VOL. LXXIII.] PHILOSOPHICAL TRANSACTIONS. 341 



necessary for the solution of lead in this acid. The dephlogisticated acid acts 

 more powerfully. 



Of precipitations of and by iron. — The mutual precipitations of iron and 

 copper from the vitriolic acid by each other, have been well explained in a gene- 

 ral manner by Mr. Monnet and Mr. Bergman ; I shall here show the reason of 

 these precipitations more distinctly. If a piece of copper be put into a saturate 

 solution of iron, fresh made, no precipitation will happen, nor will any of the 

 copper be dissolved in 1 1 hours, nor even in a longer time, if the access of air 

 to the solution be prevented ; but if the solution be exposed to the open air, the 

 addition of a volatile alkali will show the copper to have been acted on in 24 

 hours, or sooner if heat be applied, and a calx of iron is precipitated. 



With regard to a solution of iron in the marine acid, though exposed to the 

 open air, copper precipitates nothing from it in 24 hours. But if a clean piece 

 of iron be put into a solution of copper in the vitriolic acid, the copper is im- 

 mediately precipitated. It is needless to add, that copper is in the same manner 

 precipitated by iron from the nitrous and marine acids. Hence the practice of 

 extracting copper from some mineral waters by means of iron. These waters 

 therefore furnish afterwards, by evaporation, vitriol of iron ; but it is remarkable 

 that this vitriol is much paler than the common, and less fit for dyeing, 2 Schlut- 

 ter 507. The reason of which is, that it is more dephlogisticated, not only be- 

 cause old iron is chiefly used, but because copper, containing more phlogiston 

 than an equal weight of iron, deprives it of more of its phlogiston than it 

 would lose if barely dissolved in the vitriolic acid. Cast iron, according to 

 Schlutter, will scarcely precipitate a solution of copper ; and in effect Mr. Berg- 

 man has found that it contains less phlogiston than bar iron. I have always 

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

 With regard to the nitrous acid, I found that zinc does not precipitate iron ; 

 but on the contrary, iron precipitates zinc ; but in a short time the acid re- 

 dissolves the zinc, and lets fall the iron, which evidently proceeds from the too 

 great dephlogistication of the calx of iron. But zinc precipitates iron from the 

 marine, though with difficulty ; for after 24 hours the galls still struck a black. 

 Iron does not precipitate zinc from the vitriolic acid. 



Most metallic substances, precipitated by iron from the nitrous acid, are in 

 some measure re-dissolved shortly after, as the nitrous acid soon dephlogisticates 

 the iron too much, then lets it fall, and re-acts on the other metals and re-dis- 

 solves them. The precipitation of the argillaceous earth from alum by iron is 

 owing to the excess of acid in the alum which first dephlogisticates the iron ; 

 and when this is dephlogisticated, it attracts the acid more strongly. Earth of 

 alum, on the other hand, precipitates iron when the solution of iron is dephlo- 



