VOL. LXXX.] PHILOSOPHICAL TRANSACTIONS. 703 



the solution itself, or the metallic salt formed in it, yields abundance of gas when 

 mixed with water. 4. That dilution with water renders the concentrated mixture 

 less capable of dissolving silver, but more capable of acting on other metals. 

 5. That this mixture of highly concentrated vitriolic and nitrous acids acquires 

 a purple or violet colour when phlogisticated, either by addition of inflammable 

 substances, as sulphur, or by its action on metals, or by very strong impregna- 

 tion of oil of vitriol with nitrous gas*. 6. That this phlogistication was found 

 to communicate to the mixture the power of dissolving, though in small quanti- 

 ties copper, iron, zinc, and regulus of cobalt. 7. That water expels from a highly 

 phlogisticated mixture of concentrated vitriolic and nitrous acids, or of oil of 

 vitriol impregnated with nitrous gas, a great part of its contained gas; and that 

 therefore this gas is not capable of being retained in such quantity by dilute as by 

 concentrated acids. Water unites with the mixture of oil of vitriol and nitre, 

 without any considerable effervescence. 



29. To these observations I shall subjoin one other fact, namely, that when, 

 to the mixture of oil of vitriol with nitre, a saturated solution of common salt 

 in water is added, a powerful aqua regis is produced, capable of dissolving gold 

 and platina; and this aqua regis, though composed of liquors perfectly colourless 

 and free from all metallic matter, acquires at once a bright and deep yellow colour. 

 The addition of dry common salt to the concentrated mixtures of vitriolic and 

 nitrons acids produces an effervescence, but not the yellow colour; for the pro- 

 duction of which therefore a certain proportion of water seems to be necessary. 

 Part 2. On the precipitation of silver from nitrous acid by iron. 



^ 1 . Bergman relates, that on adding iron to a solution of silver in the nitrous 

 acid, no precipitation ensued; though the affinity of iron to acids in general is 

 known to be much stronger than that of silver; and though, even with regard 

 to the nitrous acid, other experiments evince the superior affinity of iron: for as 

 iron precipitates copper from this acid, and as copper precipitates silver, we must 

 infer the greater affinity of iron than of silver. In the course of his experiments 

 however, some instances of precipitation occurred, which he attributed to the 

 peculiar quality of the irons which he then employed 'j^. I was desirous of dis- 



* Dr. Priestley has noticed this colour communicated to oil of vitriol by impregnation with nitrous 

 gas or vapour, and also the effervescence produced by adding water to this impregnated liquor. See 

 Exp. and Obs. vol. 3, p. 129 and 217- — Orig. 



+ Bergman tried many different kinds of iron, and he thought he found 2 that were capable of 

 precipitating silver. But as he did not discover the circumstances according to which this precipitation 

 sometimes does, and at other times does not happen, he may have been mistaken with regard to the pe- 

 culiar quality of these 2 kinds of iron. At least the several kinds which I have tried always precipi- 

 tated silver in certain circumstances, and always failed to precipitate in certain other circumstances I 

 do not know any other author who has mentioned this subject, excepting Mr. Kirwanj who, in the 

 , conclusion of his valuable papers on the Attractive Powers of Mineral Acids, says, " I have always 

 found silver to be easily precipitated from its solution in the nitrous acid by iron. The sum of the 



