by nitre which has loft foitie of its oxygen, has not, I be- 

 lieve, been noticed, it may not be improper to mention fome ofthofe fafts relating to it which feem moft enti- 

 tled to attention. Nitre, which has-been heated fome time, precipitates gold in its metallic ftate from a folution 

 in aqua regia, if it is diluted with water. If a folution of gold in nitrous acid is dropped into pure water, the 

 calx of gold is feparated, which is of a yellow colour; but if the water contains a very fmall proportion of 

 nitre, which has loft fome of its air by heat (as one grain in fix ounces), the gold is deprived of its oxygen 

 and becomes blue. The alkali of the nitre does not affift in producing this effeft. Nitrous acid alone, which 



does 



