VOL. LXXX.] PHILOSOPHICAL TRANSACTIONS. 705 



6. The same effects were produced when vitriolic acid was added to the satu- 

 rated solution of silver, and iron afterwards applied. 



7. To some of the same nitrous acid, of which a part had been employed to 

 dissolve the silver, I added a piece of iron ; and while the iron was dissolving I 

 poured into the liquor some of the saturated solution of silver ; on which a preci- 

 pitation of silver took place instantly ; though, when the same acid had been pre- 

 viously mixed with the solution of silver, and the iron was then added to the 

 mixture, no precipitation had ensued. 



8. The quantity of vitriolic acid, or of the red fuming nitrous acid, necessary 

 to communicate to the saturated solution of silver the property of being acted on 

 by iron, varies according to the concentration, and to the degree of phlogistica- 

 tion of the acids added ; so that a less quantity than is sufficient does not produce 

 any apparent effect. Yet, when the solution of silver is by addition of these acids 

 brought nearly to a precipitable state, the addition of spirit of wine will, in a little 

 time, render it capable of acting on iron. 



9. It appears then, that a solution of silver is not precipitated by iron in cold, 

 unless it have a superabundance of phlogisticated acid.* 



10. Heat affects the action of a solution of silver on iron: for if iron be di- 

 gested with heat, in a perfectly saturated solution of silver, such as a solution of 

 crystals of nitre of silver in water, the silver will be deposited in its bright metallic 

 state on different parts of the iron, and the iron which has been acted on by the 

 solution appears in form of a yellow ochre. 



1 1 . Bergman relates, that he has sometimes observed beautiful crystallizations 

 or vegetations of metallic silver formed on pieces of iron immersed long in a solu- 

 tion of silver. I have found that no time is able to effect this deposition, unless 

 the solution be in a state nearly sufficiently phlogisticated to admit of a precipita- 

 tion by iron, but not completely phlogisticated enough to effect that purpose 

 immediately. 



12. Dilution with a great deal of water seemed to dispose the solutions of 

 silver to be precipitated by iron more easily. A solution of silver, which did not 

 act on iron, on being very much diluted, and having a piece of iron immersed 



* It was said, at | 4, that the addition of dephlogisticated nitrous acid to a saturated solution of 

 silver did not render this solution precipitable by iron. Yet, as this acid dissolves iron, such a quan- 

 tity may be added, as to overcome the counteracting quality of the solution of silver, so that the acid 

 shall be able to act on the iron ; and vi^hile this metal is dissolving, it phlogisticates the mixture, 

 which then becomes capable of being precipitated, and is in fact reduced to tlie same circumstances as 

 are described at § 7. The limits of the quantities which produce changes cannot be ascertained, be- 

 cause they depend on the degrees of concentration and phlogistication of the substances employed 3 

 and therefore, whenever a change is said to be produced by a certain substance, it means that it may 

 be produced by some proportion, but does not imply by every proportion, of that substance. Without 

 attending to these considerations, persons trying to repeat the experiments mentioned in this paper 

 will be liable to be deceived. — Orig. 



VOL. XVI, 4 X 



