700 PHILOSOPHICAL TRANSACTIONS. rANNOl790. 



earthen glazed pan; to pour on them some of the acid liquor, which may be in 

 the proportion of 8 or 10 lbs. of oil of vitriol to 1 lb. of nitre; to stir them 

 about, that the surfeces may be frequently exposed to fresh liquor, and to assist 

 the action by a gentle heat from 100° to 200° of Fahrenheit's scale. When the 

 liquor is nearly saturated, the silver is to be precipitated from it by common salt, 

 which forms a luna cornea, easily reducible by melting it in a crucible with a 

 sufficient quantity of pot-ash; and lastly, by refining the melted silver, if ne- 

 cessary, with a little nitre thrown on it. In this manner the silver will be ob- 

 tained sufficiently pure, and the copper will remain unchanged. Otherwise, the 

 silver may be precipitated in its metallic state, by adding to the solution of silver 

 a few of the pieces of copper, and a sufficient quantity of water to enable the 

 liquor to act on the copper. The property which this acid mixture possesses of 

 dissolving silver with great facility, and in considerable quantity, will probably 

 render it a useful menstruum in the separation of silver from other metals; and 

 as the alchemists have distinguished the peculiar solvent of gold under the title of 

 aqua regis, a name sufficiently distinctive, though founded on a fanciful allusion; 

 so, if they had been acquainted with the properties of this compound, they would 

 probably have bestowed on it the appellation of aqua reginae. 



•^ 3. The change of properties communicated to the mixture of vitriolic and 

 nitrous acids by phlogistication. — 20. The above-described compound acid may be 

 phlogisticated in different methods, of which I shall mention 3. 1st, By di- 

 gesting the compound acid with sulphur by means of the heat of a water-bath, 

 the liquor dissolves the sulphur with effervescence, loses its property of yielding 

 white fumes; and if the quantity of sulphur be sufficient, and if the heat ap- 

 plied be long enough continued, it exhibits red nitrous vapours, and assumes a 

 violet colour. 



2dly, If, instead of dissolving nitre in concentrated vitriolic acid, this acid be 

 impregnated with nitrous gas, or with nitrous vapour, by making this gas, or 

 vapour pass into the acid, this compound will be phlogisticated, as it contains 

 not the entire nitrous acid, but only its phlogisticated part, or element, the 

 nitrous gas, without the proportion of pure air is necessary to constitute an acid. 

 This impregnation of oil of vitriol with nitrous gas, or nitrous vapour, was first 

 described, and some of the properties of the impregnated liquor noticed, by Dr. 

 Priestley. See Exp. and Obs. on Air, vol. 3, p. 129 and 21 7. 3dly, By sub- 

 stituting nitrous ammoniac instead of nitre in the mixture with oil of vitriol. 



21. The compound prepared- by any of these methods, but especially by the 

 1st and 2d, differs considerably in its properties with regard to its action on 

 metals from the acid described in the first section. It has been observed, that 

 the latter compound has little action on any metals but silver, tin, mercury, and 

 nickel. On the other hand, the phlogisticated compound not only acts on these. 



